A chemical reaction can be shown to be endothermic if it absorbs heat and causes a decrease in the temperature of its surroundings. Illustrative lab experiments performed in schools to demonstrate endothermic reactions often use the reaction occurring between vinegar and baking soda or the reaction between barium hydroxide and ammonium nitrate. The latter reaction, if taking place in a flask resting in a small amount of water, will absorb enough heat from the water to cause it to freeze.
The baking soda and vinegar reaction will cause a temperature decrease of about 10 degrees Fahrenheit, which is a less dramatic illustration than the water freezing in the barium hydroxide and ammonium nitrate reaction, but it can still be measured with a thermometer. Although requiring significantly different amounts, both of the illustrative endothermic reactions require an input of energy, which is absorbed in the form of heat.
Chemical reactions require the bonds in the reactants to be broken and the bonds in the products to be formed. Endothermic reactions are those which require a greater amount of energy to break the bonds of the reactants than the amount of energy required to form the bonds of the reaction's products. Energy, therefore, needs to be pulled into the reaction so that it can take place. The absorption of energy required to break the bonds of the reactants pulls heat away from the environment and results in a temperature decrease. This is the opposite of what occurs in an exothermic reaction. In an exothermic reaction, less energy is needed to form the bonds of the products than the amount released when the bonds of the reactants are broken. In the case of an exothermic reaction, the excess energy is released in the form of heat.