The Amazing World of Solutes, Solvents, and Solutions | CBSE Class 8 Science Notes

This note covers uniform and non-uniform mixtures, solutes and solvents, saturation, concentration, solubility, the effects of temperature, density and relative density, measurement of mass and volume, water displacement, and the effects of heating and pressure on density.

What makes a mixture a solution?

A mixture contains two or more substances together. Its components are the substances that make it up. In a uniform mixture, the components are evenly distributed throughout. A solution is a uniform mixture formed when two or more substances mix.

Salt or sugar mixed with water can form a solution. The dissolved material is spread throughout the water, rather than remaining in separate visible portions. The components of a solution are not visible separately.

How do uniform and non-uniform mixtures differ?

In a non-uniform mixture, the components are not evenly distributed. They can be seen separately with the naked eye or with a magnifying device. Chalk powder, sand and sawdust mixed with water are examples.

Mixing substances is therefore not sufficient by itself to identify a solution. The important feature is whether the components become evenly distributed. Sand and water remain a mixture, but they do not form a solution.

Why can each sip contain the same components?

When sugar and salt dissolve in water, both are distributed throughout the mixture. This explains why different sips can taste the same instead of one tasting salty and another sweet. The uniformity concerns the distribution of the substances through the liquid.

A beaker is connected to a magnified circular schematic. Yellow circles labelled solute are distributed among smaller blue circles labelled solvent. The drawing represents the even distribution of the dissolved material.

See Fig. 9.3 in your NCERT textbook

How are solute and solvent identified?

When a solid dissolves in a liquid to form a solution, the solid is called the solute and the liquid is called the solvent. The solute is the substance being dissolved. The solvent is the substance in which it dissolves.

Definition: In a solution made by dissolving a solid in a liquid, the solid component is the solute and the liquid component is the solvent.

For salt dissolved in water, salt is the solute and water is the solvent. For sugar dissolved in water, sugar is the solute and water is the solvent. The complete uniform mixture is the solution.

Does the smaller amount always identify the solute?

When two liquids form a solution, it is not always clear which substance is dissolving the other. In this case, the substance present in the smaller amount is called the solute. The substance present in the larger amount is called the solvent.

This rule for two liquids should not replace the rule for a solid dissolved in a liquid. Chashni, the sugar syrup surrounding gulab jamuns, contains a large amount of solid sugar dissolved in a small amount of liquid water. Sugar remains the solute and water remains the solvent.

The terms describe the roles of the components in a solution. They should be identified from the kind of mixture being considered, rather than from an assumption that every solute must be present in a smaller amount than its solvent.

How do saturation, concentration and solubility differ?

An unsaturated solution can dissolve more solute at a given temperature. A saturated solution has dissolved the maximum amount of that solute and cannot dissolve more at that temperature. Temperature is part of both definitions.

How can saturation be investigated?

1. Half-fill a clean glass tumbler with water.
2. Add one spoon of salt and stir until it dissolves completely.
3. Continue adding salt a spoonful at a time, stirring after each addition.
4. Observe when added salt stops dissolving completely and some remains at the bottom.

Initially, the salt dissolves. After further additions, the water reaches its limit for dissolving salt under those conditions. The undissolved salt at the bottom shows that further additions have exceeded what the solution can dissolve at that temperature.

What does concentration describe?

Concentration is the amount of solute present in a fixed quantity of solution or solvent. A dilute solution contains less solute in the quantity being compared; a concentrated solution contains more. Dilute and concentrated are relative terms.

For the same starting amount of water, the solution made with one spoon of salt is dilute compared with one made with two or more spoons. In another comparison, four spoons of salt in 50 millilitres of water are more concentrated than two spoons in 100 millilitres.

A millilitre, written mL, is a unit of volume, or space occupied. Solubility is the maximum amount of solute that dissolves in a fixed quantity of solvent at a particular temperature. Unlike concentration, it describes the dissolving limit.

Note: “Concentrated” compares the amount of dissolved solute, whereas “saturated” identifies a dissolving limit at a particular temperature. State the temperature condition whenever defining saturation.

How does heating affect the solubility of solids?

For most substances, solubility increases with an increase in temperature. A demonstration with baking soda shows how a solution that has reached its dissolving limit can dissolve more solute after heating.

