Elements, Compounds and Mixtures (experimental techniques) | ICSE Class 7 Chemistry Notes

This note covers elements and compounds, symbols and formulae, mixtures and their types, differences between mixtures and compounds, evaporation, distillation, separating funnels, sublimation, fractional distillation, paper chromatography, and the choice and order of separation techniques.

What are elements, compounds and pure substances?

Matter is anything that has mass and occupies space. Mass describes the quantity of matter in an object. A physical process changes physical features without producing a new substance. Matter is made of tiny particles. To classify a material, consider what its particles are and whether different substances are present together.

Definition: A pure substance consists of the same type of particles and cannot be separated into other substances by a physical process. Elements and compounds are both pure substances.

What makes a substance an element?

An element contains one kind of atom and cannot be broken down into simpler substances by chemical methods. An atom is a tiny particle of an element that retains its chemical identity. Iron, aluminium, copper, hydrogen and oxygen are elements.

A molecule is a particle made of atoms chemically joined together. The atoms of most elements cannot exist independently. Two hydrogen atoms form a hydrogen molecule; two oxygen atoms form an oxygen molecule. These remain elements because each contains just one kind of atom.

What makes a substance a compound?

A compound contains different elements chemically combined in a fixed ratio, a comparison of quantities. Its properties differ from those of its constituent elements. Here, constituents means the substances or elements from which something is formed. Water contains hydrogen and oxygen chemically combined.

The number of hydrogen atoms to oxygen atoms in water is in the ratio two to one. Water is therefore a compound even though a sample of pure water contains the same type of water particles throughout.

Common salt, also called sodium chloride, is another compound. Sugar is a compound containing carbon, hydrogen and oxygen. The presence of several kinds of atoms does not make a pure compound a mixture.

How do symbols and formulae help us identify substances?

A chemical symbol is the short letter representation of an element. A chemical formula uses element symbols to show the composition of a substance. The first letter of a symbol is capitalised, and a second letter, when present, is lower case.

The symbols used below are Fe for iron, Al for aluminium, Cu for copper, H for hydrogen, O for oxygen, Na for sodium, Cl for chlorine, and Ca for calcium. Read each complete symbol together: the two letters in Fe represent one element.

What do the small numbers mean?

A subscript is a small number written below the normal line after a symbol. In a molecular formula it shows the number of atoms of that element in a molecule. When no subscript is shown, one atom is understood.

In H₂O, the subscript applies to hydrogen. The formula represents two hydrogen atoms and one oxygen atom in each water molecule. In NaCl and CaO, the formula gives the simplest ratio of the constituent particles, one to one; it need not describe a separate molecule.

Note: Count kinds of elements, not letters or total atoms. O₂ contains two atoms but only one kind of element. H₂O contains two kinds of elements and is a compound.

A mixture has no single fixed chemical formula describing its overall composition. Listing the formulae of its components identifies what has been mixed; it does not turn those components into a new compound.

How does a mixture differ from a compound?

A mixture forms when two or more substances are brought together without a chemical change. Its components are the individual substances present. A chemical change produces a new substance with different properties; simply mixing substances need not produce one.

The components of a mixture retain their individual properties. They may be present in different proportions. In contrast, the elements in a particular compound are chemically combined in a fixed ratio. Changing that ratio does not simply produce a different sample of the same pure compound.

A solution is a uniform mixture formed when one substance dissolves in another.

Which differences are useful for identification?

A physical change changes a physical feature, such as state, without making a new substance. Evaporating water from salt solution separates the components physically. The salt recovered remains sodium chloride; it has not been separated into sodium and chlorine.

Likewise, boiling water changes liquid water into water vapour. Vapour is the gaseous form of a substance such as water. It does not mean that water has separated into its constituent elements. Cooling the vapour can return it to liquid water.

Appearance alone is insufficient for deciding whether something is a compound. A mixture may look uniform. Sugar solution looks the same throughout, but sugar and water remain separate substances at the particle level. Composition and the behaviour of the components provide the stronger evidence.

What does the iron and sulphur experiment demonstrate?

