PSLE Science TOTW ~ Effect of impurities on melting point


Although the PSLE syllabus does not require students to know the effect of impurities on the freezing/melting point of a substance, this concept is commonly introduced in many schools and learning centres. (Here's the link to the latest MOE Science syllabus)

There's nothing wrong with this, if not for the fact that it's either mis-explained or inadequately explained by well-meaning educators.

Let's use salt (the usual unwitting accomplice in such cases) as an example of an impurity in our discussion.

Fact: Salt lowers the freezing/melting point of a substance.

Scenario:

Say, the room temperature is -5 degrees Celsius. Any water present would have turned into ice (because ice freezes at 0 degree Celsius). Say, we have ice block A and ice block B. What happens if you sprinkle salt liberally on only ice block A?

Observation:

Ice block A will start melting whereas ice block B will remain frozen.

Explanation:

The reckless addition of salt lowers the freezing point of ice to below -5 degrees Celsius. For the sake of argument, say it lowers the melting point of ice to -10 degrees Celsius. Now that the surrounding temperature is higher than the melting point, ice starts to melt.

This explanation seems to 'hold water', until someone asks, "But how did the salt interact with the frozen ice?".

Now, if we're adding salt into water, the salt dissolves into the water to form a salt solution, which will have a freezing point lower than pure water, so the part about salt solution not freezing at -5 degrees Celsius can be easily understood. But we can't dissolve salt in ice (the salt merely 'sticks' to the surface), so how does salt actually cause ice to melt?

Well, what actually happens is this. There is always a thin film of water surrounding ice. In a steady state, the rate at which ice melts is equal to the rate at which water freezes (all this happens even though the surrounding temperature is below the freezing point of pure water), so the amount of this thin film of 'water skin' remains more or less unchanged.

When salt is added, this thin film of water turns into a salt solution. This salt solution, which now has a lower freezing point than the surrounding temperature, can't freeze back into ice. The ice, however, continues to melt slowly. Thus gradually, all the ice turns into water.

Still, this is not the full story, because the dissolving of salt in water gives rise to chemical reactions that could either release (exothermic) or absorb (endothermic) heat energy, but we'll save that for another discussion...

Sounds complicated?

Here's the good news: this explanation is not required according to the PSLE syllabus! Still, it's good to know what actually happens, in case you're ever asked by your precocious child.

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