Experiments That Actually Work: Latent heat of fusion

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There used to be a sign outside my physics classroom:

If it’s green and wriggles, it’s biology.
If it’s green and bubbles, it’s chemistry.
If it’s green and doesn’t work, it’s physics.

This is unfortunately very true. Many classroom experiments end with me saying “what you should have seen…” or “what should have happened…”.

In this series I’m going to list experiments that actually work as intended – experiments that can be relied to end without a “should” in the explanation.

The Cooling Curve of Stearic Acid

In this experiment we’re attempting to show that the freezing process gives out energy; or alternatively that the melting process requires an input of energy. This energy is called the latent heat of fusion.

This experiment is often carried out using stearic acid (C18H36O2), which is particularly suitable as it’s fairly non-toxic and has a melting point of 69.5°C which is easily achievable with a water bath and a Bunsen burner. What pupils usually do is heat a test tube of solid stearic acid in the water bath, taking temperature readings every minute until all the stearic acid has melted.

What this data should show when plotted on a graph is a flat section where the thermal energy from the Bunsen burner is going into weakening bonds between particles rather than raising the temperature. Unfortunately, this rarely works. Pupils find it very difficult to isolate any particular section as flatter than the rest.

My version makes two important changes:

  1. Pupils start off with a test tube full of liquid stearic acid in a hot (80°C) water bath.
  2. Pupils use two thermometers: one in the stearic acid and one in the water bath, and record both temperatures at the  same time.

This creates a graph that looks like this (I cheated and used a datalogger to record my data):

cooling-curve-stearic-acid

The blue line is the temperature of the water bath and the red line is the temperature of the stearic acid. At about 68°C the two lines begin to diverge. The stearic acid stays hotter than the surrounding water because thermal energy is being released by the bond-forming process as liquid turns to solid. Once all the stearic acid has become solid this release of heat ceases and the two temperatures equalise again.

This experiment requires a bit more preparation, and a little bit more work as pupils have to record two sets of temperature data, but I think it’s worth it.

Following instructions

One of the key skills that a scientist needs is the ability to write accurate, unambiguous instructions.

I gave one of my classes an exercise to improve their instruction writing skills. Each person got a card with a symbol on it and had to create a set of instructions for a partner to follow so that they could recreate the symbol.

Some were more successful than others:

following-instructions7

following-instructions5

following-instructions6

following-instructions4

following-instructions3

following-instructions2

following-instructions

Estimating density

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It turns out that estimating density is something that pupils find really hard.

density-questions

I asked a class of pupils to estimate the density of four substances: air, water, mercury and wax. Before you read any further, try to estimate (in kilograms per metre cubed) the density of these four.

density-estimations

In case you can’t read the photograph, their guesses (in kg/m3) were as follows:

  • Water – 1000, 60, 100
  • Air – 1, 1, 10
  • Mercury – 100000, 200, 100000
  • Wax – 10000, 40, 75

And the correct values:

density-answers

They were much closer to the correct answer for air than for anything else, but probably only because we’ve discussed it before when looking at mass. Interestingly, taking the geometric mean of the suggested values for mercury gives a value of 12600kg/m3; very close to the correct value of 13580kg/m3, but this is almost certainly a fluke.

So why is estimating density so difficult?

Pupils have far less trouble estimating mass, and less trouble estimating volume (though converting from cm3 to m3 is something they find very taxing). I think the problem comes from a lack of reference points – they’ve all held a kilogram in their hand, and have used metre rulers frequently so can envision a box 1m×1m×1m. The main point of this exercise is to give them reference points. For example: if they calculate the density of a material to be 1200kg/m3 then they know it should sink in water, and something with a density of 1kg/m3 should be lighter than air.