Class 8 › Science › Chapter 8
Nature of Matter: Elements, Compounds, and Mixtures
Mixtures and their types, pure substances, elements and compounds, how a mixture differs from a compound (iron and sulfur), uses of these substances, and minerals.
1. What Are Mixtures?
Mixture: two or more substances mixed together, where each substance retains its own properties. The substances in it are its components, and they do not react chemically with each other.
Non-uniform
Components visible
Components can be seen with the naked eye or a magnifying device (e.g. sprout salad, poha, sand in water).
Uniform
Evenly distributed
Components cannot be seen separately even with a microscope (e.g. sugar in water, air, brass).
- Alloys: uniform mixtures of metals (or metals with a little non-metal). Stainless steel = iron + nickel + chromium + a little carbon; brass = copper + zinc; bronze = copper + tin.
- Heritage: Mishraloha was the ancient Indian name for a mixture of metals with properties different from its constituents. Bronze (Kamsya: 4 parts copper to 1 part tin) was used in medicine (Charaka and Susruta Samhita).
- In everyday language a mixture's components can be mixtures themselves (poha), but in science the components of a mixture must be pure substances.
2. Is Air a Mixture?
- Air is a uniform mixture of mainly nitrogen (about 78%), oxygen, argon, carbon dioxide and water vapour.
- Oxygen is needed for life and combustion; nitrogen does not take part in combustion.
- Water vapour: when warm air touches a cool surface, vapour condenses into droplets.
- CO₂ test (lime water): calcium oxide (quick lime) + water gives calcium hydroxide (lime water). On exposure to air it turns milky because calcium hydroxide reacts with CO₂ to form insoluble calcium carbonate.
Lime water test
Calcium hydroxide + Carbon dioxide → Calcium carbonate (milky) + Water
- Dust in air (seen on a black sheet left near a window) is not part of air; it is a pollutant. Major pollutants: particulate matter (dust, soot), carbon monoxide, ozone, nitrogen dioxide, sulfur dioxide. The Air Quality Index (AQI) describes air quality.
3. Types of Mixtures
| Type | Example | Uniform / Non-uniform |
|---|---|---|
| Gas + gas | Air | Uniform |
| Gas + liquid | Soda water; oxygen dissolved in water | Uniform |
| Solid + gas | Carbon particles (smoke) in air | Non-uniform |
| Liquid + liquid | Vinegar (acetic acid in water) | Uniform |
| Liquid + liquid | Oil and water | Non-uniform |
| Solid + liquid | Sand and water | Non-uniform |
| Solid + liquid | Seawater | Uniform |
| Solid + solid | Baking powder (baking soda + tartaric acid) | Non-uniform (particles of each can be told apart under a lens) |
| Solid + solid | Alloys | Uniform |
- Separation: in daily life we separate a mixture to keep the component we want; in science the aim is to obtain pure substances.
4. What Are Pure Substances?
| Everyday meaning of "pure" | Scientific meaning | |
|---|---|---|
| Meaning | Unadulterated (no cheap or poor-quality substances added illegally) | Contains only one kind of particle; nothing else present |
| Example | "Pure" milk, ghee, spices | Sugar, baking soda, gold. Milk, fruit juice and soil are mixtures. |
- Pure substance: kind of matter that cannot be separated into other kinds of matter by any physical process; all its particles are identical.
- A pure substance is either an element or a compound.
Classification of matter.
5. Elements
Activity: passing electricity through water (teacher demonstration)
Water with a few drops of dilute sulfuric acid; two water-filled test tubes inverted over the terminals of a 9 V battery. Gas bubbles collect in both tubes.
Hydrogen: a burning candle gives a pop sound. Oxygen: the flame glows brighter.
The gases are not water vapour (vapour would condense back). So water is made of hydrogen and oxygen: Water → Hydrogen + Oxygen.
Element: a pure substance made of identical particles called atoms (different from the atoms of any other element); it cannot be broken down into simpler substances. Elements are the building blocks of all matter. Examples: hydrogen, oxygen, gold, silver, sulfur, carbon.
- Molecule: atoms of most elements cannot exist alone; two or more combine into a stable particle. Two hydrogen atoms form one hydrogen molecule; two oxygen atoms form one oxygen molecule.
- Classification: metals (gold, silver, magnesium, iron, aluminium), non-metals (carbon, sulfur, hydrogen, oxygen) and metalloids (silicon, boron) with intermediate properties.
- Facts: 118 known elements, most solid. 11 are gases at room temperature (all non-metals). Only 2 are liquid at room temperature: mercury (metal) and bromine (non-metal). Gallium and caesium are solids that melt at around 30 °C.
- A mobile phone uses more than 45 different elements.
6. Compounds
Compound: formed when different elements combine chemically in a fixed ratio to form something entirely new. Its properties are different from those of its elements, and the elements cannot be separated by physical methods.
| Compound | Made of | Fixed ratio | Notes |
|---|---|---|---|
| Water | Hydrogen + oxygen | 2 : 1 (atoms) | Hydrogen is a fuel, oxygen supports combustion, water extinguishes fire |
| Sodium chloride (salt) | Sodium + chlorine | 1 : 1 | Soft metal + hazardous gas make a harmless, essential substance |
| Sugar | Carbon + hydrogen + oxygen | - | Heating sugar gives brown then black char (carbon) plus water droplets |
- Salt from water by evaporation is a physical separation of a mixture; it does not split salt into sodium and chlorine.
