Heat is ______ when limestone is burnt to obtain lime.

Endothermic reaction:

• The change of one or more substances into other substances having different compositions and different properties is called a chemical reaction.
• During a chemical change, energy is either evolved or absorbed.
• The formula of Limestone is ${\mathrm{CaCO}}_3$.
• When Limestone is burnt, it absorbs heat to form quicklime (or lime) and Carbon dioxide.

$\underset{\mathrm{Limestone}}{{\mathrm{CaCO}}_3\left(\mathrm{s}\right)}\stackrel{\mathrm{Heat}}{\to }\underset{\mathrm{quicklime}(\mathrm{Or}\mathrm{lime})}{\mathrm{CaO}\left(\mathrm{s}\right)}+\underset{\mathrm{Carbon}\mathrm{dioxide}}{{\mathrm{CO}}_2\left(\mathrm{g}\right)}$

• Since in the above reaction heat energy is absorbed such reactions are called an endothermic reaction.

Hence, Heat is absorbed when limestone is burnt to obtain lime.