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.
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