Chemical reactions
 A chemical reaction occurs when
substances react to form a new substance
with different properties.
 Example: combustion (burning) reactions
Reactants vs. products
 Reactants: substances that are used up
during a reaction; starting substances
 Products: substances that are produced
during a reaction; ending substances
Reactants
Products
Describing chemical
reactions
 Chemists use equations to describe chemical
reactions.
 There are 2 different kinds of equations:
 Word Equations
 Chemical Equations
Word Equations
 Word Equations: names of the reactants
(starting substances) and products (ending
substances) are written out in full.
 Example:
Iron
+
sulfur
reactants

iron(II) sulfide
products
+
energy
Chemical Equations
 Chemical Equations: chemical formulas
of the reactants and products are used.
 Example:
Fe (s) +
S (s)

FeS (s)
+
energy
Similarities between word
and chemical equations
 An arrow indicates the direction in which the
chemical reaction is going. The arrow is read
as “yields,” “forms,” or “produces.”
 Substances to the left of the arrow are called
reactants.
 Substances to the right of the arrow are called
products.
 “+” signs are placed between the reactants or
products if more than one are involved or
formed.
State symbols
Tell us the state, or form, of each substance in
a chemical equation
State Symbol
Meaning
(s)
Solid
(l)
Liquid
(g)
Gaseous
(aq)
Aqueous (dissolved in water)
Example:
 Write the word and chemical equation for the
following:
Magnesium metal burns in oxygen gas with a
bright white light to make a white powder called
magnesium oxide.