What is matter?
Anything that has mass and occupies space (has volume) is matter. Air, water, a stone and your body are matter. Feelings, light and sound are not.
Early Indian philosophers grouped matter into five basic elements, the Panch Tatva: air, earth, fire, sky and water. Modern science classifies matter by its physical properties and its chemical nature.
Matter is made of particles
Dissolve a few crystals of potassium permanganate in water. The whole beaker turns purple, and even after diluting it many times the colour remains. A tiny crystal must contain millions of tiny particles that spread through the water.
Properties of the particles of matter:
They have space between them. Sugar dissolves in water without raising the level much, because its particles fit into the spaces between water particles.
They are always moving. They have kinetic energy, which increases with temperature. Incense smell spreads through a room by diffusion, the intermixing of particles; diffusion is faster in hot liquids and fastest in gases.
They attract each other. The force of attraction holds particles together. It is strongest in solids and weakest in gases. That's why it is hard to break an iron nail but easy to move your hand through air.
The three states
Property | Solid | Liquid | Gas |
|---|---|---|---|
Shape | fixed | takes the shape of its container | fills the whole container |
Volume | fixed | fixed | not fixed |
Compressibility | negligible | very low | high |
Space between particles | least | more | very large |
Force of attraction | strongest | weaker | weakest |
Ability to flow (fluidity) | none | flows | flows |
Diffusion | very slow | faster | fastest |
Gases are highly compressible. That is why large volumes of CNG (compressed natural gas) and LPG can be stored in small cylinders.
A sponge is a solid even though it can be pressed, because the holes in it are filled with air; the solid material itself doesn't compress. A rubber band changes shape when stretched but returns to it, so it is a solid too.
Changing state
Melting. When a solid is heated, its particles vibrate faster until they overcome the forces holding them in place. The temperature at which a solid melts at atmospheric pressure is its melting point. Ice melts at 0 °C (273 K).
Boiling. The temperature at which a liquid boils at atmospheric pressure is its boiling point. Water boils at 100 °C (373 K).
To convert temperatures: K = °C + 273.
Latent heat. While ice is melting, the thermometer reading stays at 0 °C even though heat is being supplied. The heat is used to overcome the forces between particles, not to raise the temperature. This hidden heat is latent heat.
Latent heat of fusion: heat needed to change 1 kg of solid into liquid at its melting point.
Latent heat of vaporisation: heat needed to change 1 kg of liquid into gas at its boiling point.
Steam at 100 °C causes worse burns than water at 100 °C, because steam carries extra latent heat that it releases when it condenses on the skin.
Sublimation. Some solids change directly into gas without becoming liquid: camphor, naphthalene and ammonium chloride. The reverse, gas directly to solid, is called deposition.
Pressure. Applying pressure and lowering temperature can turn gases into liquids. Solid carbon dioxide (dry ice) changes directly to gas when pressure is reduced to 1 atmosphere.
Evaporation
Evaporation is the change of a liquid into vapour at any temperature below its boiling point, happening only at the surface. Wet clothes dry without boiling.
The rate of evaporation increases with:
larger surface area (clothes are spread out to dry),
higher temperature,
lower humidity (water vapour already in the air),
higher wind speed.
Evaporation causes cooling. The particles that escape take energy from the remaining liquid and its surroundings. Examples:
Water in an earthen pot (matka) stays cool because water seeps out through the pores and evaporates.
Acetone (nail polish remover) on your palm feels cold.
We sweat more in summer; the sweat evaporates and cools the body.
Cotton clothes are best in summer because they absorb sweat and let it evaporate.
Summary
Matter is made of tiny particles that have space between them, keep moving and attract each other.
Solids, liquids and gases differ in shape, volume, compressibility and particle arrangement.
During melting and boiling the temperature stays constant because of latent heat.
Evaporation is a surface process that causes cooling.