Q. Why does water expand on cooling?
Ans. A water molecule is formed of an oxygen atom that is slightly negatively charged and two hydrogen atoms each of which acquires slight positive charge. In water, different molecules join together forming weak bonds (called hydrogen bonds) between the negatively charged oxygen atom of one molecule and positively charged hydrogen atom of other molecule. As the water cools, these hydrogen bonds adjust themselves to hold the negatively charged oxygen atoms apart and hence to form a more stable crystalline structure. Thus in the solid form of water there are less molecules in the same amount of space than those were in the liquid form, hence giving ice a more expanded structure.