Nature of Matter: Elements, Compounds, and Mixtures | CBSE Class 8 Science Notes

This note covers matter, mixtures and their components, uniform and non-uniform mixtures, air, pure substances, elements, atoms and molecules, compounds, physical separation, the water, sugar and iron-sulfur activities, everyday uses of materials, and minerals.

What is matter, and how can it be classified?

Matter is anything that has mass and occupies space. Mass describes how much matter an object contains. Water, air, food, clothes, books and trees are examples of matter. These materials are made of tiny particles.

Light, heat and electricity are not matter. Thoughts and emotions are not matter either. Something can be an important part of our experience without being a material substance that has mass and takes up space.

What are the main categories?

A pure substance contains the same type of particles and has no other substance present in it. A mixture contains two or more substances that retain their properties. Most things around us contain more than one substance.

Pure substances include elements, which cannot be broken down into simpler substances, and compounds, in which different elements combine chemically in a fixed ratio. A ratio compares quantities, such as the numbers of atoms belonging to different elements. An atom is a particle of an element; atoms of one element differ from those of other elements.

The distinction concerns what a material contains and how its constituents, or the substances making it up, are combined. The number of ingredients visible to the eye is useful evidence in some cases, but visibility alone does not establish purity.

What makes a mixture uniform or non-uniform?

Definition: A mixture contains two or more substances mixed together without a chemical reaction between them. A chemical reaction is a change that produces new substances. Each substance retains its properties, and the individual substances are called components.

In everyday language, poha and sprout salad are mixtures of ingredients. A sprout salad may contain green gram, chickpeas, onion and tomato. Its ingredients remain recognisable after mixing. Sugar dissolved in water, soups and lemonade are other familiar examples of mixing.

How are components distributed?

In a non-uniform mixture, different components are generally visible with the naked eye or with a magnifying device. A sprout salad illustrates this: its ingredients can be distinguished rather than appearing the same throughout.

In a uniform mixture, the components are evenly distributed and cannot be distinguished separately, even with a microscope. Sugar dissolved in water illustrates this kind of mixture. The inability to see sugar separately does not mean that the sugar is absent.

There is a distinction between everyday and scientific usage. Ingredients in an everyday mixture may themselves be mixtures. In the scientific description of a mixture, its components are pure substances. Sugar and common salt dissolved in water provide examples of this usage.

Note: Uniform appearance does not prove that a material is a pure substance. Sugar water contains more than one substance even though its components cannot be distinguished separately.

How do mixtures differ according to the states of their components?

A physical state is a form of matter, such as solid, liquid or gas. Mixtures can contain components in different states. Classifying a mixture by these states is a separate task from deciding whether its distribution is uniform or non-uniform.

For example, air is a gas-and-gas mixture. Sand and water form a solid-and-liquid mixture. These descriptions identify the states of the components; they do not mean that a mixture must contain components in different states.

What are alloys?

Alloys are uniform mixtures containing metals, such as stainless steel, brass and bronze. Stainless steel contains iron, nickel, chromium and a small amount of carbon. Its components are mixed so uniformly that the individual substances cannot be seen separately.

Brass contains copper and zinc, while bronze contains copper and tin. Both illustrate solid mixtures. A material can therefore contain several substances while appearing the same throughout its bulk.

The names of similar materials need care. Baking powder is a mixture of baking soda and tartaric acid. Baking soda and baking powder should not be treated as interchangeable names when describing what a material contains.

Why is air a mixture, and how can its components be detected?

Air is a uniform mixture mainly of nitrogen, oxygen, argon, carbon dioxide and water vapour. Water vapour is water in its gaseous state. The gases coexist as components of a mixture rather than forming one compound.

Nitrogen makes up about 78% of air, meaning about 78 parts in every hundred, and does not take part in combustion. Combustion means burning. Oxygen supports combustion and is required by most living beings to stay alive.