Iron and sulphur show the difference between mixing and chemical combination. Iron filings are small pieces of iron. Before heating, iron filings mixed with sulphur powder form a mixture in which the components retain their properties.

How are the samples prepared and compared?

1. Examine iron filings and sulphur powder, then mix them thoroughly. Keep some of this unheated mixture for comparison.
2. For the demonstration, heat a portion containing the elements in the required reacting proportion until a black mass forms.
3. Allow the product to cool. Compare its appearance with the original mixture.
4. Bring a magnet near each sample and compare whether iron can be drawn out.

In the unheated mixture, the black iron particles and yellow sulphur particles can be seen. A magnet attracts the iron filings, allowing separation from sulphur. This demonstrates that iron retains its magnetic behaviour when it is simply mixed with sulphur.

The black product formed on heating is iron sulphide, a compound of iron and sulphur. Its colour and texture are uniform throughout. It is not attracted by the magnet in this demonstration, and the original iron cannot be drawn out by magnetic separation.

The word description of the reaction is: iron combines with sulphur on heating to form iron sulphide. A new substance with different properties has formed. Grinding the cooled product does not change the compound back into the original mixture.

Note: Heating this mixture is a supervised laboratory demonstration. Use the required reacting proportion and suitable apparatus in a well-ventilated area. Do not inhale gases produced during the activity.

The useful comparison is between the original components and the new product. Mixing puts substances together; chemical combination produces a substance whose properties cannot be inferred simply by adding the properties of its starting elements.

What are the different types of mixtures?

A homogeneous mixture has a uniform composition throughout. A heterogeneous mixture has a non-uniform composition. Composition means what a material contains and the proportions of those constituents. These classifications concern distribution, rather than whether the components are solids or liquids.

How can uniform and non-uniform mixtures be recognised?

A solution is a homogeneous mixture. In sugar solution, sugar is the solute, the substance dissolved, and water is the solvent, the substance that dissolves it. Dissolved sugar is distributed uniformly, so separate sugar particles cannot be seen.

Sand and water form a heterogeneous mixture. Sand particles remain visible and settle with time. Kerosene and water also form a heterogeneous mixture because they separate into distinct liquid layers. A material need not contain a solid to be heterogeneous.

Alloys are homogeneous mixtures of two or more metals, or of a metal and a non-metal. Brass contains copper and zinc; bronze contains copper and tin. These examples show that a solid material can also be a mixture.

How are mixtures grouped by physical state?

Most solid-solid mixtures are heterogeneous. Alloys show why “most” must not become “all”. Equally, liquid-liquid mixtures may be uniform or may form separate layers. The state of the components alone cannot decide which type of mixture is present.

Note: Milk appears uniform but is a colloid, a heterogeneous mixture containing very small dispersed particles. “Dispersed” means distributed through another substance. Looking uniform to the unaided eye does not by itself prove that a mixture is a solution.

How can the concentration of a mixture be calculated?

Concentration describes the amount of solute in a given amount of solution or solvent. A percentage compares the amount of solute with 100 parts of the whole solution. Always identify whether the quantities are masses or volumes.

Which quantities belong in each formula?

Let $m$ be the solute mass, $w$ the solvent mass and $M$ the total solution mass, all in grams. Let $v$ be the solute volume and $V$ the final solution volume, both in millilitres.

For a solution containing a solute and a solvent, $M = m + w$. The denominator in a percentage calculation is the amount of solution, including the solute, rather than the amount of solvent alone.

Mass by mass percentage, $C_m$, gives grams of solute per 100 g of solution: $C_m = \frac{m}{M}\times100$.

Mass by volume percentage, $C_{mv}$, gives grams of solute per 100 mL of solution: $C_{mv} = \frac{m}{V}\times100$. Use mass in grams and volume in millilitres for this convention.

Volume by volume percentage, $C_v$, gives millilitres of solute per 100 mL of solution: $C_v = \frac{v}{V}\times100$. The final solution volume is the volume after mixing.

How are the formulas applied?

Worked example 1. Calculate the mass by mass percentage when 10 g of salt is dissolved in 90 g of water.

Formula: $M = m + w$; $C_m = \frac{m}{M}\times100$.