- Heating sugar: it decomposes into carbon and water (the water comes from the sugar, not the air), so sugar is a compound, not an element.
7. Iron + Sulfur: Mixture vs Compound
Setup (teacher demonstration, use a well-ventilated area)
Sample A: 5.6 g iron filings + 3.2 g sulfur powder, simply mixed.
Sample B: half of Sample A heated with stirring until a black mass forms, cooled and ground. This is iron sulfide.
| Test | Sample A (mixture) | Sample B (iron sulfide, compound) |
|---|---|---|
| Appearance | Black iron and yellow sulfur particles visible; non-uniform | Same black colour and texture throughout |
| Magnet | Iron filings are attracted, so iron and sulfur can be separated | Not attracted |
| Dilute HCl | Iron reacts: iron chloride + hydrogen. Gas is colourless, odourless and burns with a pop. Sulfur is left as a yellow solid. | Reacts: iron chloride + hydrogen sulfide. Gas is colourless with a rotten-egg smell. |
| Components | Retain their own properties | New substance; iron and sulfur cannot be separated again |
Reactions
Iron + Dilute hydrochloric acid → Iron chloride + Hydrogen
Iron sulfide + Dilute hydrochloric acid → Iron chloride + Hydrogen sulfide
Iron sulfide + Dilute hydrochloric acid → Iron chloride + Hydrogen sulfide
- Safety: never smell a gas directly; waft it towards your nose.
8. How Do We Use Elements, Compounds and Mixtures?
- Examples: air (mixture of gases); water (compound of hydrogen and oxygen); iron and aluminium (elements used for bridges, buildings, vehicles).
- Chemists: study how elements combine to make compounds such as medicines, vaccines and fertilisers (which raise crop production).
- Engineers and material scientists: design alloys such as stainless steel, stronger and more durable than pure iron. Wood, steel and concrete are mixtures.
- Graphene aerogel: carbon-based, said to be among the lightest materials; highly porous with high absorbing capacity, so it could clean up oil spills and be used in energy-saving devices and coatings.
- Dhokra art (Bihar, Odisha): wax model coated in clay; wax melted out and the mould filled with molten brass or bronze (alloys).
- Not everything is matter: light, heat, electricity, thoughts and emotions are not made of matter. Matter has mass and takes up space.
9. Minerals
- Most rocks are mixtures of minerals.
- Minerals: natural solid substances found on Earth with a fixed chemical composition. Most are compounds; some are pure elements (native minerals).
- Native minerals: metals like gold, silver, copper and non-metals like sulfur and carbon.
- Common minerals: quartz, calcite, mica, pyroxene, olivine, talc.
- Uses: cement is made from calcite, quartz, alumina and iron oxide (minerals or obtained from minerals); talcum powder is made from talc; metals are extracted from minerals.
- So gold is both a mineral (found naturally in Earth) and a metal (an element).
10. Quick Revision
| Element | Compound | Mixture | |
|---|---|---|---|
| Type | Pure substance | Pure substance | Not pure |
| Composition | One kind of atom | Different elements chemically combined in a fixed ratio | Two or more pure substances, any proportion |
| Properties | Own properties | Different from its elements | Components keep their properties |
| Separation | Cannot be broken down | Only by chemical methods | By physical methods (e.g. magnet, evaporation) |
| Examples | Iron, sulfur, oxygen, gold | Water, salt, sugar, iron sulfide, CO₂ | Air, seawater, brass, soil, milk |
Exam tip
Classify quickly
Ask: one kind of particle? If yes, pure (element if it cannot be broken down, else compound). If not, it is a mixture.
Remember
Mixture vs compound test
Magnet and dilute HCl on iron + sulfur show the difference: the mixture keeps both properties; the compound has new ones.
Common mistakes:
• Calling air or seawater a compound (they are mixtures), or calling water an element (it is a compound).
• Treating "pure" milk, ghee or juice as scientifically pure (they are mixtures).
• Saying a compound's elements keep their properties (they do not).
• Saying elements in a compound combine in any ratio (the ratio is fixed, e.g. 2:1 in water).
• Counting dust as a component of air.
• Forgetting that alloys are uniform mixtures, not compounds.
• Confusing physical separation (evaporation of salt water) with splitting a compound into elements.
• Calling air or seawater a compound (they are mixtures), or calling water an element (it is a compound).
• Treating "pure" milk, ghee or juice as scientifically pure (they are mixtures).
• Saying a compound's elements keep their properties (they do not).
• Saying elements in a compound combine in any ratio (the ratio is fixed, e.g. 2:1 in water).
• Counting dust as a component of air.
• Forgetting that alloys are uniform mixtures, not compounds.
• Confusing physical separation (evaporation of salt water) with splitting a compound into elements.