Introduction to reactions
The Law of the Conservation of Matter/Mass
In the 18th century, a Frenchman named Antoine Lavoisier stated that in a chemical reaction, matter is neither created nor destroyed. This is the Law of the Conservation of Matter, which forms the basis for balancing equations in chemical reactions. From this law, it can be assumed that if matter is neither created nor destroyed, the products from the reaction have the same mass as the reactants involved, which is why the law is sometimes known as the Law of the Conservation of Mass.
Lavoisier was not the first to conduct a number of experiments to prove this law. A couple of decades before Lavoisier, Mikhail Lomonosov, a Russian scientist, made a similar observation after conducting his experiments. However, Lavoisier was a better- known chemist than Lomonosov, which is why his name is credited most frequently.
What is a chemical reaction?
A chemical reaction is the process by which elements combine and rearrange their components to make new substances. Reactants are the ingredients of a chemical reaction, the elements used to form the new substance. In a chemical reaction, the products of the reaction must be different from the reactant ingredients. See image 1.
Physical change versus chemical change
A physical change occurs when a substance changes form but retains its chemical structure. Physical changes include when a substances dissolves, is crushed or changes state from solid to liquid or from liquid to gas and vice versa. In a physical change, the substance or substances involved remain chemically identifiable and the process is usually reversible.
If salt is dissolved in water, the solution still contains identifiable salt compounds (such as sodium chloride, NaCl), which are separate from the water compounds (H2O). It is possible to reclaim the salt and the water by boiling and condensing the steam, leaving the salt behind. Therefore, dissolving salt in water is a physical change. See animation 1.
A chemical change occurs when substances are altered such that the properties of those substances change, resulting in new products. Some properties affected can include colour, melting point, solubility and physical structure. The atoms rearrange their electron shells when reacting with other atoms, breaking and forming bonds in a process that is difficult to reverse.
From the above example, sodium (Na) and chlorine (Cl) are the reactants. Sodium is a highly reactive metal whereas chlorine is a toxic gas, but together they form a stable compound that is edible - table salt. Sodium loses its single valence electron to chlorine, and both end up with complete outer shells. Because the change involves rearranging electron shells, making the product different from the reactants, the combination of these two elements is therefore a chemical reaction. See animation 2.