Substitute: $M = 10\,\mathrm{g}+90\,\mathrm{g}=100\,\mathrm{g}$. Then $C_m = \frac{10}{100}\times100=10$.

Answer: The solution contains 10 per cent salt by mass. Its total mass is 100 g.

Worked example 2. Calculate the mass by volume percentage of a solution made by dissolving 5 g of glucose in water to make 100 mL of solution.

Formula: $C_{mv} = \frac{m}{V}\times100$, with mass in grams and final solution volume in millilitres.

Substitute: $C_{mv} = \frac{5}{100}\times100=5$.

Answer: The glucose concentration is 5 per cent by mass by volume, meaning 5 g of glucose per 100 mL of solution.

Worked example 3. A pesticide spray contains 1 mL of liquid pesticide in a final solution volume of 100 mL. Calculate its volume by volume percentage.

Formula: $C_v = \frac{v}{V}\times100$.

Substitute: $C_v = \frac{1}{100}\times100=1$.

Answer: The pesticide concentration is 1 per cent by volume, meaning 1 mL of pesticide per 100 mL of solution.

Worked example 4. Talcum powder contains 4 per cent zinc oxide by mass. Find the mass of zinc oxide in 300 g of powder.

Formula: $C_m = \frac{m}{M}\times100$. Rearranging gives $m = \frac{C_m M}{100}$, where $M$ is the total mass of powder.

Substitute: $m = \frac{4\times300}{100}\,\mathrm{g}=12\,\mathrm{g}$.

Answer: The powder contains 12 g of zinc oxide.

Worked example 5. Two tablespoons of orange juice concentrate are mixed with water to make 150 mL of juice. Each tablespoon holds 15 mL. Find the volume by volume percentage of concentrate.

Formula: $v = n t$; $C_v = \frac{v}{V}\times100$, where $n$ is the number of tablespoons and $t$ is the volume per tablespoon.

Substitute: $v = 2\times15\,\mathrm{mL}=30\,\mathrm{mL}$. Then $C_v = \frac{30}{150}\times100=20$.

Answer: The juice contains 20 per cent concentrate by volume.

Worked example 6. Student A dissolves 20 g of sugar in 80 g of water, B dissolves 20 g in 100 g of water, and C dissolves 30 g in 80 g of water. Calculate their mass percentages and identify the most concentrated solution.

Formula: $M = m + w$; $C_m = \frac{m}{M}\times100$.

Substitute for A: $M = 20\,\mathrm{g}+80\,\mathrm{g}=100\,\mathrm{g}$; $C_m = \frac{20}{100}\times100=20$.

Substitute for B: $M = 20\,\mathrm{g}+100\,\mathrm{g}=120\,\mathrm{g}$; $C_m = \frac{20}{120}\times100\approx16.67$.

Substitute for C: $M = 30\,\mathrm{g}+80\,\mathrm{g}=110\,\mathrm{g}$; $C_m = \frac{30}{110}\times100\approx27.27$.

Answer: A contains 20 per cent sugar by mass, B approximately 16.67 per cent and C approximately 27.27 per cent. C is most concentrated because sugar makes up the greatest fraction of its total solution mass.

How should a separation technique be chosen?

Separation uses a difference in the physical properties of components. First identify the mixture, then identify the useful difference, and finally decide which component must be collected. Naming an apparatus without explaining the property does not explain why the method works.

Soluble means able to dissolve in a stated solvent; insoluble means unable to dissolve appreciably in that solvent. Salt dissolves in water, whereas sand does not. This difference allows water to help separate a mixture of sand and salt.

Which properties guide the main methods?

Density is mass per unit volume, meaning the mass contained in a given amount of space. A boiling point is the temperature at which a liquid boils at a given pressure. Each term describes a property that can help distinguish components.

The same starting mixture may allow different choices depending on the intended product. Evaporation is useful when salt is wanted. Distillation is useful when water must be collected too. The purpose of the separation is therefore part of the reason for choosing the method.

Some mixtures require several steps because no single property separates every component. A sound plan explains what remains after each step and where the next useful component is located.

How does evaporation recover a dissolved solid?