Baking soda is sodium hydrogen carbonate. A laboratory thermometer is an instrument for measuring temperature. The symbol °C means degrees Celsius, the temperature unit used in this demonstration.

What happens in the baking soda demonstration?

1. Take about 50 mL of water in a glass beaker and measure its temperature, say 20 °C.
2. Add baking soda while stirring until some remains undissolved at the bottom.
3. Heat the contents to 50 °C while stirring. The previously undissolved baking soda dissolves.
4. Add more baking soda at 50 °C until some again remains undissolved.
5. Heat further to 70 °C while stirring. The undissolved baking soda dissolves again.

The water at 70 °C dissolves more baking soda than at 50 °C. The amount dissolved at 20 °C is even less. The observation connects a change in temperature with a change in the maximum amount that can dissolve.

The labelled apparatus includes a laboratory stand, laboratory thermometer, glass rod, beaker containing water and baking soda, wire gauze, tripod stand and spirit lamp. The lamp is beneath the supported beaker.

See Fig. 9.7 in your NCERT textbook

Why is the temperature condition necessary?

In this demonstration, a saturated solution behaves as an unsaturated solution when its temperature is increased. The earlier limit applies to the earlier temperature. Heating changes the conditions under which the dissolving capacity is being tested.

This is a demonstration activity involving a heating device, which must be used carefully. The conclusion should retain the phrase most substances; it must not be changed into a claim that every substance becomes more soluble when heated.

How does temperature affect gases dissolved in water?

Many gases, including oxygen, dissolve in water. Dissolved oxygen is oxygen present in solution in water. Oxygen dissolves only to a small extent, but even these minute quantities sustain aquatic life, including plants, fishes and other organisms.

Gases dissolved evenly in water form a uniform mixture. A solution therefore need not be restricted to a solid dissolved in a liquid. The even distribution of its components is the key feature.

Why does cold water hold more oxygen?

The solubility of gases generally decreases as temperature increases. More oxygen can dissolve in cold water. When the water warms, oxygen becomes less soluble. This connects temperature with the supply of dissolved oxygen available to aquatic organisms.

The two trends should be distinguished. The baking soda result cannot be used to conclude that warming water makes every kind of solute more soluble. For gases, the general temperature trend is in the opposite direction.

Does dissolving a gas make the mixture non-uniform?

No. A mixture of gases dissolved evenly in water is uniform. The classification depends on how the components are distributed, rather than on whether the solute began as a solid or a gas.

Keep the qualification generally when describing the temperature trend for gases. Also distinguish a small dissolved amount from an unimportant amount: dissolved oxygen occurs in minute quantities but has a vital role in aquatic life.

What do mass, volume, density and relative density mean?

Mass is the quantity of matter in an object. Matter is anything that has mass and occupies space. Volume is the space occupied by an object or substance. Density is the mass present in a unit volume of a substance.

$\text{Density} = \frac{\text{Mass}}{\text{Volume}}$. The sign = means “equals”. The fraction means that mass is divided by volume. Density compares mass for equal volumes, rather than comparing the total masses of objects of different sizes.

A wooden stick and an iron rod can have the same size, yet the iron rod feels much heavier. Density helps describe this difference. The density of a substance is independent of its shape or size, but depends on temperature and pressure, the force acting per unit area. A force is a push or pull on an object arising from interaction with another object.

Which units are used?

SI means the International System of Units. The SI unit of mass is the kilogram, written kg. The SI unit of volume is the cubic metre, written m³, which is the volume of a cube with sides one metre long.

The SI unit of density is kilogram per cubic metre, written kg/m³. The slash in a unit means “per”. Other convenient units are gram per millilitre, g/mL, and gram per cubic centimetre, g/cm³. Here g means gram and cm³ means cubic centimetre.

One millilitre is equivalent to one cubic centimetre. A litre, written L, is equivalent to one cubic decimetre, written dm³. At room temperature, 1 mL of water has a mass close to 1 g: 100 mL therefore has a mass of approximately 100 g.

What is relative density?

Relative density compares a substance's density with the density of water at the same temperature. $\text{Relative density} = \frac{\text{Density of substance}}{\text{Density of water at that temperature}}$. It is a number without units.

Worked example 1. An aluminium block has mass 27 g and volume 10 cm³. Calculate its density.

Answer: $\text{Density} = \frac{27\,\mathrm{g}}{10\,\mathrm{cm}^{3}} = 2.7\,\mathrm{g/cm}^{3}$. Using water's approximate density of 1 g/cm³, aluminium's relative density is 2.7, without units.

How is mass measured correctly with a balance?

A balance is an instrument used to measure mass. A digital weighing balance displays the reading directly. Before using it, check its starting reading so that an unwanted initial reading does not become part of the object's measured mass.

The tare or reset button brings the display to zero. A watch glass is the shallow glass dish used here to hold the object on the balance. Butter paper can also be used as the support.

What is the correct measuring sequence?

1. Switch on the digital weighing balance and check the initial reading.
2. If the display does not show zero, press the tare or reset button.
3. Place a dry, clean watch glass or butter paper on the pan and note the reading.
4. Press tare or reset again so that the display returns to zero with the support in place.
5. Place the solid object carefully on the support and record the displayed mass.

In the stone example, the displayed mass is 16.400 g. Resetting after placing the watch glass means that the final reading gives the stone's mass. To measure a liquid, replace the watch glass with a beaker before adding the liquid.

The three drawings show the digital balance, the watch glass placed on its pan while the tare button is pressed, and the solid object placed in the watch glass.

See Fig. 9.12 in your NCERT textbook

How does mass differ from weight?

Weight is the force with which Earth attracts an object or substance. The SI unit of force is newton, written N. Since weight is a force, the SI unit of weight is newton too. Mass is measured in grams or kilograms.

Most balances, except two-pan balances, actually measure weight but have scales marked in mass units. They therefore show grams or kilograms. Although mass and weight are often used interchangeably in everyday language, their scientific meanings differ.

How are liquid volumes and measuring-cylinder readings obtained?

A measuring cylinder is a narrow transparent container, open at one end, with volume markings on its side. Its capacity is the maximum volume it can measure. The spacing of the scale divisions determines the smallest volume reading.

How is the smallest scale division found?

Find the difference between two numbered marks and count the smaller divisions between them. Divide the volume difference by the number of divisions. Count the intervals represented by those divisions carefully.

Worked example 2. A measuring cylinder has ten small divisions between the 10 mL and 20 mL marks.

Answer: The volume difference is $20\,\mathrm{mL} - 10\,\mathrm{mL} = 10\,\mathrm{mL}$. Divide by ten: $\frac{10\,\mathrm{mL}}{10} = 1\,\mathrm{mL}$. Each small division represents 1 mL. The symbol − means subtraction.

The smallest readable volume usually depends on the cylinder's capacity. Smaller cylinders of 10 mL or 25 mL usually read 0.1 mL; a 100 mL cylinder reads 1 mL, a 250 mL cylinder 2 mL, and a 500 mL cylinder 5 mL.

To measure 70 mL in one step, a 100 mL cylinder is the best choice among the 50 mL, 100 mL, 250 mL and 500 mL cylinders discussed here. A 50 mL cylinder needs two measurements; the larger cylinders have less fine divisions.

Where should the eye be placed?

The meniscus is the curved surface of liquid inside the cylinder. For water or other colourless liquids, read the mark at the bottom of the meniscus. Keep the eye level with that bottom while taking the reading.

1. Place a clean, dry measuring cylinder on a flat surface.
2. Pour water slowly towards the required mark.
3. Use a dropper to add or remove small amounts if necessary.
4. Check the bottom of the water meniscus at eye level, then transfer the required volume.

Three eye positions are drawn beside a cylinder scale. Dashed sight lines approach the meniscus from above, at its level and from below. The middle eye is level with the bottom of the water meniscus.

See Fig. 9.18 in your NCERT textbook

For coloured liquids, the cylinder mark should coincide with the top of the meniscus. The appropriate reading point must therefore be identified before recording the volume.

How are solid volumes and densities calculated?

A regular solid can have its volume calculated from measured dimensions. A cuboid is a box-shaped solid with rectangular faces. For a cuboid, use length, width and height. An irregular solid, such as a stone, needs a different method.

How is the volume of a cuboid found?

