Laws of Chemical Change

Observations of chemical reactions are summarized in certain generalizations called the laws of chemical change.  The three laws of chemical change are;

1)  Law of Conservation of Mass

2)  Law of Constant Composition

3)  Law of Multiple Proportions

Law of Conservation of Mass

• States that during a chemical reaction, matter is neither created or destroyed

Mass of reactants = mass of products

Law of Constant Composition

• The percentage composition by mass in particular compounds never varies
• In other words, in the formation of a compound, the mass of 1 element needed to combine with a given mass of another is always the same
• E.g. 39.37% Na : 60.73% Cl for compound NaCl

Law of Multiple Proportions

• Atoms of 2 elements can combine in more than 1 way

e.g.   elements H & O can form water (H₂O) and hydrogen peroxide                               (H₂O₂)

—both have different physical and chemical properties despite containing the same atoms i.e. H₂O₂ bleaches most dyes while H₂O does not

• When the proportions (by mass) in which 2 elements combine to form 2 different compounds are divided by each other, the result is a fraction composed of small whole numbers

Or in other words…..

• When 2 elements, A and B, form more than one compound, the mass of element A divided by the mass of element B in one compound, compared with mass of A divided by the mass of B in the second compound gives a ratio of small whole numbers