Question

when water unfreezes (melts), what happens to the volume of the sample? it increases. it stays the same. it decreases.

Explanation:

When water freezes, it expands (volume increases). When it melts (unfreezes), the opposite occurs. Based on the property that ice (frozen water) has a lower density than liquid water (due to its crystalline structure in ice which has more space between molecules compared to liquid water), when ice melts to liquid water, the molecules pack more closely. Density formula is \(d=\frac{m}{V}\) (where \(d\) is density, \(m\) is mass, \(V\) is volume). Mass remains constant during phase change (assuming no evaporation etc.). Since density of liquid water (\(d_{liquid}\)) is greater than density of ice (\(d_{ice}\)) and \(m = d\times V\) (so \(V=\frac{m}{d}\)), as \(d\) increases (from ice to liquid water), \(V\) (volume) decreases.

Answer:

It decreases.