Unit 11:  Heat and temperature                                    1   2   3   4   5   6

4. Effects of heat

Heat produces two main effects on matter:

- Changes of volume

- Changes of physical state

4.1. Changes of volume

Matter changes volume when it exchanges energy with its surroundings. Bodies expand or contract when they gains or lose thermal energy.

      - Expansion.

          This is the increase in volume which occurs when a body gains heat.

          Its particles start to move faster and therefore occupy more space.

           - Contraction.

          This is the decrease in volume which occurs when a body losts heat.

          It is particles start to move slower and therefore occupy less space.

Normally, gases expand more than liquids and liquids more than solids.

This is because the forces among particles in solids are very strong

and it isnecessary more energy to separate them, while in liquids, and specially

in gases, the forces among particles are very weak and it is not necessary so much energy to separate them.

4.2. Changes of state

They are changes in the aggregation state of the matter. That is the arrangement of particles in matter.

- In solids, particles occupy fixed positions (they vibrate but not displace) and the distance among them are also fixed.

- In liquids, particles do not occupy fixed position although they keep fixed the distances among them.

- In gases, particles move freely and they try to keep so separate as possible.

When a body in any state of matter gains or loses enough heat , it undergoes

a change of state, that is a change in the arrangement of its particles.

- The changes produced by absorption of thermal energy are melting, evaporation and sublimation.

- The changes produced by heat loss are solidification, condensation and regressive sublimation.

Every substance changes from one state to another at a specific temperature:

- From solid to liquid and vice versa this temperature is called

  melting or solidification point.

- From liquid to gas and vice versa this temperature is called

   boiling or condensation point.

When a substance is changing its physical state its temperature does not change, in spite of the fact that it is absorbing energy.

This is because the heat is used to break the forces that keep join together the particles of the matter during the change of state. This energy is called latent heat and it is different to each substance.

4.1. Answer the questions:

         a. Why are small gaps placed between sections of a road?

         b. Why does the density of a liquid or a gas decrease when heated?

4.2. We put a certain amount of a substance in solid state within a container with a thermometer and start to heat it. The initial temperature is 15 ºC

      and it increases little by little until to reach 90 ºC at 5 minutes from the beginning of the experiment. In this moment the temperature remains constant and the solid starts to melt. 2 minutes later, temperature starts to rise up again. When it reaches 130°C, 6 minutes later it remains steady while the liquid boils.

a. Represent the graph.

b. In which state will the substance be at 50ºC? And at 150ºC?

c. Indicate the boiling point and the melting point of the substance

d. What is happening during the time that temperature does no increase?