When warm air touches a cool surface, its water vapour turns into liquid water, forming tiny droplets. This change from vapour to liquid is called condensation. The droplets provide evidence for water vapour in air.

How does lime water reveal carbon dioxide?

Lime water is a solution of calcium hydroxide in water; a solution is a uniform mixture. Calcium oxide, also called quick lime, reacts vigorously with water, produces calcium hydroxide and releases heat. This preparation step must be carried out carefully.

1. Water is placed in a glass tumbler, and a small amount of calcium oxide is added slowly.
2. The contents are stirred continuously to prepare the calcium hydroxide solution and then filtered.
3. The colourless lime water is left in a petri dish, a shallow laboratory dish, for a few hours and stirred at regular intervals.
4. The solution becomes milky because carbon dioxide reacts with calcium hydroxide, forming tiny white particles of calcium carbonate and water.

Calcium carbonate is insoluble, meaning it does not dissolve in the water here. Its particles cause the milky appearance. In the word equation below, the plus sign separates substances, and the arrow means “react to form”.

Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water

Two drawings show petri dishes viewed from above. The first labels clear lime water; the second labels lime water that has turned milky.

See Fig. 8.5 in your NCERT textbook

What about dust in air?

A clean black sheet left near an open window or in a garden for a few hours may collect tiny particles. These show that dust particles are suspended in air. Their nature and number may vary with time and place.

Dust is not an integral component of air and is considered a pollutant, a substance that contaminates the environment. Particulate matter includes dust and soot. The air quality index, abbreviated AQI, is a tool for describing air quality.

How does scientific purity differ from everyday purity?

In everyday usage, a product called pure is understood to be unadulterated. Adulteration means adding cheaper or poorer-quality substances to a product. It is usually done to increase quantity or reduce manufacturing cost, but it reduces quality and can make the product hazardous to health.

Scientific purity asks a different question: does the material contain just one substance? A product can be unadulterated while still containing several substances. Thus, an everyday claim of purity does not establish that the product is scientifically a pure substance.

What identifies a pure substance?

A pure substance contains the same type of particles. Its constituent particles behave identically. It cannot be separated into other kinds of matter by a physical process, a process that does not form new substances.

Both elements and compounds belong to this category. Water and sugar are compounds, yet they are pure substances. The presence of different elements chemically combined within a compound does not make that compound a mixture.

Separation of mixtures also has different purposes in different settings. Everyday separation often obtains a useful component while discarding others. In science, separation aims to obtain pure substances. The purpose explains why identifying every component matters.

Note: “Pure” does not mean “element”. A compound can be pure because it is one substance, even though more than one element forms that substance.

What does passing electricity through water demonstrate?

Cooling water produces ice, while boiling produces water vapour. Heating ice or cooling water vapour gives liquid water again. In these changes of state, the water particles remain the same; new substances are not produced.

Passing electricity through water provides a different result. It breaks water down into hydrogen and oxygen, which are different substances. This is a chemical change, a change in which new substances form.

How is the demonstration arranged?

The apparatus uses a beaker or glass tumbler, two small test tubes and a 9 V battery. V means volt, the unit of voltage. Voltage describes the electrical push supplied by the battery. A few drops of dilute sulfuric acid are added to the water. Dilute means that the acid solution contains a small amount of acid relative to water.

1. The beaker is filled two-thirds with water, and the dilute sulfuric acid is added.
2. Both test tubes are filled completely with water taken from the beaker.
3. The battery is placed in the beaker, and the water-filled test tubes are carefully positioned over its terminals, the battery's electrical connection points.
4. Gas bubbles collect in the tubes. After 10 to 15 minutes, the collected gases are tested separately with a burning candle.

This is a teacher-supervised demonstration: sulfuric acid must be handled carefully, a lithium-ion battery must not be used, and a safe distance must be maintained during gas testing.

