Changes in a material's temperature or state of matter are caused by changes to the internal energy. The energy required by different materials depends on their 'heat capacity' and 'latent heat'.

Specific heat capacity relates only to the energy required for a change in temperature. Specific latent heat relates only to the energy required for a change in state. If a change in internal energy of a material will cause it to change temperature and change state, both equations can be used.

What happens when 1 kilogram (kg) of water at 75 degrees Celsius (°C) is heated with 2.5 megajoules (MJ) (2,500,000 J)?

1. Some of the energy is used to raise the temperature of the water to 100°C, so the energy needed to raise 1 kg of water by 25°C is:

2. Some of the remaining 2,395,000 J is then used to turn the boiling water into steam, so the energy needed to change 1 kg of water at 100°C into steam at the same temperature is:

The final amount of energy 2,500,000 - 2,260,000 - 105,000 = 135,000 J, is used to raise the temperature of the steam, and as steam has a specific heat capacity of 1859 J/kg°C, the final temperature of the steam would be:

The steam started at 100°C and heats up by 72.6°C so is now 172.6°C.

- Question
If 0.5 kg of water at 80°C is changed into steam at 110°C, how will the energy be used?

Energy will go into three places.

1. Raising the temperature of the water to 100°C. So the amount of energy needed in this case would be:

2. Turning the water into steam. So the amount of energy needed in this case would be:

Raising the temperature of the steam from 100°C. So energy needed in this case would be:

So the total amount of energy needed to change 0.5 kg of water at 80°C into steam at 110°C would be:

Total amount of energy = 42,000 + 1,130,000 + 9,295 = 1,181,295 J