Evaporation is the change of a liquid into vapour from its surface. In a salt solution, water is the solvent and common salt is the dissolved solute. When water evaporates, salt is left behind.

What happens during the separation?

1. Place the salt solution in a suitable evaporating dish, a heat-resistant dish used to evaporate liquid.
2. Allow the water to evaporate, or use controlled heating in a supervised demonstration to make evaporation faster.
3. Observe that the liquid decreases as water passes into the surrounding air as vapour.
4. Collect the salt remaining after the water has been removed and the apparatus has cooled.

The method depends on the different behaviour of water and salt during the process. Water leaves as vapour. Salt does not leave with the evaporating water under these conditions. The salt was present before evaporation; heating has not manufactured it.

Salt can also be obtained from seawater through removal of water. Seawater contains dissolved salts and is therefore a mixture. The separation changes the distribution and physical state of components, rather than converting water into salt.

What is the limitation of this method?

In ordinary evaporation, the water vapour escapes into the surroundings. The solvent is therefore not collected as liquid water. If water is the required product, letting it escape defeats the purpose of the separation.

Evaporation should also not be described as a universal way of obtaining a pure solid from any mixture. A mixture may contain other dissolved substances. Removing water does not, by itself, explain how those dissolved substances would be separated from one another.

The useful conclusion is specific: evaporation can separate dissolved common salt from water. For recovering the water as well, the escaping vapour must be cooled and collected. This additional stage leads to distillation.

How does distillation collect a pure liquid?

Distillation separates a liquid by vaporising it and then cooling its vapour back into liquid. Condensation is the change from vapour to liquid. The liquid collected after condensation is called the distillate.

For water containing dissolved common salt, water is vaporised from the solution. Its vapour enters a condenser, an apparatus that cools vapour so that it becomes liquid. The condensed water is collected separately while salt remains in the original vessel.

What are the main stages?

1. Place the solution in the distillation flask, the vessel in which the mixture is heated.
2. Heat it in a properly assembled laboratory set-up so that water vapour forms.
3. Guide the vapour through the condenser, where cooling causes condensation.
4. Collect the distillate in a separate receiving vessel.

Distillation uses both heating and cooling. Simply boiling a solution in an open dish is not enough to collect the solvent. A path for the vapour and a cooled collection stage are essential parts of the method.

A distillation flask containing acetone and water stands above a burner. A thermometer, an instrument for measuring temperature, is fitted above the flask. A sloping water condenser connects it to a conical flask labelled acetone. Water inlet and outlet tubes and supporting stands are labelled.

See Fig. 5.12 in your NCERT textbook

Acetone is the liquid named in this diagram. It mixes with water and has a lower boiling point. The apparatus illustrates how vapour travels from the heated flask to a cooled region and then into the receiver.

Can distillation separate two liquids?

Miscible liquids mix uniformly with each other. Simple distillation can separate suitable miscible liquids whose boiling points differ by at least about 25 °C, where °C means degrees Celsius, a unit of temperature. Liquids with relatively close boiling points require fractional distillation.

Distillation can recover water from a solution containing a non-volatile dissolved solid such as common salt. Do not assume that distillation separates every possible impurity from every liquid equally well.

How does a separating funnel separate two liquid layers?

Immiscible liquids do not mix to form one uniform liquid. Kerosene and water are an example. On standing, they form separate layers. For immiscible liquids of different densities, the denser liquid forms the lower layer.

A separating funnel is a vessel with a narrow outlet controlled by a tap called a stopcock. Opening the tap allows the lower layer to flow out. Closing it controls where collection stops.

How is the funnel used?

1. Place the mixture in the separating funnel and leave it undisturbed until the liquid layers are distinct.
2. Place a receiving vessel below the outlet and slowly open the stopcock to drain the lower layer.
3. Close the stopcock when the lower layer is almost completely drained.
4. Collect separately the small boundary portion that may contain both liquids, then collect the upper liquid separately.

In the mustard oil and water example, mustard oil forms the upper layer and water forms the lower layer. The lower water layer is therefore collected first. Separating the boundary portion helps avoid mixing the two collected liquids again.

