When solid calcium chloride (chemical formula CaCl?) is placed in water, the calcium chloride dissolves and liberates heat in the process. Calcium chloride is consequently one of the ingredients in instant “hot packs” sold in retail stores. Chemistry instructors demonstrate the dissolution of calcium chloride as an illustrative example of an exothermic (heat-producing) reaction, and chemistry students run experiments to measure the amount of heat produced.
Calcium chloride is a water-soluble ionic compound. Consequently, when it is mixed with water, it undergoes a process known as “dissociation,” in which it separates into calcium ions (Ca²?) and chloride ions (Cl?).
Chemical processes either release heat into or absorb heat from their surroundings; these processes are referred to as being either “exothermic” or “endothermic,” respectively. Heat is measured in units of joules (J).
Calcium Chloride + Water
The dissolution of calcium chloride in water releases 81,350 joules per mole of calcium chloride. Moles of a substance are calculated by dividing by the compound’s formula weight (which is 110.98g/mol for calcium chloride). Therefore, dissolving 1 gram of calcium chloride would release
1 g / (110.98g/mole) = 0.0090 moles calcium chloride
(0.0090 moles) x (81,350 J/mole) = 733 J of heat
Determining what 733 J of heat actually mean in terms of temperatures requires consideration of water’s specific heat capacity, which is 4.184 joules per gram degree Celsius (J/g°C). Therefore, 4.184 joules of heat will increase the temperature of 1 gram of water by 1 degree Celsius. Dissolving 1g of calcium chloride in, for example, 100g of water would increase the temperature of the water by
733 J / (100g x 4.184 J/g°C) = 1.8 °C
In addition to its use in instant heat packs, calcium chloride is widely used as a desiccant (moisture-absorber) because it is capable of absorbing moisture from the air. Some concrete mixes incorporate calcium chloride to decrease drying time.
Calcium chloride is also used as a de-icer. Like sodium chloride (table salt), calcium chloride will decrease the freezing point of water. Thus, ice, which would normally be solid at 0 degrees Celsius, will convert to liquid form if calcium chloride is dissolved in it.