Let l represent length, w width and h height. Then $\text{Volume} = l \times w \times h$, where × means multiplication. A centimetre, written cm, is the length unit used in the notebook example.

Worked example 3. A notebook is 25 cm long, 18 cm wide and 2 cm high.

Answer: $\text{Volume} = 25\,\mathrm{cm} \times 18\,\mathrm{cm} \times 2\,\mathrm{cm} = 900\,\mathrm{cm}^{3}$. All three dimensions are expressed in centimetres, so the volume is 900 cm³.

How does water displacement measure an irregular solid?

Water displacement measures an object's volume from the increase in the water reading when the object is immersed. “Immersed” means placed within the water. The final reading includes the volume displaced by the object; subtract the initial reading to find that volume.

1. Record the initial volume of water in a measuring cylinder, such as 50 mL.
2. Tie the stone with thread and lower it slowly into the water.
3. Record the final reading after the level rises, such as 55 mL.
4. Subtract the initial reading from the final reading and express the solid's volume in cm³.

Two cylinders are shown side by side. The first contains water without the object; the second contains a stone suspended by thread and has a higher water level.

See Fig. 9.19 in your NCERT textbook

Worked example 4. A stone has mass 16.400 g. It raises the water reading from 50 mL to 55 mL, and 1 mL equals 1 cm³. Formula: Volume = final reading − initial reading; Density = mass ÷ volume. Substitute: use the two readings and the measured mass. Answer: Volume = 55 mL − 50 mL = 5 mL = 5 cm³. Density = 16.400 g ÷ 5 cm³ = 3.28 g/cm³.

How can two objects be compared?

Worked example 5. Object A has mass 200 g and volume 40 cm³; object B has mass 240 g and volume 60 cm³. A and B are labels for the two objects. Formula: Density A = mass of A ÷ volume of A; Density B = mass of B ÷ volume of B. Substitute: divide each supplied mass by its corresponding volume. Answer: Density of A = 200 ÷ 40 = 5 g/cm³. Density of B = 240 ÷ 60 = 4 g/cm³. Object A is denser despite having the smaller mass.

Worked example 6. An iron block has mass 600 g and density 7.9 g/cm³. Answer: Rearranging density = mass ÷ volume gives volume = mass ÷ density. Volume = 600 ÷ 7.9 ≈ 75.95 cm³. The symbol ≈ means approximately equal to.

Worked example 7. An object has mass 400 g and volume 40 cm³.

Answer: $\text{Density} = \frac{400\,\mathrm{g}}{40\,\mathrm{cm}^{3}} = 10\,\mathrm{g/cm}^{3}$. The density of 10 g/cm³ expresses how much mass is present per cubic centimetre.

How do temperature and pressure affect density and floating?

Generally, density decreases on heating and increases on cooling. As temperature rises, the particles of a solid, liquid or gas tend to move apart and spread out. Particles are the tiny units that make up matter.

The volume increases while the mass stays unchanged. Since density is mass divided by volume, the density decreases. Hot air rises because it is less dense than the surrounding cool air. A hot-air balloon works on this principle.

How does pressure affect different states of matter?

Increasing pressure brings gas particles closer together, reducing the gas's volume and increasing its density. The effect depends on the state of matter.

Why does ice float on water?

Water has its highest density at 4 °C. When water freezes at 0 °C, its particles arrange themselves in a structure occupying more space. This increase in occupied space is called expansion.

The same amount of water occupies a larger volume as ice, so its density is lower. Ice therefore floats on liquid water. The floating ice layer keeps the water underneath warm enough for fish and other creatures to survive extremely cold weather.

Does density explain everything about floating?

Sand sinks in water while sawdust floats. Oil floats on water, indicating that water is denser than oil. These observations connect density with floating and sinking, but they do not make density a complete explanation of every situation.

Note: Density is not the only factor that decides whether an object floats or sinks in a particular liquid. Use density comparisons without replacing this qualification with an absolute rule.

Changing a substance's shape does not by itself change its density. This differs from heating, where the material's volume generally changes, or compressing a gas, where its particles move closer together.

