Latent Heat

2. Latent heat,

During a change in state the heat energy flow is used to change the bonding between the molecules. In the case of melting, added energy is used to break the bonds between the molecules. In the case of freezing, energy is given out as the molecules bond to one another, into less energetic states. These energy exchanges are not changes in kinetic energy. They are changes in bonding energy between the molecules.

Latent heat is the heat energy supplied to a substance to change state. There is no rise in temperature at this stage.

use Cetyl Alcohol to plot the

The STS of Latent Heat (Applications)

  • An ice-cube is more effective in cooling a drink on a summer’s day than would be an equal mass of cold water. One could say that a little ice cools a lot of water.
  • If one wets one’s finger in one’s mouth and holds it up in the wind, the wind direction can be determined by the evaporation and subsequent cooling that takes place on the skin’s surface.
  • A scald from steam is always more painful than that from boiling water. This is because steam condenses on the skin, and in so doing releases the latent heat of vaporisation. This heat energy causes vibrations which traumatise the skin tissue, creating the sensation of pain. (It is always advisable to cool the area with cold running water.)
  • Large containers of water are sometimes left in a cellar in which apples, vegetables or tinned foods are stored. If the cellar temperature were to fall to, or below, 0 °C, the water in the containers would freeze before the fluids contained within the food. As the water in the container freezes, the latent heat emitted may be enough to prevent the foods from freezing.
  • Aftershave or skin tonic applied to the face makes the wearer feel fresher. Because these preparations contain alcohol, which is a volatile liquid, evaporation takes place quickly, so the skin feels cooler and fresher.
  • Campers often realise that it is more effective to wrap a damp cloth around bottles or cartons of milk to keep them cool, rather than placing them in a jar of water. The water evaporating from the cloth around the milk takes heat energy from the milk, thereby cooling it.
    • The same idea was used by the Egyptians thousands of years ago. They realised that water could be kept cool by placing it in porous earthenware vessels. These containers allow sufficient liquid to escape and evaporate, and in so doing cool the water remaining. The same idea has been used with wine coolers, and also with pieces of muslin draped over jars.
  • Very often a nurse or doctor, before giving a patient an injection, will rub the area first with a volatile liquid. This acts as an antiseptic but also, as the liquid evaporates quickly, the skin is left cooler and slightly anaesthetised.
  • To help a person with a very high temperature, which is more effective: to sponge the forehead with cold water or with tepid water? The tepid water is better. Because of its higher temperature, the tepid water will evaporate at a faster rate. It is then the latent heat absorbed from the body which cools the patient down.
  • Perspiration. When we sweat / perspire we secrete a solution of salty water from our sweat gland. Droplets of this mixture sit on our skin for a short time. This sweat actually keeps us cool by taking in energy from our hot bodies and changing state from a liquid to a vapour. This vapour diffuses through the air, but critically it has removed a great deal of excess heat energy from our bodies.
  • fruit farmers often spray the delicate fruit on the trees with water if they think there will be a late spring frost. In this way, as the water on the fruit freezes at night, it gives out its LH of fusion, which can often be enough to keep the underlying fruit "warm"
  • This is also practiced in farming where cattle etc are kept in shed overnigh on cold winter nights. A large trough full of water will freeze and give out its :H of fusion which in turn will keep the stable marginally warmer!

specific latent heat

Definitions and units.

Appropriate calculations.

In order to extract the maximum flavor in the shortest amount of time, your local fast food purveyor has decided to brew its coffee at 90 ℃ and serve it quickly so that it has only cooled down to 85 ℃. While this may be economically sensible, it is negligent and dangerous from a health and safety standpoint. Water (which is what coffee mostly is) at 85 ℃ is hot enough to cause third-degree burns (the worst kind) in two to seven seconds. You decide to add ice cubes to your coffee to cool it down to a more reasonable 55 ℃ so you will be able to drink it sooner. (Watery brew be damned. You need your caffeine now.) How many 23.5 g ice cubes at −18.5 ℃ should you add to your 355 ml cup of coffee to accomplish your thermal goal?

3. Measurement of the specific latent heat of fusion of ice.

Heat experiment 2 Latent heat of fusion

The ice is added to water above room temperature because

  1. The final temp would be less than room temp would be less heat transferred from the air therefore smaller percentage error
  2. The ice will melt more quickly, less time for the air temp to alter the values
  3. A higher final temperature means less chance of condensation
  4. heat taken from the room is equivalent to the heat lost to the room

The Ice is crushed as a large block of ice would have a colder core temperature than the outside

The ice is dried as to measure only the latent heat of ice, without adding excess water that woujld distort the readings

Common Sources of error

Thermometer takes heat out

Lack of insulation

Spillage of water

Ice not crushed / dried

4. Measurement of the specific latent heat of vaporisation of water.

Heat experiment 3 Latent heat of vaporisation

Heat pump,

Homemade heatpump pic here !!!

needs editing!!

A fridge works using a heat pump. This system allows a liquid that has a low boiling point (this means it will readily turn into a gas) to be pumped around a coil. When the liquid enters the less presurised part of the system, it expands, it turns into a gas. This lower pressure, lowers the boiling temperature of the coolant. The liquid boils at a lower temperature at the lower temperature.

The change of state still requires energy, and so the coolant liquid takes in the energy from the air surrounding it, the fridge.

It is essential that you understand that even if something is cool or even cold it still has heat energy. Some of the remaining heat energy in the cold volume of a fridge or freezer is given to the coolant, thus further removing the heat from the cool region.

This coolant however must complete a cycle and so is taken out of the insulated unit and is compressed back into a liquid, as we can see from the diagram above this releases energy in the form of heat out the back of the unit.

The fridge cools slowly.

The same effect can also be used to heat a space, if you can remove heat from somewhere and relocate the energy somewhere else this can reduce heating costs. The 'social housing' project the Oliver Bond flats on the Quays (behind the petrol station) was supposed to be heated using this type of heat pump. In the early 1970's the council objected to using the 'heat' from the River Liffey. Now it is becoming a standard low running cost and maintainance method of heating homes.


A Question

Mid-ocean ridges and other volcanic areas emit a large amount of energy due to the cooling of magma. In this question you will calculate the amount of energy that could be obtained from this source. The answer will depend on how much lava there is, its initial temperature and its physical properties (such as specific heat capacity and the specific latent heat of solidification).

Consider a particular example of a lava flow with a volume of 5.2 × 107 m3 and a density of 2.7 × 103 kg m–3 which erupts at a temperature, Terupt, of 1145 °C and cools to a final temperature, Tfinal, of 10 °C. Assume that the specific heat capacity, c, of both the lava and the rock formed is 1.5 × 103 J kg–1 °C–1, and that the average specific latent heat, L, released from the lava in solidifying into a crystalline rock is 4 × 105 J kg–1.

(i) Calculate the mass, m, of lava. (Show all of your working.) (ii) Write an equation, using symbolic notation, expressing the total energy, q, released from the lava as a result of cooling and crystallisation.

(iii) Calculate the total amount of energy, q, released by the lava. (Show all of your working.)

Check Out the Mpenda effect especially the guy that it is named after ...

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