Chemical Reactions
A chemical reaction is the process by which one or more substances change
into one or more new substances whose chemical and physical properties
differ from those of the original substances. In any chemical reaction, the
original substances, which can be elements or compounds, are known as
reactants. The substances created are called products. A common example
of a chemical reaction is shown in Figure 1.
It’s not always easy to tell that a chemical change is happening, but there are
some signs to look for, which are summarized in table 1. For example,
certain signs indicate that wood burning in a campfire is undergoing a
chemical change. Smoke rises from the wood, and a hissing sound is
made. Energy that lights up the campsite and warms the air around the
fire is released. The surface of the wood changes color as the wood
burns. Eventually, all that remains of the firewood is a gray, powdery
ash.
Figure 1. Chemical changes
occur as wood burns. Two
products formed are carbon
dioxide and water.
Table 1. Evidence of Chemical Change
Changes in Energy
Release of energy as heat
Release of energy as light
Production of sound
Change in temperature
Absorption or release of electrical
energy
Formation of a New Substance
Formation of a gas
Formation of a precipitate
(an insoluble solid)
Change in color
Change in odor
Sometimes, the evidence for a chemical change is indirect. When you place a new battery in a flashlight,
you don’t see any changes in the battery. However, when you turn the flashlight on, electrical energy
causes the filament in the bulb to heat up and emit light. This release of electrical energy is a clue that a
chemical reaction is taking place in the battery.
All chemical reactions involve certain changes in the substances and energy. There are basic six types of
chemical reactions. Here is some interesting information about the types of chemical reactions.
During any chemical reaction, there is a conversion of the reactants into a single or many products. A
reactant means a substance or substances that are involved in a chemical reaction. The chemical reactions
occur under the appropriate conditions of pressure and temperature in the presence of a catalyst. The
catalyst plays a significant role in increasing the rate of a chemical reaction without actually getting
involved in that reaction.
Types of chemical reactions are characterized by the type of chemical changes. Any chemical reaction
yields a single or more products, which are quite different from the reactants. The chemical reactions
include some changes that involve the motion of electrons during the formation and breakage of chemical
bonds. The chemical reactions could be written in a symbolic form. Chemical equations are used to
describe a chemical transformation of elementary particles, which takes place during the reaction. The
chemical reactions involve a change in energy; either released or absorbed. Chemical reactions are
described as exothermic reactions (in which energy is released) or endothermic reactions (in which
energy is absorbed).
Types of Chemical Reactions
There are 6 basic types of chemical reactions such as synthesis reaction, decomposition reaction, single
replacement reaction, double replacement reaction, combustion reaction and acid-base reaction.
Synthesis Reaction: In this reaction, two or more simple substances (reactants) combine together to
yield a more complex substance. For example, hydrogen gas combines with oxygen to form a more
complex product, water. The chemical equation of this reaction looks as follows:

2 H2 + O2  2 H2O

Decomposition Reaction: In this type of chemical reaction, a complex substance breaks down into
some simple substances. A single reactant produces two or more products. For example, a water
molecule can be broken down into hydrogen and oxygen. This chemical reaction can be presented as:
2 H2O  2 H2+ O2

Single Replacement or Displacement Reaction: In this kind of reaction, a less active element is
replaced by a more active element present in a compound. Two reactants produce two products. For
example, when zinc reacts with hydrochloric acid, hydrogen molecule is replaced by zinc to form
ZnCl2. This reaction looks like:
Zn + 2 HCl  ZnCl2 + H2

Double Replacement or Displacement Reaction: In this chemical reaction, the cations and anions of
two different substances switch their places to yield two totally different compounds. For example,
when silver nitrate reacts with sodium chloride, sodium and silver switch their places and there is a
formation of sodium nitrate and silver chloride. The chemical equation of this reaction is:
AgNO3 + NaCl  AgCl + NaNO3

Combustion: During this chemical reaction, a hydrocarbon is burnt in the presence of oxygen to form
carbon dioxide (in complete combustion), or carbon monoxide (in partial combustion due to a limited
amount of oxygen). When reading a description of a combustion reaction, you will typically see the
phrase “…burns in air….” This reaction can be presented as:
C10H8 + 12 O2  10 CO2 + 4 H2O

Acid-base Reaction: It is a special type of double displacement reaction, which is characterized by the
reaction between an acid and a base. In this chemical reaction, H+ ion in the acid reacts with OH- ion
present in the base, leading to formation of water. Generally, the end products of this reaction are
water and some ionic salts. For example, hydrobromic acid reacts with sodium hydroxide to yield
water and NaBr. The equation of this chemical reaction is:
HBr + NaOH  NaBr + H2O
Other types of chemical reactions include organic reactions, which involve the compounds having carbon
as the major element. Some basic types of organic chemical reactions are addition reaction, redox
reaction, elimination reaction, rearrangement reaction, pericyclic reaction and substitution reaction.