Glossary

• Solution — A uniform mixture in which the component substances are evenly distributed throughout.
• Solute — The substance that dissolves in a solvent to form a uniform mixture.
• Solvent — The substance in which the solute dissolves to form a solution.
• Saturated solution — A solution that cannot dissolve any more of a particular solute at the given temperature.
• Unsaturated solution — A solution capable of dissolving more of a particular solute at the given temperature.
• Concentration — The amount of solute present in a fixed quantity of solution or solvent.
• Solubility — The maximum amount of solute that dissolves in a fixed quantity of solvent at a particular temperature.
• Mass — The quantity of matter present in an object or substance.
• Volume — The amount of space occupied by an object or a substance.
• Density — The mass present in a unit volume of a substance.
• Relative density — The ratio of a substance's density to water's density at the same temperature, expressed without units.
• Meniscus — The curved surface of a liquid observed inside a measuring cylinder.
• Tare — Resetting a balance display to zero, including after placing a supporting container on its pan.
• Water displacement — Finding an immersed object's volume from the difference between final and initial water readings.

Common errors and misconceptions

• Misconception: Every mixture is a solution. Correct: A solution is uniform. Sand and water form a non-uniform mixture.
• Misconception: The solute is always the component present in the smaller amount. Correct: In sugar syrup, solid sugar remains the solute even when there is a large amount relative to water.
• Misconception: Concentrated and saturated mean the same thing. Correct: Concentration describes the amount dissolved; saturation means no more of that solute can dissolve at that temperature.
• Misconception: Heating makes all solutes more soluble. Correct: Solubility of solids in liquids generally increases, while solubility of gases generally decreases.
• Misconception: The heavier object must be denser. Correct: Density compares mass per unit volume. Both mass and volume are needed.
• Misconception: Mass and weight are identical scientific quantities. Correct: Mass is the quantity of matter; weight is Earth's attractive force on an object.
• Misconception: A water reading can be taken from any eye position. Correct: The eye must be level with the bottom of the meniscus.
• Misconception: Heating always lowers density and density alone explains floating. Correct: The heating trend is general, water has its highest density at 4 °C, and density is not the only factor in floating.

Exam-style questions with model answers

Q1. A sugar syrup contains a large amount of solid sugar dissolved in a small amount of liquid water. Identify the solute and solvent, explaining each choice. [2 marks]

1. Sugar is the solute because it is the solid component dissolved in the liquid.
2. Water is the solvent because it is the liquid in which the sugar dissolves, even though its amount is smaller.

Q2. Explain three differences between saturation, concentration and solubility. Include the temperature condition wherever it is needed. [3 marks]

1. Saturation describes whether a solution has reached its dissolving limit. A saturated solution cannot dissolve more of that solute at the given temperature.
2. Concentration describes the amount of solute present in a fixed quantity of solution or solvent. Dilute and concentrated are relative descriptions.
3. Solubility is the maximum amount of solute that can dissolve in a fixed quantity of solvent at a particular temperature.

Q3. Baking soda remains undissolved in water at 20 °C but dissolves when heated to 50 °C. After more is added until some remains, that solid dissolves on heating to 70 °C. Explain these observations and contrast them with the general temperature trend for gases. [4 marks]

1. At 20 °C, the solution has reached its dissolving limit for baking soda, so some added solid remains undissolved.
2. Heating to 50 °C allows more baking soda to dissolve. The solution's earlier saturation condition no longer applies at the higher temperature.
3. The further dissolving at 70 °C shows that more baking soda can dissolve there than at 50 °C, illustrating the trend for most substances.
4. For gases, solubility generally decreases as temperature increases. More oxygen can therefore dissolve in cold water than in warm water.

Q4. Describe how to measure a stone's mass and volume, then calculate its density. The tared balance reads 16.400 g. Water rises from 50 mL to 55 mL when the stone is immersed. Use 1 mL = 1 cm³. [5 marks]

1. Switch on the digital balance, check the initial display, and reset it to zero if necessary before placing a supporting watch glass on its pan.
2. Reset the display again with the clean, dry watch glass in place. Add the stone and record its mass as 16.400 g.
3. Record the initial water reading of 50 mL in a measuring cylinder. Lower the stone slowly by thread and record the final reading of 55 mL.
4. Subtract the initial reading from the final reading: displaced water = 55 − 50 = 5 mL. The stone's volume is therefore 5 cm³.
5. Use density = mass ÷ volume. Substitution gives 16.400 g ÷ 5 cm³ = 3.28 g/cm³, the density of the stone.

