Why is change in volume of the solution zero in case of an ideal solution?
If you have two component mixture of A and B, most important reason for volume of mixing being zero is that when solution is ideal heterogeneous interactions (A - B) are the same as homogeneous (A - A and B - B). If interactions are the same than volume doesn't change when you mix components. Other reasons which contribute to non - ideality of mixing is size and shape of the molecules. If you mix water with some liquid polymer, you can expect that  due to big difference in size and shape of water and polymer molecules, volumes will not be additive because water molecules being much smaller than polymer molecules can easily fit between them in free volume of polymer liquid. Such packing/fitting of water molecule is quite different than if you just look at water molecules in pure water or if you look at pure polymer since bulky polymer molecules can't just fit easily in free volume of other polymer molecules like water can.