When this happens, heat energy is released. The heat released by the rusting of the iron causes the mercury in the thermometer to expand and rise. Parent's Note . Chemical reaction is a process in which one substance is chemically converted to another. Chemical reactions include the rusting of iron and the digestion of food.
Most chemical reactions give off heat.
For example, chemical reactions that occur in digestion give off heat that keeps our bodies warm and functioning. Chemists use chemical equations to express what occurs in chemical reactions. Chemical equations consist of chemical formulas and symbols that show the substances involved in chemical changes. The chemical reaction for the rusting of iron shows that four atoms of solid iron react with three molecules of oxygen gas to form two units of solid rust. Experiments demonstrate that iron and oxygen react in these proportions in air at room temperature.