- All matter is either a pure substance or a mixture. Pure substances split further into elements and compounds.
- An element cannot be broken into anything simpler (gold, oxygen, iron). A compound is two or more elements joined chemically in a fixed ratio (water, salt, sugar).
- A mixture is two or more substances simply mixed — they keep their own properties and do not react. Mixtures may be uniform (air, sugar water, alloys) or non-uniform (sprout salad, sand and water).
- Compounds need a chemical method to separate; mixtures can be separated by physical methods (magnet, evaporation, filtration).
- Key experiment: iron + sulfur mixed = a mixture (magnet works, sulfur stays yellow); after heating = iron sulfide, a compound (no magnet, new properties).
- Exam weightage: ~4–5 marks — usually a classify-these (element/compound/mixture) question, the iron-sulfur comparison, and an assertion–reason on "air is a mixture".
1. The big map of matter
Everything around you — the staircase, the air, the water in your bottle, your shoes, the trees — is matter: it has mass and takes up space. But not everything is matter: light, heat, electricity, even thoughts and emotions are real and important, yet they are not matter.
Matter divides neatly into two big families, and this whole chapter is just this one diagram:
Matter → (b) Mixtures → either Uniform or Non-uniform
Most things around us are not made of just one substance — they are two or more substances mixed together. So we begin with mixtures, then peel inwards to pure substances, elements and compounds.
2. What are mixtures? (Section 8.1)
When two or more substances are mixed, and each substance keeps its own properties, it is called a mixture. The individual substances that make up a mixture are its components. Two things are always true of a mixture:
- The components do not react chemically with each other.
- There is no fixed ratio — you can add more of any component.
Everyday examples: poha, sprout salad, sugar dissolved in water, soups, lemonade.
3. Uniform vs non-uniform mixtures
- Non-uniform mixture: the components are visible separately with the naked eye or a magnifying glass — like green gram, chickpeas, onion and tomato in a sprout salad. The mixture looks different at different points.
- Uniform mixture: the components are evenly distributed and cannot be distinguished, even under a microscope — like sugar dissolved in water. It looks the same throughout.
Stainless steel contains iron, nickel, chromium and a little carbon, mixed so uniformly that you cannot see the separate metals. Such uniform metal mixtures are called alloys. Brass (copper + zinc) and bronze (copper + tin) are other alloys. Heritage note: the ancient name for such a metal mixture was Mishraloha; bronze (Kamsya) was made of Copper (Tamra, 4 parts) and Tin (Vanga, 1 part).
4. Is air a mixture? (Section 8.1.1)
Yes — air is a uniform mixture of mainly nitrogen, oxygen, argon, carbon dioxide and water vapour. Key facts: nitrogen ≈ 78% of air and does not take part in combustion; oxygen is needed by living beings and helps combustion.
Add calcium oxide (quick lime) to water — it reacts vigorously, releasing heat, to form calcium hydroxide. Filter to get a colourless solution called lime water. Leaving it exposed to air (stirring now and then) turns it milky. Why? CO₂ from the air reacts with the calcium hydroxide to form insoluble white calcium carbonate:
The milkiness proves carbon dioxide is present in air.
A clean black sheet left near an open window for a few hours collects tiny particles (seen better with a magnifying glass). This shows dust particles are suspended in air — they are pollutants, and their amount varies by time and place. Major air pollutants: particulate matter (dust, soot) and gases like carbon monoxide, ozone, nitrogen dioxide, sulfur dioxide. The Air Quality Index (AQI) describes air quality.
5. Types of mixtures (Section 8.1.2 — Table 8.1)
In science, the components of a mixture must themselves be pure substances. Mixtures are classified by the physical states of their components:
| Mixture type | Example | Uniform / Non-uniform |
|---|---|---|
| Gas + gas | Air | Uniform |
| Gas + liquid | Aerated (soda) water; oxygen in water | Uniform |
| Solid + gas | Carbon particles in air | Non-uniform |
| Liquid + liquid | Vinegar (acetic acid in water) — uniform; oil + water — non-uniform | Both possible |
| Solid + liquid | Sand + water — non-uniform; seawater — uniform | Both possible |
| Solid + solid | Baking powder (baking soda + tartaric acid); alloys | Uniform / both |
You separate mixtures using physical methods (filtration, evaporation, magnet, etc.). In everyday life we separate to keep the useful part; in science the aim is to obtain pure substances.
6. What are pure substances? (Section 8.2)
The word "pure" means two different things:
- Common usage: "pure" = unadulterated. Adulteration is the illegal adding of cheaper or poor-quality substances to a product (to cut cost or increase quantity) — it lowers quality and can be hazardous.
- Science: a pure substance has no other substance in it — it is made of the same type of particles only, and cannot be separated into other kinds of matter by any physical process. So even "pure" milk or juice is impure to a scientist, because it contains more than one substance.
7. Splitting water — elements appear (Section 8.3)
Fill a beaker 2/3 with water + a few drops of dilute sulfuric acid; fill two test tubes with this water and invert them over the two terminals of a 9 V battery. After 10–15 minutes, gas bubbles collect — but the volumes are not equal. Testing with a burning candle:
- One gas gives a "pop" sound → hydrogen.
- The other makes the flame glow brighter → oxygen.
So water is made of two different constituents. (Note: this breaking down by electricity is a chemical change.)
8. Elements (Section 8.3.1)
The hydrogen and oxygen formed above are elements: pure substances that cannot be broken down into anything simpler. They are the building blocks of all matter. Each element is made of identical tiny particles called atoms, different from the atoms of every other element. Other examples: gold, silver, sulfur, carbon.
- Molecules: the atoms of most elements cannot exist alone. Two or more combine to form a stable particle called a molecule — e.g. two hydrogen atoms make one H₂ molecule; two oxygen atoms make one O₂ molecule.
- Metals, non-metals, metalloids: elements are classified into metals (gold, silver, magnesium, iron, aluminium) and non-metals (carbon, sulfur, hydrogen, oxygen). A few like silicon and boron have in-between properties — they are metalloids.
There are 118 known elements; most are solids. Eleven are gases at room temperature (all non-metals, e.g. oxygen, helium, nitrogen). Only two are liquids: mercury (a metal) and bromine (a non-metal). Gallium and caesium are solids but melt at around 30 °C (303 K). A mobile phone uses more than 45 different elements (aluminium, copper, silicon, cobalt, lithium, gold, silver, etc.).
9. Compounds (Section 8.3.2)
Why can't we separate the hydrogen and oxygen in water by physical means? Because their particles are tightly attached chemically. That is what makes water a compound. A compound is formed when different elements combine chemically in a fixed ratio to form something entirely new. Three rules:
- The properties of a compound are different from the elements that form it. (Hydrogen is a fuel, oxygen supports burning — yet water extinguishes fire!)
- The elements are joined in a fixed ratio — in water the atom ratio of hydrogen to oxygen is 2 : 1.
- A compound cannot be separated by any physical method — only a chemical change can.
Common salt (sodium chloride): sodium (a soft, reactive metal) + chlorine (a hazardous gas) combine in a 1 : 1 ratio to form a harmless, tasty, life-essential compound. (Dissolved salt in water is only a mixture — that can be separated by evaporation.)
Heat a teaspoon of sugar gently in a boiling tube. It turns brown, then chars to black. Water droplets appear near the open end (they came from the sugar, not the air, since we are heating). Left behind is charcoal (carbon). So sugar decomposes into carbon and water — meaning sugar is a compound made of the elements carbon, hydrogen and oxygen.
10. The star experiment — iron + sulfur (Activity 8.5)
This is the single most important activity for exams. Take 5.6 g iron filings (black) and 3.2 g sulfur powder (yellow).
