Why temperature stays constant during a change of state, specific latent heat of fusion and vaporisation, and reading heating and cooling graphs.
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1.What happens to temperature during a change of state?
It stays constant, even though energy is still being supplied.
2.Why does the temperature not change during melting or boiling?
The energy supplied is used to break the bonds between particles, changing their potential energy rather than their kinetic energy.
3.What is latent heat?
The energy needed to change the state of a substance without changing its temperature.
4.Define specific latent heat.
The energy needed to change the state of one kilogram of a substance with no change in temperature.
5.State the equation for energy in a change of state.
energy for a change of state = mass x specific latent heat
6.What is specific latent heat of fusion?
The energy needed to change one kilogram from solid to liquid, that is to melt it.
7.What is specific latent heat of vaporisation?
The energy needed to change one kilogram from liquid to vapour, that is to boil it.
8.What are the units of specific latent heat?
Joules per kilogram.
9.Calculate the energy to melt 2 kg of ice. Latent heat of fusion is 334000 J/kg.
2 x 334000 = 668000 J.
10.Calculate the energy to boil 0.5 kg of water. Latent heat of vaporisation is 2260000 J/kg.
0.5 x 2260000 = 1130000 J.
11.Why is the latent heat of vaporisation larger than that of fusion?
Boiling requires the particles to be separated completely, whereas melting only needs enough energy for them to move past one another.
12.Describe the shape of a heating graph for ice being heated to steam.
A rising line as the ice warms, a flat section at 0 C while it melts, a rising line as the water warms, a flat section at 100 C while it boils, then rising again.
13.What does a flat section on a heating graph represent?
A change of state, where energy is being supplied but temperature is not changing.
14.What does a sloping section represent?
The substance warming up in a single state, with kinetic energy and temperature increasing.
15.Why is the flat section for boiling longer than for melting?
More energy is needed per kilogram to vaporise than to melt, so at a steady power it takes longer.
16.What happens to energy during condensing?
Energy is released to the surroundings as the particles come closer together.
17.Is freezing exothermic or endothermic?
Exothermic. Energy is released as the particles form a fixed arrangement.
18.Why does steam cause worse burns than boiling water at the same temperature?
As the steam condenses on the skin it releases a large amount of latent heat in addition to the energy from cooling.
19.Why does sweating cool you down?
Evaporating sweat takes latent heat from your skin, lowering its temperature.
20.What is the difference between evaporation and boiling?
Boiling happens throughout the liquid at a fixed temperature. Evaporation happens only at the surface and at any temperature.
21.What is sublimation?
A change directly from solid to gas without passing through the liquid state.
22.How would you find the latent heat of fusion experimentally?
Supply a measured amount of energy to melt a known mass of ice, and divide the energy by the mass melted.
23.Why should the ice already be at 0 C at the start?
So that all the energy supplied goes into melting, rather than partly warming the ice up first.
24.Which requires more energy, melting 1 kg of ice or warming 1 kg of water by 50 C?
Melting the ice. 334000 J compared with 4200 x 50 which is 210000 J.
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