A separating funnel is supported on a laboratory stand above a conical flask. Mustard oil is labelled in the upper layer and water in the lower layer. The stopcock is labelled near the outlet; the glass stopper is shown separately beside the apparatus.

See Fig. 5.16 in your NCERT textbook

This method depends on distinct liquid layers. Alcohol and water do not form such layers, so opening a funnel tap would not separate them into alcohol and water. Their miscibility makes a boiling-point method more appropriate.

A separating funnel therefore uses both lack of mixing and a density difference. It does not work merely because a mixture contains two liquids, nor does it require one component to evaporate.

How does sublimation separate a solid mixture?

Sublimation is the direct change of a solid into vapour below its melting point, without passing through a liquid state. The melting point is the temperature at which a solid changes into a liquid.

The reverse change, from vapour directly to solid, is called deposition. Together, these changes allow a suitable solid to leave a mixture as vapour and then be collected as a solid on a cooler surface.

What happens with camphor and sand?

Camphor is a solid that sublimes. In a mixture of camphor and sand, gentle heating allows camphor to pass into vapour while sand remains. On a cooler surface, the vapour forms solid camphor again.

1. Place the dry mixture of crushed camphor and sand in a clean, dry china dish.
2. Cover the dish with an inverted glass funnel whose narrow opening is plugged with cotton.
3. Heat gently in a supervised laboratory demonstration so that camphor changes into vapour.
4. Look for solid deposits on the cooler inner wall of the funnel while sand remains in the dish.

You may find white camphor deposits on the inner wall. The observations connect the method to the useful property: camphor sublimes, whereas sand does not sublime during this heating. The two solids therefore follow different paths.

Ammonium chloride is another solid used in school demonstrations of separation by sublimation. When describing this technique, identify the solid collected on cooling and the non-subliming material remaining in the original vessel.

Evaporation and sublimation must not be confused. Evaporation begins with a liquid; sublimation begins with a solid. In a sublimation separation, collecting the solid deposit is as important as heating the starting mixture.

Why is fractional distillation used for alcohol and water?

Fractional distillation separates miscible liquids with relatively close boiling points. Alcohol and water are a suitable example. Here, alcohol refers to ethanol, the alcohol used in this laboratory example. The liquids mix uniformly, so a separating funnel cannot separate them.

What does the fractionating column do?

A fractionating column is fitted between the heated flask and the condenser. It provides repeated opportunities for vapour to condense and liquid to vaporise again. These repeated changes improve separation compared with a single vaporisation and condensation stage.

The vapour travelling upwards becomes richer in the more volatile component, meaning the component that vaporises more readily under the conditions. For alcohol and water, this is alcohol. Vapour reaching the condenser is cooled, and the resulting liquid is collected.

A collected portion is called a fraction. The name “fractional distillation” refers to collecting portions separated through differences in boiling behaviour. It does not mean cutting up the liquid or filtering out visible particles.

How does this differ from simple distillation?

The condenser still has the same role: it cools vapour into liquid. The fractionating column performs the additional separation before that final cooling. Keeping these roles separate helps explain the apparatus clearly.

Do not describe the first collected fraction as necessarily completely pure alcohol. The useful introductory explanation is that fractional distillation improves separation and produces a fraction richer in the more volatile liquid. The method relies on boiling behaviour, not on a visible boundary between layers.

How does paper chromatography separate the colours in ink?

Paper chromatography separates dissolved components because they travel at different rates along paper with a moving solvent. An ink that looks like one colour may contain several colouring substances. These are often called dyes, substances used to give colour.

What are the stationary and mobile phases?

A phase here means a part of the separation system with a particular role. The stationary phase stays in place, while the mobile phase moves and carries the sample components. The paper supports the stationary phase; the solvent rises through it.

In ordinary paper chromatography, water held by the paper acts as the stationary phase. The mobile phase is the moving solvent or solvent mixture. At this level, the key idea is interaction with the paper system compared with travel in the moving liquid.

Components interact differently with the solvent and the paper. They therefore move at different rates and may separate into coloured spots. The separation is not simply caused by one colour being darker than another.

How is a suitable sketch-pen ink tested?

