L2 - States of Matter

objectives

Understand the interconversions between the three states of matter, their names, how they are achieved and the changes in arrangement, movement and energy of particles

Changing state

The above diagram represents the general changes of state and their names - it also shows that we have to apply more energy in order to separate the particles

Melting

„Melting When a solid is heated, its particles get more energy and vibrate more. This makes the solid expand.

„At the melting point, the particles vibrate so much that they break away from their positions. The solid turns into liquid.

Freezing

Boiling

„Boiling When a liquid is heated, its particles get more energy and move faster. They bump into each other more often, and bounce further apart. This makes the liquid expand. At the boiling point, the particles get enough energy to overcome the forces between them. They break away to form a gas:

Condensing

Evaporation

„Evaporating Some particles in a liquid have more energy than others. Even well below the boiling point, some have enough energy to escape and form a gas. This is called evaporation. It is why puddles of rain dry up in the sun.

How much Heat Energy is needed?

„The amount of heat needed to melt or boil a substance is different for every substance. That’s because the particles in each substance are different, with different forces between them.

„The stronger the forces, the more heat energy is needed to overcome them. So the higher the melting and boiling points will be.

Exceptions

—Under some conditions, some solids turn straight into a gas – sublimation

¡e.g. solid CO₂ (dry ice), only exists as a liquid under high pressure

¡e.g. iodine – purple/black crystals to purple vapour

—Gases will solidify if pressure is increased enough because the particles are moved closer together - this is called deposition

Reversing the Changes

„You can reverse those changes again by cooling. As a gas cools, its particles lose energy and move more slowly. When they collide, they do not have enough energy to bounce away. So they stay close, and form a liquid. On further cooling, the liquid turns to a solid.

„Look at this diagram for water: