Matter and particles, Year 10: latent heat, heating curves and gas pressure

Specific latent heat

HintThe middle word comes from the Latin for 'hidden', because a thermometer does not show this energy going in.

The whyIts unit is joules per kilogram (J/kg) and its symbol is L. There are two values for each substance: one for melting, called fusion, and one for boiling, called vaporisation. Energy for a change of state = mass × this quantity.

167

HintThe equation for a change of state has only two quantities multiplied together; then look again at the unit asked for.

The whyEnergy for a change of state = mass × specific latent heat, or E = m L, so E = 0.5 × 334 000 = 167 000 J, which is 167 kJ. There is no temperature term, because the temperature does not change during melting. This equation is given on the AQA Physics equation sheet.

Fusion

HintThe same word is used for joining things by melting them together.

The whyThe value for the change from liquid to vapour is called the specific latent heat of vaporisation. For water the vaporisation value is several times larger than the melting one, which is why a scald from steam is so severe.

2 260 000 J/kg

HintFind how much energy a whole kilogram would need.

The whyRearranging E = m L gives L = E ÷ m = 45 200 ÷ 0.02. Dividing by a number smaller than one makes the answer bigger, as it should: a whole kilogram needs fifty times the energy of 0.02 kg. The equation is on the equation sheet.

Specific heat capacity is about changing temperature; specific latent heat is about changing state

HintOne goes with a thermometer reading that climbs, the other with a reading that stays put.

The whySpecific heat capacity is the energy to raise the temperature of one kilogram by one degree Celsius, with no change of state. Specific latent heat is the energy to melt or boil one kilogram with no change of temperature. AQA expects pupils to be able to tell the two apart.

The solid is melting

HintEnergy is still going in, but it is being spent on something other than warming.

The whyA flat section on a heating graph marks a change of state. The first one met when heating a solid is melting. Once all the solid has become liquid, the temperature starts to climb again.

Changing the state of the substance: it raises the internal energy without raising the temperature

HintThe particles are being rearranged, not sped up.

The whyOn a heating graph this is why the line goes flat at the melting point and again at the boiling point. The energy supplied is called latent heat. It increases what is stored by the particles by changing how they are arranged, and only when the change is complete does the reading climb again.

80

HintStarting from a solid, which change of state must come first?

The whyThe first level section, at the lower temperature, is where the solid turns to liquid. The second, at 218 °C, is where the liquid boils. Each flat section sits at the temperature of the change of state it shows.

Liquid

HintWork out which two changes of state the chosen temperature falls between.

The whyBelow the first flat section the substance is solid; above the second it is gas. At 150 °C it has finished melting but has not yet reached its boiling point, so it lies on the sloping part of the graph between the two level sections.

The substance is turning from liquid to solid

HintIt is the heating graph run backwards.

The whyA level section on a cooling graph is a change of state too. As the liquid solidifies it gives out energy, which keeps its temperature steady even though the tube is still losing energy to the room. Once it is all solid, the temperature falls again.

The particles move faster and hit the walls harder and more often

HintWarming a gas does not add anything to it, so ask what changes about what is already there.

The whyHeating the air gives its particles more energy, so they travel faster. Faster particles strike the inside of the bottle more often, and each collision pushes harder. Both effects raise the force on every square centimetre of the wall.

increases

HintThink of an aerosol can and the warning printed on it about heat.

The whyWith the container's size fixed, hotter gas means faster particles and more forceful collisions with the walls. This is why sealed cans must be kept away from fires and why tyre pressures are checked cold.

Their average kinetic energy

HintHotter means faster; which quantity in physics goes with speed and mass?

The whyThe molecules of a gas move at a wide range of speeds, so no single molecule sets the temperature. What a thermometer responds to is the average. Heat the gas and that average goes up; cool it and it comes down.

Constant, random motion

HintThey never stop, and there is no pattern to where they are heading.

The whyGas molecules travel in straight lines at high speed until they collide with each other or with the walls of the container, then head off in a new direction. The directions and speeds are random. The collisions with the walls are what cause the pressure of the gas.

A change in volume would also change the pressure

HintAsk what else, apart from heating, could alter how often the molecules strike the walls.

The whyPressure depends on how hard and how often the molecules hit the walls. If the container could expand, the molecules would have further to travel between collisions and the pressure would change for that reason as well. Keeping the volume constant means any change in pressure is caused by the temperature alone.