It is clear that, to melt ice and boil water, you need to add heat... but where does that energy go?
It seems to be hidden, hence the name latent heat. This is in contrast to sensible heat, where a temperature change is observed with the addition of heat (as could reasonably be expected!).
Key Concepts
When a material is heated, it goes through several stages:
- \(A \rightarrow B\) is the heating of the solid. It increases in temperature. Specific heat capacity of the solid could be calculated for this region from the gradient as \(\Delta Q=mc\Delta \theta \Rightarrow mc ={1\over gradient}\).
- \(B \rightarrow C\) is the melting of the solid into liquid. This is the latent heat of fusion stage and the temperature remains constant: \(\Delta Q= mL_f\)
- \(C \rightarrow D\) is the heating of the liquid. It increases in temperature. I have shown this with a different gradient to the solid as the liquid with have a different specific heat capacity.
- \(D \rightarrow E\) is the boiling of the liquid into gas. This is the latent heat of vaporisation stage and the temperature remains constant: \(\Delta Q= mL_v\)
- \(E \rightarrow F\) is the heating of the gas, which increases in temperature, again shown with a different gradient.
Notice that the unit on the horizontal axis is 'J' (Joules) indicating that heat is being added to the system. Graphs like this one will look the same even if the horiztonal axis is 'time', provided that heat is supplied at a constant rate.
An electrical heater with constant power can provide this. To calculate the heat added from the Electrical power:
\(\Delta Q = IV \Delta t\)
Measuring latent heat of fusion of water
The specific latent heat is the amount of heat required to change the state of one kilogram of a substance:
\(Q = m L\)
We make the assumption that changes in state occur with no change in temperature.
Use quizzes to practise application of theory.
START QUIZ!
When a solid changes to a liquid energy is required.
Which is the best description of where that energy goes.
Chemists talk about breaking bonds we talk about increasing PE, it would also be bonds between the molecules rather than atoms but I'd rather not get into that.
Temperature doesn't change so no change in KE
Water molecules actually get closer.
100g ice is melted at 0°C in 5 minutes by an electric heater.
The latent heat of fusion of ice is approximately 3 x 105 Jkg-1
The amount of heat required to melt the ice = kJ
The power of the heater = W
Q = mL, P = Q/t careful with units
The temperature is measured as some water is boiled in a 2kW electric kettle.
The latent heat of vaporisation of water is about 2 x 106 Jkg-1
Temperature T = °C
The heat required to boil the water = kJ
The mass of water = kg
Q = Pt
m=Q/L
If you get melted sugar on your skin it burns very badly.
Fill the gaps to give the explanation.
Word list: constant, energy, solid, liquid, temperature, release, state, power
When sugar changes from to it must release , if in contact with your skin this will be given to you. remains constant as the sugar changes so it remains at melting point until all the is released. during this time the sugar will be damaging your skin.
It does too.
A type of salt is used to store solar energy collected in the day time so it can be used at night. The power station collects 1 MW.
The latent heat of fusion of the salt is 3 x 106 Jkg-1
The energy collected in 10 hours of sun light = MJ (no commas)
The mass of salt melted in one day (10 hours of sunlight) = kg
MJ is 106 J
How much of Latent heat have you understood?
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