1. Draw a horizontal pencil line near the lower end of a strip of chromatography paper or filter paper.
2. Put a small spot of suitable black sketch-pen ink at the centre of that line.
3. Place a shallow layer of water in a container and dip the lower end of the paper into it. Keep the ink spot above the water level.
4. Allow the water to rise through the paper and observe whether the ink separates into different coloured spots.

The first strip shows a pencil line near its lower end, and the second has an ink spot on that line. The third drawing shows the strip in a gas jar with a shallow water layer below the ink spot and a watch glass above.

See Fig. 5.15 in your NCERT textbook

The starting spot must remain above the liquid surface. Otherwise, the ink can dissolve directly into the liquid in the container instead of travelling from a small starting spot along the paper.

Different coloured spots provide evidence that the ink contains different components. A single visible spot does not necessarily prove purity, because components may not separate under the chosen conditions. The choice of solvent therefore matters.

Water is not suitable for every sample. Some separations need alcohol or a mixture of solvents. Chromatography can also separate coloured substances from flower petals or leaf extracts, helping reveal components that were not distinguishable in the original mixture.

How can several techniques be used in the right order?

A separation plan must follow the components from one stage to the next. For a mixture of sand and common salt, the useful first difference is their behaviour in water: salt dissolves, while sand remains undissolved.

Filtration separates a suitable insoluble solid from a liquid by passing the mixture through a filtering material. The solid retained is the residue. The liquid passing through is the filtrate. These names describe where each material ends up.

How can sand and salt be recovered?

1. Add water to the mixture and stir so that the salt dissolves. Sand remains as an insoluble solid.
2. Filter the mixture through suitable filter paper. Sand remains on the paper as residue.
3. Collect the salt solution below the filter as the filtrate. Dissolved salt passes through with the water.
4. Evaporate the water from this filtrate to recover the salt. Use distillation instead if the water must also be collected.

The order matters. Evaporating the water before removing sand would leave salt and sand together again. Filtration first separates sand from the solution, so the later evaporation acts on a different, simpler mixture.

Filtering salt solution alone does not recover its dissolved salt. The salt is not present as the kind of undissolved particles retained by ordinary filter paper. This is why the filtrate still requires another method.

What should an experimental explanation include?

State the starting components, the property used, the method, and the material collected at each stage. Include relevant conditions, such as keeping a chromatography spot above the solvent or allowing liquid layers to settle before using a separating funnel.

For any multistep separation, check that the chosen sequence keeps the wanted components available for collection. A method is justified by the behaviour of the actual mixture and the required product, rather than by memorising an apparatus name alone.

Glossary

• Element — A pure substance containing one kind of atom and not chemically separable into simpler substances.
• Compound — A substance containing different elements chemically combined in a fixed ratio, with properties different from its elements.
• Mixture — Two or more substances present together without chemical combination, retaining their individual properties.
• Homogeneous mixture — A mixture whose components are uniformly distributed, giving the same composition throughout.
• Heterogeneous mixture — A mixture with non-uniform composition, in which components are not evenly distributed throughout.
• Solute — The substance dissolved in a solvent to form a homogeneous mixture called a solution.
• Solvent — The substance that dissolves a solute, such as water in sugar solution.
• Evaporation — The change of a liquid into vapour from its surface, allowing a suitable dissolved solid to remain.
• Distillation — Separation involving vaporisation of a liquid followed by condensation and collection of the resulting liquid.
• Immiscible liquids — Liquids that do not mix to form a single uniform liquid, such as kerosene and water.
• Sublimation — The direct change of a solid into vapour without passing through the liquid state.
• Fractional distillation — Separation of miscible liquids with relatively close boiling points using repeated vaporisation and condensation.
• Chromatography — Separation based on different rates of movement of components through a system with stationary and mobile phases.
• Residue — The solid retained on the filtering material during the filtration of a suitable mixture.
• Filtrate — The liquid that passes through the filtering material and is collected below it.

