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.

Your answers seem great so far, except for a tiny issue: With the ionic symbols, try to get into the habit of using "+", with metals, like sodium, and try to use the integer first. So, for example, a potassium ion would be K^+, while an oxide ion would be O^2-

Let's take aluminium as an example I'll work through:

Aluminium, with it's atomic number of 13, will have an electronic configuration of 1s2 2s2 2p6 1s2 2p1. So it would have 2, 8, 3 electrons in the first three energy levels, respectively.

Usually, if an elemental atom has a valence electron (highest energy level electron) count less than 4, it almost always will lose electrons. Since aluminium has 3, it will also lose the electrons.

It loses the 3 valence electrons, and so will end up with 10 electrons.

Since the atomic number also tells how many protons it has, we know that an aluminium atom has 13 protons, which doesn't change.

Since the size of the charges of a proton and an electron are the same, with protons being positive and the electrons being negative, an aluminium ion would have a charge of +3, and the Ionic symbol would be Al^3+

Hope I helped! xx