Question

Why is sand insoluble but sugar soluble as the atoms present in the water have gap between them then any substance can fill those gaps .

Answer 1

The solubility of any substance in water is depending on two factors

1. The substance has to be broken into very small size particles so that it can sit in between the gaps of water.
2. There should be bond between water and the substance so that water can hold it in gaps and not allowing it to settle.

In our case sand cannot be broken into very small invisible particles but sugar can be broken in to very small size particles by water. The second reason is there will be bond between sugar and water but there is no bonding between sand and water.
So sugar will dissolve but sand will not dissolve in water.