Chemistry · Some Basic Concepts Of Chemistry · NEET
There are TWO separate systems, and students mix them up. System 1 is by physical STATE: solid, liquid, gas (this is about how the particles are arranged). System 2 is by COMPOSITION: pure substance (element or compound) or mixture (about what the matter is made of). Water can be ice (solid), water (liquid) or steam (gas) — that is System 1 — but in all three it stays the compound H2O — that is System 2. NEET can ask either, so keep them apart.
Element = made of only ONE kind of atom (like O2, Na, Fe). Compound = two or more elements joined in a FIXED ratio, and you need a chemical reaction to break it (like H2O, CO2, NaCl). Mixture = two or more substances mixed in ANY ratio, and you can separate them by PHYSICAL methods like filtering or evaporating (like sugar in water, air). One-line test: fixed ratio + chemical change to separate = compound; any ratio + physical separation = mixture.
Air is a MIXTURE (a homogeneous mixture). It contains nitrogen, oxygen, argon, carbon dioxide and water vapour in no fixed ratio, and each gas keeps its own properties. Because the ratio can change (more CO2 in a crowded room), it cannot be a compound. NCERT names air as a classic homogeneous mixture.
Water (H2O) is a COMPOUND: hydrogen and oxygen are chemically bonded in a fixed 2:1 ratio, and you cannot separate them by filtering or boiling — you need electrolysis (a chemical change). Sugar dissolved in water is a MIXTURE: the sugar and water are not chemically bonded, the sweetness of sugar and the properties of water stay, and you can get the sugar back by simply evaporating the water. Fixed ratio + chemical bond = compound; any ratio + physical separation = mixture.
A HOMOGENEOUS mixture has the same composition throughout — the parts are uniformly mixed and you cannot see the boundary (sugar solution, air, salt water). A HETEROGENEOUS mixture does NOT have uniform composition — you can see separate parts (sand and salt, oil and water, a mixture of iron filings and sulphur). Quick test: if it looks the same everywhere = homogeneous; if you can point to different regions = heterogeneous.
Solid, liquid and gas are the three physical STATES of matter, not the composition classification. In a solid, particles are packed closely in an orderly way and barely move. In a liquid, particles are still close but can move around, so a liquid flows and takes the shape of its container. In a gas, particles are far apart and move fast, so a gas has no fixed shape or volume and fills the whole container. State depends on temperature and pressure — the same water can be all three.
In which of the following is the number of water molecules maximum?
The highest number of helium atoms is present in
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Solid, liquid and gas. In a solid, particles are packed close and orderly with little movement. In a liquid, particles are close but can slide past each other, so it flows. In a gas, particles are far apart and move freely, so it fills its container. State can change with temperature and pressure.
A pure substance has a fixed composition and definite properties — it is either an element or a compound (like copper or water). A mixture is two or more substances physically mixed in any ratio, and each keeps its own properties (like air or salt water). Mixtures can be separated by physical methods; compounds cannot.
Element: oxygen gas (O2) or iron (Fe) — one kind of atom. Compound: water (H2O) or carbon dioxide (CO2) — different elements in a fixed ratio. Mixture: air, sugar solution or a sand-salt mix — substances combined in no fixed ratio.
A mixture can be either. Homogeneous means uniform throughout with no visible boundary (sugar solution, air). Heterogeneous means you can see different parts (sand and iron filings, oil and water). NEET often asks you to sort examples into these two types.
No. A compound is held together by chemical bonds, so it needs a chemical change (like electrolysis or heating that causes a reaction) to break it into its elements. Only mixtures can be separated by physical methods such as filtration, evaporation or distillation.
