Name two substances which contract on heating.

Hint:Here, you are asked to find substances that when heated, do not expand but contract and name any two substances. This is an unusual behavior that when substances are heated, they actually contract. The reason behind this is the value of coefficient of expansion. The coefficient of expansion of some substances is negative for a certain range of temperature. So, for that particular range of temperature, the substance will actually contract on being heated up.

Complete answer:
Water usually behaves as other compounds, that is, it expands when heated. But it is observed that for a certain range of temperature, that is from 0 degree Celsius to 4 degree Celsius, it actually contracts. When ice is taken and it is heated, the ice molecules gain kinetic energy and the ice expands till it starts melting. Once it is melted, that is, the temperature reaches 0 degree Celsius, expansion stops.

Now, if further the temperature is increased, the density also starts to increase and eventually it contracts until maximum density is reached, that is 4 degree Celsius. The coefficient of expansion is negative for this range of temperature and hence, water contracts on heating. So, one substance will be water. The other substance which exhibits the same behavior is the silver iodide.

Hence, two substances which contract on heating are water and silver iodide.

Note:You need to remember the property that water exhibits in the temperature range of 0-4 degree Celsius. Also, the contraction happens due to unique crystal structure and this property is known to be negative thermal expansion. Remember that the coefficient of thermal expansion gives you the information that whether the substance will expand or contract within a range of temperature by its sign.