Chemistry · Some Basic Concepts Of Chemistry · NEET
| Composition | Fixed and definite, always the same | Variable, components can be in any ratio |
| Number of components | Only one type of particle | Two or more substances mixed together |
| Separation | Cannot be separated by physical methods | Can be separated by physical methods (filter, evaporate, distil) |
| Properties | Fixed properties (fixed melting/boiling point) | Properties vary with composition |
| Examples | Copper, pure water, glucose | Air, tea, sugar solution, sand in water |
Pure water (H2O) is a pure substance. Every particle in it is the same, and it always contains hydrogen and oxygen in a fixed ratio (2:1 by atoms). But 'tap water' or 'sea water' is a mixture, because it also has dissolved salts and gases in variable amounts. For NEET, remember: the word matters. 'Water' = pure substance; 'salt water' or 'mineral water' = mixture.
Air is a mixture. It contains nitrogen, oxygen, carbon dioxide, water vapour and other gases, and their amounts can change from place to place. Because the ratio is not fixed, air cannot be a pure substance. It is a homogeneous mixture (the gases are evenly spread and you cannot see separate parts).
Ask two questions. (1) Is the composition fixed and always the same? If yes, pure substance; if it can change, mixture. (2) Can you separate it into simpler parts by a physical method like filtering, evaporation, or distillation? If yes, it is a mixture. A pure substance cannot be separated by physical methods.
Milk is a mixture. It contains water, fat, proteins, sugar (lactose) and minerals. Because you can separate cream from milk, and the amounts vary, it is not a pure substance. It is a heterogeneous mixture (tiny fat droplets are spread in water but are not one uniform phase).
No. This is a common trap. Even if a mixture looks clean and clear, like sugar dissolved in water, it is still a mixture because it has two components (sugar and water) in a changeable ratio. 'Homogeneous' (looks uniform) does not mean 'pure'. A homogeneous mixture is still a mixture, not a pure substance.
No. A pure substance is the bigger group. It splits into two types: elements (like copper, oxygen) and compounds (like water, glucose). So every element and every compound is a pure substance, but a pure substance is not always an element. For NEET, learn the tree: Matter to (Pure substance to Element or Compound) or (Mixture to Homogeneous or Heterogeneous).
Try the real previous-year questions from this chapter — each with the answer and a full solution.
A pure substance has a fixed composition and only one type of particle, and it cannot be separated by physical methods. A mixture has a variable composition, contains two or more substances, and can be separated by physical methods.
Pure substances: copper, pure water, glucose. Mixtures: air, tea, sugar solution in water. Elements and compounds are both pure substances.
No. A pure substance cannot be separated into simpler parts by physical methods like filtration or evaporation. A compound can only be broken down by chemical methods, and an element cannot be broken down at all.
Yes. Classifying matter into elements, compounds and mixtures is a listed objective of Class 11 Chapter 1. NEET often tests it indirectly through purity, percentage composition and stoichiometry problems, so getting the basic classification right builds the base for those questions.
