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Heat , when added to or removed from a substance can either change the temperature of that substance or change its phase . The three phases a substance can exist in are , a solid , a liquid or a vapour .
The best examples of these three phases are ice , water and water vapour ( or steam ).
To heat water a kettle must be switched on to provide the energy . Initially the water increases in temperature . Once it is boiling the temperature remains the same . It will only keep on boiling if the element remains switched on .
From this we can deduce that to heat the water , energy is required and to make the water boil , energy is also required .
Where does the energy go when the water is boiling because the temperature of the water does not increase ?
The heat being added when the water is boiling is used to change the water to water vapour . The heat added does not increase the temperature and cannot be measured with a thermometer , therefore it is called latent ( or hidden ) heat .
To put this in a refrigeration context , this is the process used by an evaporator ; heat is absorbed by the evaporator to boil off a refrigerant at a low temperature .
To change the temperature of a substance heat is required ; to change the phase of a substance heat is also required .
Sensible heat
• Any heat which , when added to a body or substance , increases its temperature , is called sensible heat .
• Any heat which , when removed from a body or substance , decreases its temperature , is called sensible heat .
When water is heated from 20 ° C to 100 ° C , or cooled from 100 ° C to 20 ° C , the heat added or removed can be sensed or felt . This is sensible heat ; when you put your finger in the water you can feel the water getting hotter or colder .
Heat is energy , so the unit for sensible heat is the same as for energy , i . e . the Joule .
To decrease the temperature of the substances the same amount of heat is required .
The specific heat capacity of some familiar substances are as follows :
Substance Specific heat capacity
Water
4,2 kJ / kg . K Water vapour 1,89 kJ / kg . K Ice
2,1 kJ / kg . K
Moist air 1,012 kJ / kg . K
Latent heat
• Any heat which , when added to or removed from a body or substance changes its phase without a change in temperature , is called latent heat .
When heat is added to water at 100 ° C at sea level it will boil and evaporate and change phase ( water to water vapour ). The heat added does not increase the temperature but changes the phase . It is thus latent heat .
Similarly , if heat is added to ice at 0 ° C it will melt and change phase ( ice to water ). The heat added does not increase the temperature but changes the phase . Thus , this is also latent heat . The unit is the same as for heat i . e . the Joule . The latent heat required to melt a solid is referred to as the
“ heat of fusion ”.
The latent heat required to evaporate a liquid is called its “ heat of vaporisation ”.
Change of phase
• When a substance is of the same nature throughout its mass , it is said to be in phase .
Water has the same nature throughout and is therefore said to be in phase . The same applies to ice and water vapour .
Again , the three phases in which matter can exist are - solid , liquid and vapour or gas - and thus different changes of phase can occur which have different names .
Specific heat capacity
• The specific heat capacity of a substance is the amount of heat required to raise ( or lower ) the temperature of unit mass ( 1 kg ) of that substance by 1K .
Different substances require different amounts of heat to raise their temperature by a certain amount .
For instance , to increase the temperature of 1kg of water from 15 ° C to 16 ° C requires
4,2 kJ ( 4 200 Joules ), but to increase the temperature of 1kg of air at 20 ° C by 1K only , requires 1,012 kJ .
Solid to liquid and liquid to solid .
• Changing a solid to a liquid ( like melting ice ) is called melting or fusion .
• Changing a liquid to a solid ( like freezing water ) is called freezing .
The heat to be added to melt ( or fuse ) one kilogram of a solid as well as the heat to be removed to freeze or solidify one kilogram of a liquid are both called heat of fusion .
The amount of heat required to melt and to freeze it are exactly the same .
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