The drawing shows two inverted test tubes above a battery in a beaker. Labels identify hydrogen gas, oxygen gas, gas bubbles, water containing a few drops of sulfuric acid, and the 9 V battery.

See Fig. 8.8d in your NCERT textbook

How are the gases identified?

Hydrogen gives a pop sound when tested with a flame. Oxygen makes the burning candle's flame glow brighter. These different test results identify different gases, rather than water vapour from a change of state.

Water → Hydrogen + Oxygen

Here the arrow means “breaks down to form”, with electricity passed through the water. Water is therefore made from two different constituent elements. Boiling water and electrically breaking it down must not be confused.

What are elements, atoms and molecules?

An element cannot be broken down into simpler substances. Hydrogen and oxygen are elements, as are gold, silver, sulfur and carbon. Elements provide the basic building blocks from which matter is formed.

Each element contains identical particles called atoms. The atoms of one element differ from those of another element. Thus, hydrogen atoms and oxygen atoms are not the same kind of particle, even though both are atoms.

How can atoms form molecules?

The atoms of most elements cannot exist independently. Two or more such atoms combine to form a stable particle called a molecule. Two hydrogen atoms form one hydrogen molecule; two oxygen atoms form one oxygen molecule.

A molecule containing several atoms is not automatically a compound. A hydrogen molecule still contains hydrogen alone. A water molecule, by contrast, contains atoms of two different elements chemically combined. The kinds of atoms present are therefore essential to classification.

How are elements grouped?

Metals and non-metals are groups of elements distinguished by their properties. Gold, silver, magnesium, iron and aluminium are metals. Carbon, sulfur, hydrogen and oxygen are non-metals.

Metalloids have properties intermediate between those of metals and non-metals. Silicon and boron are examples. These categories describe elements; they should not be confused with the separate classification into elements, compounds and mixtures.

Most elements exist in the solid state. At room temperature, eleven elements are gases, and all eleven are non-metals. Oxygen, helium and nitrogen are examples. Only two elements are liquid at room temperature: mercury, a metal, and bromine, a non-metal.

The qualification at room temperature matters. It states the condition under which the examples are being compared. An element's state and its identity as an element are different aspects of its description.

How do compounds acquire fixed composition and new properties?

A compound forms when different elements combine chemically in a fixed ratio. Its properties differ from those of the elements forming it. Its constituent elements cannot be separated using a physical method.

In water, hydrogen and oxygen particles are so tightly attached that it is generally impossible to pull them apart using physical methods. The ratio of hydrogen atoms to oxygen atoms in water is 2:1, meaning two hydrogen atoms for each oxygen atom.

The drawing shows repeated groups of one red sphere joined to two smaller pale spheres. Arrows label oxygen and hydrogen, distinguishing the two kinds of atom in each group.

See Fig. 8.11 in your NCERT textbook

Why do the original properties not remain?

Hydrogen is a fuel, meaning a substance that can burn. Oxygen supports combustion. Water has different properties and is used to extinguish fire. Chemical combination therefore produces a substance whose behaviour differs from its constituent elements.

Sodium chloride, the chemical name for common salt, provides another example. Sodium is a soft metal, whereas chlorine is a hazardous gas. They combine in a 1:1 ratio, meaning one sodium particle for each chlorine particle, to form the familiar taste-enhancing compound.

The fixed ratio describes particles within the compound. It is different from the quantities of substances placed together when making a mixture. A compound's identity involves chemical combination as well as the presence of more than one element.

What does evaporation separate?

Evaporation is the slower formation of vapour at a liquid's surface. Vapour forms at the surface even below the temperature at which the liquid boils. It can separate dissolved common salt from water. The salt recovered is still sodium chloride: the process has separated a mixture, not split the salt into sodium and chlorine.

How does heating sugar show that it is a compound?

Sugar looks like one substance, but it contains more than one element. A heating activity reveals its chemical nature. Decomposition means the breaking down of a substance into other substances. Sugar decomposes when heated.

What observations are made?

In the presence of a teacher, a teaspoon of sugar is placed in a boiling tube, a laboratory tube used for heating, and heated gently. The observations should be distinguished from the conclusion drawn from them.

1. The sugar first turns brown as heating proceeds.
2. It then begins to char, meaning that it becomes blackish.
3. Small water droplets appear inside the tube near its open end.
4. Charcoal, identified as carbon, remains and can be collected in a watch glass, a shallow laboratory glass dish.

Photograph: Heating sugar in a boiling tube (NCERT Class 8 Figure 8.12). The photographs show a boiling tube held above a spirit lamp, water droplets near the open end of the heated tube, and black charcoal collected on a watch glass.

What follows from these observations?

The products are carbon and water. Water contains hydrogen and oxygen. The activity therefore shows that sugar contains carbon, hydrogen and oxygen; it cannot be an element.

The water comes from the decomposition of the dry sugar. It is not evidence that the sugar was merely a wet mixture from which existing liquid water was removed. Heating has produced different substances.

This also explains why appearance is insufficient for classification. Sugar is a pure substance and a compound, even though its constituent elements cannot be distinguished by looking at it. The decomposition evidence reveals more than visual inspection of the original sugar.

How does heating iron and sulfur distinguish a mixture from a compound?

Iron filings are small pieces of iron. Mixing them with sulfur powder produces a mixture. Heating that mixture forms iron sulfide, a compound with different properties. Comparing the unheated and heated samples makes the distinction observable.

How are the samples prepared?

The demonstration begins with 5.6 grams of iron filings and 3.2 grams of sulfur powder. A gram is a unit of mass. The activity is carried out under teacher supervision in a fume hood, a ventilated laboratory enclosure, or a well-ventilated area; the gases must not be inhaled.

1. Iron filings and sulfur powder are mixed thoroughly in a watch glass. This unheated mixture is labelled Sample A.
2. Half of Sample A is placed in a china dish and heated gently with continuous stirring until a black mass forms.
3. The heated material is allowed to cool. It is then ground using a mortar and pestle, equipment for crushing solids.
4. The black material is transferred to another watch glass and labelled Sample B. The samples are compared by appearance, attraction to a magnet, an object that attracts iron, and reaction with dilute hydrochloric acid, the acid solution used in the comparison.

What do appearance and the magnet test show?

Sample A contains recognisable black iron particles and yellow sulfur particles. A magnet draws the iron filings out of the mixture. The components have retained their properties and can be separated physically.

Sample B has the same black colour and texture throughout. It is iron sulfide and is not attracted by a magnet. Iron and sulfur have combined chemically, so a magnet cannot separate them from this new substance. With heating, the word equation is: Iron + Sulfur → Iron sulfide.

Photograph: Responses of Samples A and B to a magnet (NCERT Class 8 Figure 8.17). Two photographs show a magnet held over each sample. Material is drawn towards the magnet from Sample A, while the black material of Sample B remains in its watch glass.

How do their reactions with acid differ?

Hydrochloric acid is the acid solution used in the comparison. In Sample A, iron reacts with the dilute acid to produce iron chloride and hydrogen gas. Sulfur does not react with the hydrochloric acid and remains as a yellow solid.

Iron + Dilute hydrochloric acid → Iron chloride + Hydrogen gas

The hydrogen is colourless and odourless and burns with a pop sound. In Sample B, iron sulfide reacts with dilute hydrochloric acid to give iron chloride and hydrogen sulfide, a colourless gas with a rotten egg-like odour.

Iron sulfide + Dilute hydrochloric acid → Iron chloride + Hydrogen sulfide

These equations name the substances reacting and the products formed. Different gases provide further evidence that the unheated mixture and the heated compound have different properties. Acid must be handled carefully; gases must never be smelled directly.

How can elements, compounds and mixtures be compared using evidence?