- Sample A: just mix them — a non-uniform mixture; you can still see black and yellow particles separately.
- Sample B: heat half of Sample A with stirring until a black mass forms, then grind it — this is the compound iron sulfide.
| Test | Sample A (mixture) | Sample B (iron sulfide, compound) |
|---|---|---|
| Appearance | Black + yellow particles seen separately | Uniform black; same colour/texture throughout |
| Magnet test | Iron is attracted — can be separated | No effect — iron cannot be pulled out |
| Add dilute HCl | Gas with no smell, burns with a "pop" (hydrogen); sulfur left as yellow solid | Gas with a rotten-egg smell (hydrogen sulfide) |
Reactions:
Iron sulfide + Dilute Hydrochloric acid → Iron chloride + Hydrogen sulfide
Conclusion: In Sample A the components keep their own properties and can be separated (mixture). In Sample B a new substance with completely different properties has formed and iron/sulfur can no longer be separated (compound). That is exactly why the magnet works on A but not on B.
11. Using elements, compounds & mixtures (Section 8.4)
- Air (mixture) to breathe; water (compound) for life; iron and aluminium (elements) for bridges, buildings, vehicles.
- Chemists combine elements into compounds to make medicines, vaccines and fertilisers.
- Engineers design alloys like stainless steel (stronger and more durable than pure iron). Wood, steel and concrete are all mixtures.
- Graphene aerogel — a "wonder" material made from carbon, said to be the lightest material on Earth; highly porous, used to clean oil spills and make energy-saving coatings.
12. Minerals (Section 8.5)
Most rocks are a mixture of minerals. Some minerals are native minerals — pure elements, not compounds (metals like gold, silver, copper; non-metals like sulfur, carbon). But most minerals are compounds of more than one element. Examples: quartz, calcite, mica, pyroxene, olivine, talc.
- Cement is made from calcite, quartz, alumina and iron oxide.
- Talcum powder comes from the mineral talc.
Heritage — Dhokra art (Bihar & Odisha): a beeswax model is coated in clay, the wax melted out, and the hollow filled with molten brass or bronze — alloys that make the figures strong and golden.
13. Quick revision checklist
- Matter = pure substances (elements + compounds) OR mixtures (uniform + non-uniform).
- Element: cannot be broken down; one type of atom. Compound: elements joined chemically, fixed ratio, new properties. Mixture: just mixed, own properties kept, no fixed ratio.
- Separate a mixture by physical methods; a compound only by chemical change.
- Water = 2:1 (H:O); salt = 1:1 (Na:Cl); sugar = C, H, O.
- Iron+sulfur mixed = mixture (magnet works); heated = iron sulfide compound (no magnet, rotten-egg gas with HCl).
- Air is a uniform mixture (N₂ ≈ 78%); lime water turns milky to prove CO₂.
- Light, heat, electricity, thoughts and emotions are not matter.
- Air
- Brass
- Water
- Seawater
- 1 : 1
- 1 : 2
- 2 : 1
- 3 : 1
- Magnet
- Burning candle
- 9 V battery
- Lime water
- A non-uniform mixture
- Iron sulfide, a compound
- An alloy
- Iron chloride
- Oxygen
- Nitrogen
- Carbon dioxide
- Water vapour
- Sulfur
- Bronze
- Sodium chloride
- Carbon dioxide
- Hydrogen
- Oxygen
- Carbon dioxide
- Nitrogen
- Iron
- Charcoal (carbon)
- Sulfur
- Calcium carbonate
- Air
- Water
- Light
- Iron
- Alloys
- Compounds
- Metalloids
- Minerals
- Uniform mixture
- Non-uniform mixture
- Compound
- Pure substance
- One
- Two
- Eleven
- One hundred eighteen
- No smell and a "pop" sound
- A rotten-egg smell
- A sweet smell
- A burning smell of sulfur
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