Heat changes matter

When objects are heated, their volume generally increases. This increase in size called expansion.  

When objects are cooled, their volume generally decreases.  This decrease in size is called contraction.

Heat flows from a place of high temperature to a place of lower temperature.

When a solid (metal) is heated, its particles (atoms) receive more heat energy and vibrate faster.

The particles occupy more space and move further apart from one another, keeping the same relative positions. So the solid expands

On cooling the vibrations slow down and the particles return to their original positions. So the metal contracts.

When the water is heated, its particles (molecules) receive more energy and move further apart from one another in all directions.

The water takes up more space and its volume expands.

The water level in the glass tube falls a little at first because the flask expands before the water.

a) When the air is warmed, the gas particles (molecules) receive more kinetic energy and move outwards very quickly in all directions. So the air expands very fast.

b) Gases can expand very fast because there is : 

 i. Plenty of space between the particles.

ii. Hardly any force of attraction between the particles.

c) All gases expand at about the same rate when heated.