Common errors and misconceptions

• Misconception: Anything containing more than one atom is a compound. Correct: A compound contains different elements chemically combined. An oxygen molecule contains two oxygen atoms and belongs to an element.
• Misconception: A mixture has a fixed chemical formula. Correct: Its components can occur in different proportions. Each compound has a fixed composition, but the overall mixture has no single fixed formula.
• Misconception: A uniform-looking material must be a pure substance. Correct: Sugar solution is a homogeneous mixture. Milk also looks uniform but contains dispersed particles and is a colloid.
• Misconception: Evaporation collects both salt and water from salt solution. Correct: Salt remains, but the water vapour escapes. Distillation adds cooling and collection to recover the water.
• Misconception: A separating funnel separates any pair of liquids. Correct: It separates distinct layers of suitable immiscible liquids, such as kerosene and water, rather than uniformly mixed alcohol and water.
• Misconception: Sublimation means that a solid melts and then boils. Correct: The solid changes directly into vapour without a liquid stage; cooling can produce a solid deposit.
• Misconception: A chromatography ink spot should begin under water. Correct: The paper touches the solvent, but the spot begins above its level so the sample travels along the paper.
• Misconception: Ordinary filtration removes dissolved common salt from water. Correct: The salt passes through in solution. Recover it from the filtrate by removing the water.

Exam-style questions with model answers

Q1. O is the symbol for oxygen and H is the symbol for hydrogen. Classify O₂ and H₂O as an element or a compound, giving one reason for each. [2 marks]

1. O₂ represents an element because its molecules contain only one kind of atom, oxygen, even though each molecule contains two atoms.
2. H₂O represents a compound because its molecules contain two different elements, hydrogen and oxygen, chemically combined in a fixed ratio.

Q2. An unheated mixture contains iron filings and sulphur powder. The filings are attracted by a magnet. Heating these elements in the required reacting proportion produces black iron sulphide, which is not attracted by the magnet. Explain the evidence for a mixture before heating and a compound afterwards. [3 marks]

1. Before heating, the iron retains its magnetic property. This shows that mixing has left iron present as a component with its own properties.
2. A magnet can separate the iron filings from the original mixture. The components have not become one new chemically combined substance.
3. After heating, the black iron sulphide has different magnetic behaviour. It is a new compound, and its constituent iron cannot be removed by the same magnetic method.

Q3. A solution contains only water and dissolved common salt. Under the separation conditions, water vaporises and salt remains. Choose a method to collect the water, and explain the stages and the fate of the salt. [4 marks]

1. Use distillation because the required product is water. The method includes collection of the vaporised solvent after it is cooled.
2. Heat the salt solution in the distillation flask so that water changes into vapour and leaves the liquid mixture.
3. Pass this vapour through a condenser. Cooling changes it into liquid water, which is collected separately as the distillate.
4. The common salt remains in the original flask under the stated conditions. Ordinary evaporation would leave this salt but allow water vapour to escape.

Q4. Mustard oil and water are immiscible. After standing, oil forms the upper layer and water the lower layer. Describe how to collect them separately with a separating funnel, including how to handle the boundary portion. [5 marks]

1. Put the mixture in a separating funnel and allow it to stand undisturbed. The supplied information shows that the liquids form distinct layers suitable for this method.
2. Place a receiving vessel beneath the funnel. Slowly open the stopcock, the tap controlling the outlet, to drain the lower water layer first.
3. Watch the liquid boundary as the water drains. Close the stopcock when the lower water layer has almost completely left the funnel.
4. Collect the next small boundary portion separately because it may contain both liquids. Keep this portion out of the separately collected water and oil.
5. Use another receiving vessel to collect the upper mustard oil layer separately. The separation uses immiscibility and the difference in density indicated by the layer arrangement.

Q5. A dry mixture contains sand and common salt. Salt dissolves in water, sand does not, and ordinary filter paper retains sand but allows salt solution through. Water can be evaporated while salt remains. Explain four steps to recover sand and salt separately. [4 marks]

1. Add water and stir the mixture so that the common salt dissolves. The sand remains undissolved because it is insoluble in water.
2. Filter the mixture through the stated filter paper. Sand stays on the paper as the residue, separating it from the salt solution.
3. Collect the liquid passing through as the filtrate. This liquid contains dissolved salt, so it must be kept for the next stage.
4. Evaporate water from the filtrate to recover common salt. Removing sand before evaporation prevents the salt from being left mixed with sand again.

Q6. A black sketch-pen ink contains water-soluble colouring substances. These substances interact differently with paper and move at different rates with rising water. Describe a paper chromatography test, its expected observation and the conclusion. [5 marks]

1. Draw a horizontal pencil line near the lower end of a paper strip. Place a small spot of the supplied black ink on the line.
2. Put a shallow layer of water in a suitable container and dip the lower end of the strip into it. Keep the ink spot above the initial water level.
3. Allow water to rise through the paper. The water acts as the mobile phase, carrying the dissolved colouring substances away from their starting position.
4. Observe the development of separate coloured spots as the components move at different rates. Their different interactions with the solvent and paper provide the basis of separation.
5. Conclude that the original black ink is a mixture of colouring substances. The separated spots reveal components that were not individually distinguishable in the original ink spot.

Q7. Camphor changes directly from solid to vapour on gentle heating, while sand does not. Camphor vapour forms solid deposits on a cooler surface. Name the two changes that allow camphor to be recovered from sand and explain each. [2 marks]

1. Sublimation changes solid camphor directly into vapour, allowing it to leave the sand that remains in the heated dish.
2. Deposition changes the cooled camphor vapour directly back into solid camphor, allowing collection on the cooler surface.

Q8. Alcohol and water are miscible liquids with relatively close boiling points. Alcohol vaporises more readily under the conditions used. Choose a separation method, explain the role of its column, and state what happens in the condenser. [3 marks]

1. Use fractional distillation. The liquids mix uniformly and have relatively close boiling points, so the method uses their different boiling behaviour for separation.
2. The fractionating column allows repeated condensation and vaporisation. Vapour travelling towards the top becomes richer in alcohol, the more readily vaporised component.
3. The condenser cools the vapour back into liquid, which is collected as a fraction. This fraction is richer in alcohol; the description does not establish complete purity.

Key takeaways

• Elements contain one kind of atom; compounds contain different elements chemically combined in a fixed ratio.
• Mixtures contain components that retain their properties, and their proportions can vary without creating a new compound.
• Homogeneous mixtures have uniform composition throughout; heterogeneous mixtures have non-uniform composition, even when differences are difficult to see.
• Choose a separation method by linking a difference in physical properties to the component that must be collected.
• Evaporation leaves dissolved salt behind, while distillation also cools the vapour and collects the water as a liquid.
• A separating funnel separates layers of suitable immiscible liquids; fractional distillation separates miscible liquids with relatively close boiling points.
• Sublimation changes a suitable solid directly into vapour, and deposition collects it again as a solid on cooling.
• Paper chromatography separates components by different rates of movement; keep the starting spot above the initial solvent level.
• For sand and salt, dissolve the salt, filter out sand, then remove water from the collected salt solution.

Test yourself

Why can a compound be a pure substance even though it contains different elements?

Its elements are chemically combined in a fixed ratio, and a pure sample contains the same type of constituent particles throughout.

Why is O₂ an element rather than a compound?

O₂ contains two oxygen atoms in each molecule, but both atoms belong to the same element.

Does evaporating salt solution separate sodium chloride into sodium and chlorine?

No. Evaporation separates water from dissolved sodium chloride. The recovered salt remains the same compound.

What is the difference between a distillate and a filtrate?

A distillate is liquid collected after vapour condenses. A filtrate is liquid collected after passing through filtering material.

Why must a separating-funnel mixture be allowed to stand?

Standing allows the immiscible liquids to form distinct layers so that the lower layer can be drained separately.

How do sublimation and deposition differ?

Sublimation changes solid directly into vapour. Deposition changes vapour directly into solid without an intermediate liquid stage.

What does the fractionating column add to a distillation set-up?

It allows repeated condensation and vaporisation, improving separation of miscible liquids whose boiling points are relatively close.

Why is the starting ink spot kept above the chromatography solvent?

This lets the rising solvent carry ink along the paper, instead of the ink dissolving directly into the liquid reservoir.

Why filter sand and salt solution before evaporating the water?

Filtration removes sand first. Evaporating the unfiltered mixture would leave the sand and salt together again.