Chemical change is represented by an equation that has the form reactants 
products.
A skill that you must master as you proceed through your study of chemistry is writing an equation for a chemical reaction from a description of the reaction in words.
The total mass of the reactants of a chemical reaction always equals the total mass of the products. This means that the same number and type of atoms present in the reactant substances are also present in the product substances. Equations for chemical change that are consistent with this experimental fact are said to be balanced.
products. A skill that you must master as you proceed through your study of chemistry is writing an equation for a chemical reaction from a description of the reaction in words.
For example a statement may say "Elemental hydrogen reacts with elemental oxygen to give water."
Step 1: Recognize which substances are reactants and which are products. It may help to organize your thoughts to write a word equation.hydrogen + oxygen water
Step 2: Convert the word equation to a symbolic one by writing the appropriate chemical formulae for all of the substances involved. waterH2 + O2 H2O
Note that even this simple example can be challenging because it is necessary to know that elemental hydrogen and elemental oxygen occur as diatomic molecules, and of course you need to know the formula for water (and in this case the name gives you no clues!). Usually the names of the substances give more indication as to the actual composition of the substances.
Step 3: Count atoms on both sides of the equation to decide whether it is necessary to balance the equation by adjusting the coefficients. H2ONote that even this simple example can be challenging because it is necessary to know that elemental hydrogen and elemental oxygen occur as diatomic molecules, and of course you need to know the formula for water (and in this case the name gives you no clues!). Usually the names of the substances give more indication as to the actual composition of the substances.
| H2 | + | O2 | | H2O | |
| atom count | 2 H | 2 O | 2 H 1 O |
The total mass of the reactants of a chemical reaction always equals the total mass of the products. This means that the same number and type of atoms present in the reactant substances are also present in the product substances. Equations for chemical change that are consistent with this experimental fact are said to be balanced.
| Balanced: | 2H2 | + | O2 | | 2H2O |
| atom count | 4 H | 2 O | 4 H 2 O |
A more complete equation may shows the physical state of the reactants and products
.
2H2(g) + O2(g) 2H2O(l)
.
2H2(g) + O2(g)
2H2O(l)