I did a school experiment to measure the heat of solution (enthalpy) of various salts dissolved in water. My results are completely different to the teacher's answer.

Specifically my test showed Calcium Chloride increasing the temperature of the cup of water by 6 degrees Celcius, while the Magnesium Chloride only increased the temperature of the same volume of water by 1 degree Celcius.

The teacher's answer shows Magnesium Chloride with an enthalpy more than double (twice as negative) as that of Calcium Chloride.

I'm happy to be wrong about this but could someone point me to an "official" answer so I can conclude that the experiment was done incorrectly.

  • $\begingroup$ Welcome to Chemistry SE. Please take a moment to share more specifics of how the results were obtained as it may help with context. This may increase the chances of a response. $\endgroup$
    – z1273
    Feb 26 at 12:05

It may be caused by the real content of hydrate water in the used calcium and magnesium chlorides. As there is possible disagreement between declared and true water content, due long term reaching equilibrium with air water vapour. I have once seen a glass bottle with reportedly solid $\ce{LiCl}$, sitting on an shelf for years. The content was a clear liquid of LiCl solution.(*)

Both chlorides form multiple hydrates, stable at different humidity or temperature conditions. Generally, with increasing of crystal/hydrate water content, the dissolution enthalpy raises for both salts from negative to positive values. E.g. anhydrous $\ce{CaCl2}$ warms up water when dissolved, $\ce{CaCl2 . 6 H2O}$ cools water down significantly. Note that both anhydrous chlorides are highly hygroscopic.

I suggest to make repetition of the experiments with salts of different origin and short history. Or, heating them well in an oven to get anhydrous stuff.

(*) This hygroscopicity of $\ce{LiCl(s)}$ is used in some humidity meters, measuring equilibrium electric resistence of a soaked fabric.

  • $\begingroup$ This makes sense to me. @GabbyChen134 It would be worth asking your teacher about the conditions of storage of the materials. Did any other students get results similar to yours? $\endgroup$
    – Waylander
    Feb 26 at 9:11
  • $\begingroup$ Thanks everyone for your help. I'll follow up with some class mates to see if I can get some other results. It has been extremely humid in Sydney. $\endgroup$ Feb 27 at 0:04

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