What is the term for the increase in size or volume of an object as it gains heat?

The correct term for the increase in size or volume of an object as it gains heat is expansion. When materials are heated, their particles move more vigorously due to the increase in energy, causing them to spread apart. This spreading leads to an increase in volume. This principle of thermal expansion applies to solids, liquids, and gases. For instance, metal rails expand in the heat of summer, which is why engineers design them with gaps to accommodate this expansion and prevent bending or warping.

In contrast, contraction refers to the decrease in size or volume when materials lose heat and cool down. Evaporation is a process associated with liquids turning into gas, not size increase. Fertilisation is a biological process related to reproduction, which is unrelated to heat or size changes. Understanding the concept of expansion is vital in various applications, from everyday objects to engineering and construction.