Classification combines information about composition, properties and separation. A material's composition is what it contains. The question is not simply whether the material looks uniform, but whether its components retain their identities or have combined chemically.

What does a separation result actually establish?

Removing iron filings with a magnet shows that iron remains a component of the unheated mixture. Failure to remove iron from iron sulfide reflects chemical combination. These results should be interpreted together with the samples' appearance and reactions.

Similarly, evaporating water from salt water recovers a compound from a mixture. It does not transform that compound into its constituent elements. Always identify whether the process separates substances already present or produces new substances.

An element cannot also be a compound. An element cannot be broken down into simpler substances, whereas a compound contains different elements chemically combined. A substance can, however, be both a compound and a pure substance because these descriptions are compatible.

Uniform mixtures and pure substances may both appear the same throughout. That shared appearance is why classification needs evidence about constituents and their behaviour, rather than a decision based entirely on colour or visibility.

How are these materials used, and what are minerals?

Iron and aluminium are used in bridges, buildings and vehicles. Understanding how elements combine helps chemists develop medicines, vaccines and fertilisers. Fertilisers are materials used to help crop production, while medicines and vaccines help fight diseases.

Engineers use compounds and mixtures to design materials with useful properties. Stainless steel is stronger and more durable than pure iron. Wood, steel and concrete are mixtures used as building materials.

What makes a mineral different from a rock?

Minerals are natural solid substances found on Earth with fixed chemical composition. Most often they are compounds, but rarely they can be pure elements. Most rocks are mixtures of minerals, which can be observed directly or with magnification.

Native minerals are minerals that are pure elements rather than compounds. Gold, silver and copper are metallic examples; sulfur and carbon are non-metallic examples. Gold can therefore be both a metal and a mineral without being a compound.

Quartz, calcite, mica, pyroxene and olivine are examples of minerals. Cement is made from calcite, quartz, alumina and iron oxide, which are minerals or obtained from minerals. Talcum powder is made from the mineral talc.

How do craft and new materials use this knowledge?

Dhokra art is an old craft from Bihar and Odisha. A design is shaped in beeswax and covered with clay to make a mould. After the clay hardens, the wax is melted out, leaving a hollow space.

Molten brass or bronze fills that space. These alloys make the figures strong and give them a shiny golden colour. The figures often show animals, people and nature, connecting the use of materials with craft traditions.

Graphene aerogel is a material made from carbon and is said to be the lightest material on Earth. It is highly porous, meaning it contains many spaces, and has a high absorbing capacity.

It can potentially be used to clean oil spills on land and in seas. It is also useful in making energy-saving devices and special building coatings. Its possible environmental use illustrates how material properties guide applications.

Glossary

• Matter — Anything that has mass and takes up space in its surroundings.
• Mixture — Two or more substances mixed together while retaining their individual properties without reacting chemically.
• Component — An individual substance that forms part of a mixture with other substances.
• Uniform mixture — A mixture whose components are evenly distributed and cannot be distinguished separately.
• Non-uniform mixture — A mixture whose components are generally visible directly or with a magnifying device.
• Pure substance — Matter containing the same type of particles and no other substance.
• Element — A pure substance that cannot be broken down into simpler substances.
• Atom — A particle of an element, different from the particles of other elements.
• Molecule — A stable particle formed when two or more atoms combine together.
• Compound — A substance formed from different elements chemically combined in a fixed ratio.
• Alloy — A uniform mixture containing metals, with components that cannot be distinguished separately.
• Lime water — A solution of calcium hydroxide that turns milky on reacting with carbon dioxide.
• Decomposition — The breaking down of a substance into other substances, as when sugar is heated.
• Mineral — A natural solid substance found on Earth that has fixed chemical composition.
• Native mineral — A mineral consisting of a pure element rather than a chemical compound.