Q5. Object A has mass 200 g and volume 40 cm³. Object B has mass 240 g and volume 60 cm³. Calculate both densities and explain which object is denser despite its smaller mass. [3 marks]

1. Density is mass divided by volume. For object A, density = 200 g ÷ 40 cm³ = 5 g/cm³.
2. Applying the same formula to object B gives density = 240 g ÷ 60 cm³ = 4 g/cm³.
3. Object A is denser because it has more mass per unit volume. Its smaller total mass does not make its density smaller.

Q6. A measuring cylinder has ten equal divisions between 10 mL and 20 mL. Find its smallest division and state how to position the eye when reading water. [2 marks]

1. The smallest division is (20 − 10) ÷ 10 = 1 mL.
2. Keep the eye level with the bottom of the water meniscus and read the corresponding scale mark.

Q7. Explain the general effect of heating on density, why hot air rises, why ice floats, and how pressure affects gases compared with liquids and solids. [5 marks]

1. Generally, heating makes particles move apart, increasing the volume while leaving the mass unchanged. Dividing the same mass by a larger volume gives a lower density.
2. Hot air rises because it is less dense than the surrounding cool air. Hot-air balloons work using this difference in density.
3. Water has its highest density at 4 °C. On freezing at 0 °C, its structure occupies more space, so ice is less dense than liquid water and floats.
4. Increasing pressure brings gas particles closer together. The gas occupies less volume, and its density increases.
5. Liquids are nearly incompressible, so pressure has a small effect. Solids are even less affected, and changes in their density are usually negligible.

Q8. A stone sculpture has mass 225 g and volume 90 cm³. Calculate its density, compare it with water of approximate density 1 g/cm³, and predict whether this stone sculpture floats or sinks. [3 marks]

1. Use density = mass ÷ volume. The sculpture's density is 225 g ÷ 90 cm³ = 2.5 g/cm³.
2. Its density of 2.5 g/cm³ is greater than the supplied approximate water density of 1 g/cm³.
3. The stone sculpture is therefore expected to sink in water. This prediction uses the density comparison for the stated object, while recognising that density is not the only factor in floating generally.

Key takeaways

• A solution is a uniform mixture whose components are evenly distributed and are not visible separately.
• In a solid-liquid solution, the solid is the solute and the liquid is the solvent.
• Saturation is a dissolving limit at a given temperature; concentration describes how much solute is present.
• Generally, heating increases the solubility of solids in liquids but decreases the solubility of gases.
• Density is mass divided by volume; relative density compares a substance with water and has no units.
• Tare the balance with the support in place, and read water at the bottom of its meniscus at eye level.
• An immersed stone's volume equals the final water reading minus the initial reading, with millilitres equivalent to cubic centimetres.
• Density generally decreases on heating, while increasing pressure raises gas density; density alone does not explain every floating situation.

Test yourself

Why is a sand-water mixture not a solution?

Its components are not evenly distributed. Sand and water form a non-uniform mixture rather than a solution.

What condition belongs in a definition of a saturated solution?

The temperature must be specified: no more of that solute can dissolve at the given temperature.

Why must “generally” be retained in the temperature trend for gas solubility?

The trend describes the general behaviour of gases, not an absolute statement covering every case without qualification.

Does oxygen dissolve extensively in water?

No. Oxygen dissolves only to a small extent, but the dissolved oxygen sustains aquatic life.

Why is a balance reset after the watch glass is added?

Resetting excludes the supporting watch glass from the subsequent reading, so the display gives the object's mass.

A notebook measures 25 cm by 18 cm by 2 cm. What is its volume?

Its volume is length × width × height: 25 × 18 × 2 = 900 cm³.

Water rises from 50 mL to 55 mL after a stone is immersed. What is the stone's volume, using 1 mL = 1 cm³?

The volume is 55 − 50 = 5 mL, equivalent to 5 cm³ for the stone.

Why can increasing pressure increase a gas's density?

Pressure brings the gas particles closer, reducing volume and increasing the mass present per unit volume.