Latent Heat

Latent heat is defined as the heat required to convert a solid into a liquid/vapour, or a liquid into a vapour, without a change in temperature. In other words, it is exactly enough heat to ‘power’ the conversion in state – there is no heat left over to increase the temperature. Latent heat can be observed by monitoring the temperature of a beaker of water and ice cubes, as it is heated on a hotplate. When the temperature is graphed against time, there should be a flat portion of the graph, meaning that all the heat from the hotplate is being used to convert the ice cubes to liquid water, and there is none left to increase the temperature.

I performed this experiment using 10 ice cubes (mass 215g) and just enough tap water to allow them to float, in a metal saucepan on a stovetop hotplate. The saucepan was heated on the lowest setting possible, as if it is heated too fast, the flat portion of the graph will not be noticeable. Every 30 seconds the ice cubes were stirred, and the temperature of the water was recorded using a glass thermometer, ensuring it did not touch the bottom of the saucepan. The graph below shows the results.

Latent heat graph.png

The flat portion of the graph (from about 1 minute to 3.5 minutes) corresponds to the latent heat of the ice, as there was no noticeable rise in the temperature over that time period. Additionally, most of the ice had melted by the time the temperature did begin to rise (about 4 minutes). Once the ice had all melted (about 12 minutes), the temperature rose a lot faster, as can be seen from the graph.

Although the effect of latent heat was observed in this experiment, it was not as obvious as expected. It could be more obvious with more ice cubes, as more heat would be required to melt them. Other factors that would affect it include the amount of water, the amount of heat from the hotplate (low or high temperature), and the room temperature, which would affect how fast the ice cubes melted without the heat from the hotplate. One would presume that at a lower room temperature (such as 5 or 10°C), the latent heat would be much more noticeable, but I haven’t tried it out. Testing out some of these variables would make a great experiment.

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