Common errors and misconceptions

• Misconception: A material that looks uniform must be pure. Correct: Sugar water and stainless steel are uniform mixtures containing several substances.
• Misconception: A pure substance must be an element. Correct: Compounds such as water and sugar are also pure substances.
• Misconception: Boiling water separates hydrogen and oxygen. Correct: Boiling changes water into vapour while its particles remain water particles.
• Misconception: Any molecule containing two atoms is a compound. Correct: A hydrogen molecule contains two hydrogen atoms; a compound requires different elements.
• Misconception: Evaporating salt water breaks salt into sodium and chlorine. Correct: It separates common salt from water while leaving the salt as sodium chloride.
• Misconception: Iron remains magnetically separable after it forms iron sulfide. Correct: Iron sulfide has different properties and is not attracted by a magnet in the comparison.
• Misconception: Every mineral is a compound. Correct: Most often minerals are compounds, but native minerals such as gold are pure elements.

Exam-style questions with model answers

Q1. Sugar is dissolved evenly in water, and its particles cannot be distinguished separately from water particles. Classify the material and explain whether its appearance proves scientific purity. [2 marks]

1. It is a uniform mixture because the sugar and water are evenly distributed and cannot be distinguished separately.
2. Its appearance does not prove purity: two substances are present, whereas scientific purity requires one substance containing the same type of particles.

Q2. Colourless lime water, a calcium hydroxide solution, is left exposed to air. Carbon dioxide reacts with it to produce insoluble white calcium carbonate particles and water. State the observation, explain it and give the word equation. [3 marks]

1. The colourless lime water becomes milky after exposure to air. This visible change provides evidence for the presence of carbon dioxide in the air.
2. The milkiness is caused by tiny white calcium carbonate particles. They are insoluble, so they remain as particles rather than dissolving in the water.
3. The word equation is: calcium hydroxide + carbon dioxide → calcium carbonate + water. The arrow indicates the formation of the named products.

Q3. Electricity passed through water produces hydrogen, which gives a pop sound with a flame, and oxygen, which makes a candle flame brighter. Boiling instead produces water vapour without changing the particles. Explain these results in four points. [4 marks]

1. The pop sound identifies the first collected gas as hydrogen, using its stated response when tested with a flame.
2. The brighter candle flame identifies the second collected gas as oxygen, which supports combustion rather than behaving like water.
3. The electrical process is a chemical change because water breaks down into different substances, showing that it contains hydrogen and oxygen.
4. Boiling is a physical change: it produces water vapour while the particles remain water particles, so it does not separate the constituent elements.

Q4. Sample A is an unheated iron-sulfur mixture with black and yellow particles; a magnet removes its iron. Heating produces Sample B, black iron sulfide with uniform texture, which is not attracted by a magnet. Dilute hydrochloric acid gives hydrogen from A and hydrogen sulfide from B. Compare the samples in five points. [5 marks]

1. Sample A is a mixture because its iron and sulfur remain separate substances. Sample B is a compound because heating has chemically combined these elements into iron sulfide.
2. Colour: Sample A shows both black iron particles and yellow sulfur particles, whereas Sample B is black throughout.
3. Texture: Sample A has distinguishable iron and sulfur particles, whereas Sample B has the same texture throughout.
4. Magnet test: Iron is attracted out of Sample A, allowing physical separation; Sample B is not attracted, so a magnet cannot separate its chemically combined elements.
5. The acid produces different gases: hydrogen from Sample A and hydrogen sulfide from Sample B. This further demonstrates that the compound has properties different from the mixture.

Q5. When dry sugar is gently heated, it first turns brown, then blackish. Water droplets appear near the tube's open end, and carbon remains. The sugar decomposes to form carbon and water; water contains hydrogen and oxygen. Explain five conclusions or observations from this evidence. [5 marks]

1. The first visible change is browning. Continued heating makes the sugar blackish, a change described as charring, so the observations have a definite sequence.
2. The water droplets are a product of decomposition of the dry sugar. They do not show that existing liquid water was simply removed from wet sugar.
3. The remaining black material is carbon. Its formation provides evidence that carbon is one of the elements present in the original sugar.
4. Water contains hydrogen and oxygen. Since water is another decomposition product, the evidence identifies hydrogen and oxygen as elements present in sugar as well.
5. Sugar is therefore a compound containing carbon, hydrogen and oxygen. It cannot be an element because the activity breaks it down into other substances.

