Matter
ICSE Class 8 · Chemistry · Chapter 1
What this chapter covers
- Postulates of the kinetic molecular theory
- Interparticle space
- Interparticle attraction and collisions
- Comparing the three states of matter
- Change of state explained by kinetic theory
- Law of conservation of mass
- Examples of the law of conservation of mass
Key definitions
Interparticle space is the empty gap between the particles of a substance.
Interparticle attraction is the force with which the particles of a substance pull one another together.
Melting is the change of a solid into a liquid when it is heated.
Freezing is the change of a liquid into a solid when it is cooled.
Vaporisation is the change of a liquid into a gas (vapour).
Evaporation is the slow change of a liquid into vapour that takes place only at its surface and at any temperature below its boiling point.
Boiling is the rapid change of a liquid into vapour that takes place throughout the liquid at a fixed temperature.
Condensation is the change of a gas (vapour) into a liquid when it is cooled.
Sublimation is the change of a solid directly into a gas, without first becoming a liquid.
Deposition is the change of a gas directly into a solid on cooling, without first becoming a liquid.
Key points
- Matter is anything that has mass and occupies space, and it is made of extremely small particles.
- The kinetic molecular theory says the particles have spaces between them, attract one another, are always moving and keep colliding.
- Interparticle space is smallest in solids and largest in gases; interparticle attraction is strongest in solids and weakest in gases.
- Particles in a solid vibrate about fixed positions, particles in a liquid slide past one another, and particles in a gas move freely and fast.
- Heating gives particles more kinetic energy; cooling takes it away.
- A change of state happens when particles gain or lose enough energy to change their spacing and attraction; no new substance is formed.
- Melting (fusion), evaporation, boiling and sublimation take in energy; freezing, condensation and deposition give out energy. Evaporation draws its energy from its surroundings, so it needs no heating and causes cooling.
- Temperature stays constant during melting and boiling because the heat is used to separate the particles.
- The law of conservation of mass: mass is neither created nor destroyed in a physical or chemical change.
- Barium chloride and sodium sulphate solutions react in a closed flask to give a white precipitate of barium sulphate, with no change in total mass.
- In burning, the mass of gases taken from the air (mainly oxygen), and of any gases given off, must be counted for the law to hold.
Questions answered
State any four postulates of the kinetic molecular theory of matter.
All matter is made of very small particles; there are spaces between the particles; the particles attract one another; and the particles are always moving, faster when heated and slower when cooled. (The particles also collide with one another and with the walls of the container, exchanging energy.)
Why can a gas be compressed easily, but a solid cannot?
The particles of a gas are very far apart, so there is a lot of empty interparticle space to squeeze into. In a solid, the particles are already packed closely with almost no space between them.
A liquid has a definite volume but no definite shape. Explain this using the kinetic theory.
The interparticle attraction in a liquid is strong enough to keep the particles together, so the volume stays the same. But the particles are not fixed in place and can slide past one another, so the liquid takes the shape of its container.
What happens to the particles of ice when it is heated until it melts?
They gain kinetic energy and vibrate more strongly. At the melting point they have enough energy to overcome part of the attraction, leave their fixed positions and slide past one another, so the ice becomes liquid water.
Give two differences between evaporation and boiling.
Evaporation takes place only at the surface of a liquid, while boiling takes place throughout the liquid. Evaporation happens at any temperature below the boiling point, while boiling happens at a fixed temperature, the boiling point.
Name two substances that sublime, and name the reverse change.
Camphor and naphthalene (dry ice is another). The reverse change, from gas directly to solid, is called deposition.
Why does the outside of a glass of iced water become wet on a humid day?
Water vapour in the air touches the cold glass, loses energy and condenses into tiny drops of liquid water on its surface.
State the law of conservation of mass.
Mass can neither be created nor destroyed in a physical or a chemical change; the total mass of the reactants equals the total mass of the products.