Matter, Its States and Atoms and Molecules
Matter, Its States and Atoms and Molecules
Chemistry accounts for roughly a third of the 40 science questions in Part-III. This first topic sets up the particle model and the atom-molecule vocabulary that every later chemistry question uses, so imprecision here costs marks throughout the block.
The Particle Nature of Matter
Matter is anything that has mass and occupies space. It is made of tiny particles with spaces between them, and those particles are in constant motion.
| Property | Solid | Liquid | Gas |
|---|---|---|---|
| Particle arrangement | Closely packed, fixed | Close but able to slide | Far apart, free |
| Shape | Fixed | Takes the container's shape | Fills the container |
| Volume | Fixed | Fixed | Not fixed |
| Compressibility | Almost none | Very little | High |
| Force of attraction | Strongest | Moderate | Weakest |
Diffusion — the spreading of one substance through another — is fastest in gases and slowest in solids, because the particle spacing differs. The smell of food reaching another room is the standard example.
✗ Gases have a fixed volume but no fixed shape | ✓ Gases have neither a fixed shape nor a fixed volume
Changes of State
| Change | From | To |
|---|---|---|
| Melting | Solid | Liquid |
| Freezing | Liquid | Solid |
| Vaporisation | Liquid | Gas |
| Condensation | Gas | Liquid |
| Sublimation | Solid | Gas directly |
Sublimation is asked most often, and the examples are fixed: camphor, naphthalene balls, iodine, dry ice and ammonium chloride. The temperature stays constant during a change of state even though heat is being supplied, because that heat goes into breaking the attraction between particles — the latent heat. That constancy is a reasoning question.
Evaporation happens at all temperatures and only at the surface, while boiling happens at a fixed temperature throughout the liquid. Evaporation causes cooling, which is why sweating cools the body and why water stays cool in an earthen pot.
Elements, Compounds and Mixtures
| Point | Element | Compound | Mixture |
|---|---|---|---|
| Composition | One kind of atom | Fixed ratio of elements | Any ratio |
| Separation | Cannot be broken down | Chemical methods only | Physical methods |
| Properties | Its own | New, unlike its elements | Those of its components |
| Examples | Iron, oxygen | Water, salt | Air, soil, brass |
Water is a compound because hydrogen and oxygen are chemically combined in a fixed two-to-one ratio, and water behaves like neither gas. Air is a mixture because its gases are not chemically joined and their proportions vary. This pair is asked constantly.
Atoms and Molecules
- An atom is the smallest particle of an element that takes part in a chemical reaction.
- A molecule is the smallest particle of a substance that can exist independently.
- John Dalton proposed the atomic theory; the term atom comes from a Greek word meaning indivisible.
- An atom has a nucleus of protons and neutrons, with electrons around it.
- Atomic number is the number of protons; mass number is protons plus neutrons.
Note that a molecule need not have two different elements: an oxygen molecule is two oxygen atoms of the same element. Candidates often assume a molecule must be a compound.
तत्व एक ही प्रकार के परमाणुओं से बनता है, यौगिक दो या अधिक तत्वों से।
TRE pointer: Part-III chemistry is examined at NCERT class 8 depth and gives roughly 13 of the 40 science questions, so it is nearly as valuable as physics. The water is a compound, air is a mixture pair and the sublimation examples are the two items that repeat every year. Expect a reasoning question on why temperature stays constant during melting, since that is exactly the misconception a class 7 teacher must be able to fix. With −1/3 negative marking, secure the definitions first.
60-Second Recap
- Solids have fixed shape and volume; liquids fixed volume only; gases neither.
- Diffusion is fastest in gases; evaporation causes cooling and happens at all temperatures.
- Sublimation is solid to gas directly — camphor, naphthalene, iodine, dry ice.
- Temperature stays constant during a change of state; the heat is latent heat.
- A compound has a fixed ratio and new properties; a mixture has neither.
- Atomic number counts protons; a molecule may be of one element, as in oxygen.
Frequently Asked Questions
Why is water a compound but air a mixture?
Because hydrogen and oxygen in water are chemically combined in a fixed two-to-one ratio and water behaves like neither of them. Air's gases are merely mixed, their proportions vary, each keeps its own properties, and they can be separated by physical means.
Which substances undergo sublimation?
Camphor, naphthalene balls, iodine, dry ice and ammonium chloride are the standard examples. Sublimation is the direct change from solid to gas without passing through the liquid state, and it is also used as a technique to separate such a solid from a mixture.
Why does temperature stay constant while ice melts?
Because the heat supplied is used to overcome the forces of attraction between the particles rather than to raise their energy of motion. This is called latent heat. Once the whole solid has melted, further heating begins to raise the temperature again.
Is every molecule made of different elements?
No. A molecule is simply the smallest particle of a substance that can exist on its own, and it may consist of identical atoms. An oxygen molecule has two oxygen atoms, so it is a molecule of an element, not of a compound.