Q6. Water contains hydrogen and oxygen atoms in a 2:1 ratio. Hydrogen is a fuel, oxygen supports combustion, and water is used to extinguish fire. Explain why water is a compound and how its properties compare with its constituent elements. [3 marks]

1. Water is a compound because it contains two different elements, hydrogen and oxygen, chemically combined rather than merely mixed together.
2. The fixed atom ratio is two hydrogen atoms for every oxygen atom. The ratio describes water's composition, not separate gases remaining as mixture components.
3. Its properties differ from those of its elements: hydrogen burns as a fuel and oxygen supports burning, while water is used to extinguish fire.

Q7. Most rocks contain several minerals. Minerals are natural solids with fixed chemical composition; most often they are compounds, but native minerals are pure elements. Gold is both a native mineral and a metal. Explain these classifications in four points. [4 marks]

1. Most rocks are mixtures because they contain several minerals together, rather than consisting of one substance with a single fixed composition.
2. A mineral is identified here as a natural solid with fixed chemical composition. This differs from the mixture of minerals forming a rock.
3. Most often minerals are compounds, but the native-mineral category provides an exception: its members are pure elements rather than compounds.
4. Gold can be both a mineral and a metal. “Native mineral” describes its occurrence as a pure element, while “metal” identifies its group of elements.

Q8. Sodium chloride is a compound of sodium and chlorine. Evaporation separates dissolved sodium chloride from water. Identify what this process separates and explain what it does not separate. [2 marks]

1. Evaporation separates the components of salt water, a mixture, allowing sodium chloride to be recovered as water changes into vapour.
2. It does not separate sodium chloride into sodium and chlorine. The recovered salt remains a compound containing its chemically combined elements.

Key takeaways

• Matter has mass and occupies space; light, heat, electricity, thoughts and emotions are not matter.
• Mixture components retain their properties, while uniform distribution can make those components impossible to distinguish separately.
• Scientific purity requires the same type of particles, so both elements and compounds can be pure substances.
• Elements cannot be broken down into simpler substances; compounds contain different elements chemically combined in fixed ratios.
• Passing electricity through water forms hydrogen and oxygen, whereas boiling changes water's state without changing its particles.
• Heating sugar produces carbon and water, providing evidence that sugar contains carbon, hydrogen and oxygen.
• Iron can be magnetically separated from its mixture with sulfur, but this separation does not work on iron sulfide.
• Most rocks contain mixtures of minerals; most often minerals are compounds, while native minerals are pure elements.

Test yourself

Why is sugar water a mixture even when the sugar cannot be seen?

It contains sugar and water as separate component substances, evenly distributed throughout the mixture.

Which metals make up brass and bronze?

Brass contains copper and zinc, while bronze contains copper and tin.

What causes the milky appearance when lime water reacts with carbon dioxide?

Tiny white, insoluble calcium carbonate particles form and make the solution appear milky.

What is the difference between an atom and a hydrogen molecule?

An atom is a particle of an element; a hydrogen molecule consists of two hydrogen atoms combined.

What does the 2:1 ratio in water describe?

It describes two hydrogen atoms combined with each oxygen atom in water.

How is hydrogen identified in the water demonstration?

Hydrogen gives a pop sound when the collected gas is tested with a flame.

What remains unchanged when ice melts into water?

The particles remain water particles even though the physical state changes.

Why can a native mineral be an element?

Native minerals consist of pure elements rather than compounds; gold and sulfur are examples.