The Latent Heat of Fusion of Water and Ice         

  

Purpose:   To study the process of calorimetry and to determine the heat of fusion of water.

 

Apparatus: Calorimeter Styrofoam cup, Thermometer, Spring scale, Ice, Water, and Paper towel

 

Theory:     Upon application of sufficient heat, substances change their physical state. Generally but not always, these changes of state proceed from the solid to the liquid state and then from the liquid to the gaseous state as the substance is heated. In this experiment these changes of state will be studied using water as the substance under study.

When a solid changes to a liquid, the process is called fusion and in the case of water under standard pressure the change of phase takes place at 0oC. When a liquid changes to a gas, the process is called valorization and in the case of water under standard conditions, the change of state takes place at 100 oC.

To consider what happens when a sample of ice is continuously heated under standard pressure conditions, reference will be made to the diagram below.

 

 

 

 

                                                                                                                                                                                                                                                                                                                                                   

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The diagram above deals with a 1kg of ice at an initial temperature of -10oC and heat is being added to it causing its temperature to rise to 100oC.

At first the temperature of the ice rises until it reaches to 0oC. At 0oC, the temperature stops rising and the ice begins to melt. More and more ice is melted as heat is continually added but not until the ice has turned to all water does the temperature begin to rise again. As the water becomes hotter it eventuality reaches to 100 0oC, where vigorous boiling sets in. Here again the temperature stops rising and as heat is added, more and more water is boiled away to become steam. Finally, when all the water has become steam at 100 oC, the temperature begins to rise again once more.

In a solid, the molecules are anchored in place by strong mutual attractions. Each molecule oscillate about a mean position. When heat is applied the amplitude of the oscillation increases resulting in expansion. When the melting point is reached, the oscillations are sufficiently violent to overcome the attractive forces and the rigid structure breaks down resulting in a liquid. The heat absorbed in melting, without being any rise in  temperature, represents the energy needed to break down the structure and is called the latent heat of fusion, (latent from the Latin “latens” meaning “hidden”) so called because it does not cause a rise in temperature.

 In a liquid, the molecules are free to move about at random, although they are still held in a body fixed volume by the mutual attractions between them. The energies of the molecules are free to move about a mean value at any temperature. The most energetic molecules escape from the surface (evaporation), therefore the average energy of those remaining is reduced and the liquid cools if no further heat is supplied. The rate of evaporation increases as increasing heat is applied until finally  a stage is reached when a complete breakdown occurs and the liquid begins to boil. The latent heat of vaporization absorbed represents the energy needed to break down the structure of the liquid.

            The latent heat of vaporization is defined at the amount of heat needed top change unit   mass

from the liquid to the gaseous state without any change in temperature. Again, the units are Joules per kilogram. If the liquid is vaporizing, latent is absorbed, and id a vapour condenses, latent heat is given off.

This experiment determines the heat of fusion of ice by the method of mixtures. This makes use of the principle of energy conservation. When a heat interchange takes place between two bodies which are in contact and initially at different temperatures, the two bodies will assume a common equilibrium temperature, the quantity of heat loss by the warmer body being equal to that gained by the cooler body. This is true provided no heat loss to, or gained from the surroundings.

 

 

Heat gained by ice  =  Heat loss by water + Heat loss by calorimeter cup

 

milf + micw(Tf - Ti)  =   mwcw(Tin - Tf) + mccc(Tin - Tf)

 

  Where:

Procedure

1. Weigh the empty calorimeter(Styrofoam cup) making sure it is clean and dry

2. Fill the the calorimeter to about full of water at about 40oC  and then weigh it

    using the spring scale..

3. Place the calorimeter in the calorimeter jacket, and place the lid.

4. Use a thermometer to measure the temperature of the water and record.

5. Immediately, dry some small pieces of ice with a paper towel and add to the water a couple pieces at a time.

     Do not touch the ice with your fingers use a paper towel.

6. Stir well using the thermometer, and do not add more ice until the previous piece has melted.

7. Add more ice until the cup is three quarters full.

8. Record the final equilibrium temperature, and weigh the calorimeter and the contents again.

9. Tabulate all data.

 

Analysis, Calculations and Questions

 

1. With the data collected, calculate the heat of fusion for ice.

2. Compare your value with the accepted value and determine the percentage error.

3. In determining the heat of fusion of ice, why was it necessary to use small pieces of ice?

4. Why is it necessary not to touch the ice with your fingers?

5. If the ice were not dry, how would the calculated value of the heat of fusion be affected? 

 

Conclusions:   1. Comment on sources of errors.

                        2. State conclusions.

 

Latent Heat of Fusion Data Sheet