Why is it not to use resolidified sample in melting point determination

It is advisable to use a fresh sample and not a re-solidified one when determining melting point of a substance because when the sample is melted once, it changes its crystal lattice structure to a certain extent. This can cause it to melt at a different temperature the second time around, which will not be an accurate reading.

Explanation:

The temperature at which a substance changes state from its solid form to its liquid form is known as the melting point for that substance. At this temperature, a pure substance will have both its solid as well as liquid states existing in equilibrium with each other.

When a pure, crystalline sample is melted the first time to determine its melting point, its crystal lattice structure gets distorted to a certain extent with the rising temperature. Even if cooled and re-solidified, this original crystalline structure will not melt at the same temperature it did earlier. This is because its crystal lattice is different this time around, leading to an incorrect melting point reading.

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