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Types of chemical reactions

Grade 8 Science Worksheets

Chemical Reactions

Chemical reactions are occurring all around us. They occur naturally, or they experiment within chemistry laboratories, or they are used in the manufacturing and chemical industries. They occur when one or more substances (Reactants) chemically convert to one or more other substances (Products). The physical and chemical properties of the products are different from those of the reactants. Chemical reactions are represented by chemical equations. (See also: Basics of Chemical Reactions).


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Some examples with their accompanying chemical equations –


  1. Rusting, where iron combines with oxygen in the presence of water to form a red-colored oxide, is a naturally occurring process.

4Fe + 3O2 → 2Fe2O3

  1. Tarnishing of copper pennies because of exposure to air

4Cu + O2→ 2Cu2O

  1. Combustion or burning process, where a fuel such as propane, a cooking gas, reacts with oxygen at high temperature to release heat and light energy

C3H8 + 5O2 → 3CO2 + 4H2O


  1. An Acid-Base reaction, such as when vinegar (acetic acid) and baking soda (sodium bicarbonate) combine to form a bubbling mixture of sodium acetate with the release of carbon dioxide gas

NaHCO3     +          CH3COOH →   CH3COONa + H2O + CO2

(Sodium bicarbonate) +    (acetic acid)             (sodium acetate)

  1. An Acid-Metal reaction, which dissolves the metal to form a salt, such as when magnesium is dissolved in hydrochloric acid to produce magnesium chloride, a salt

Mg + 2HCl → MgCl2 + H2

Evidence of Chemical Reaction

Rusted Gate
Rusted Teapot

Rusted Gate and Rusted Teapot

Chemical reactions usually produce telltale signs that a change has occurred.

The most common signs are –

  1. Change in color – the Statue of Liberty has turned green from its original copper color due to oxidation
  2. Change in temperature – adding calcium chloride to water heats the solution due to the formation of hydrochloric acid
  3. Release of an odor – a rotting egg or burning wood both produce odor
  4. Produce bubbles (formation of gas) – adding dry yeast to hydrogen peroxide solution produces bubbling and fizzing due to formation of oxygen gas
  5. Precipitate in a liquid (formation of a solid) – calcium chloride and sodium carbonate solutions when mixed produce calcium carbonate, a precipitate (solid)

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Exothermic and Endothermic Reactions

Most chemical reactions involve the release of energy in the form of heat or light. Such reactions are called Exothermic Reactions. Some reactions, however, involve absorption of energy (heat) and these are called Endothermic Reactions.


Some examples –


  1. Any combustion reaction such as lighting a candle or a firecracker is exothermic
  2. Dissolving a salt in water, such as ammonium chloride, potassium chloride or ammonium nitrate, is endothermic
  3. A Neutralization reaction between an acid and a base to form water and a salt is exothermic
  4. Formation of nitric oxide from the reaction between nitrogen and oxygen is endothermic
  5. Thermite or welding process to join iron pieces is exothermic

Oxidation and Reduction Reactions


Oxidation and reduction reactions are those in which there is a transfer of electrons between the participating reactants and products. In an Oxidation reaction, a substance loses electrons and forms positive ions. In a Reduction reaction, a substance gains electrons and forms negative ions. The reduction is the reverse process of oxidation. The degree of oxidation of an atom in a chemical compound is referred to as its Oxidation State or Number.

Oxidation is always accompanied by reduction because the electrons released by one substance during oxidation must be captured by another substance during reduction. The combined transfer of electrons is referred to as a Redox Reaction.


For example, heating iron and sulfur together causes each iron atom to lose two electrons and become an Iron Ion, which is picked up by the sulfur atom to create a Sulfide Ion. The sulfide ions and iron ions combine to form iron sulfide, a compound.


Fe + S       →        FeS

(iron) (sulfur)                  (Iron sulfide)

Redox reactions often include the gain or loss of oxygen, such as when a metal reacts with oxygen to form an Oxide. For example, magnesium metal combines with oxygen to form magnesium oxide. Its reverse, in which magnesium oxide reacts with carbon at very high temperature to form magnesium metal and carbon monoxide, is a reduction reaction.


2Mg + O2 → 2MgO (oxidation reaction)

MgO + C → Mg + CO (reduction reaction)

In a reaction between lead oxide and ammonia gas, lead oxide is reduced to the metal lead, while ammonia is oxidized to nitrogen gas.

3PbO + 2NH3 → N2+ 3H2O+ 3Pb

Other redox examples include rusting, combustion, photosynthesis, and decomposition. 

Combination and Decomposition Reactions


A Combination Reaction is one in which the reactants combine to form a product. For example, carbon, when burned in the presence of oxygen, produces carbon dioxide.

                Δ (heat)

C + O2 → CO2

A Decomposition Reaction is one in which a reactant breaks down or Dissociates to form two or more products. The hydrolysis of water is a common example.


                        2H2O → O2 + 2H2

Check Point

  1. Heat or light is released in the form of energy in an _______ reaction.
  2. A reaction between acid and base to form salt and water is called a _______ reaction.
  3. A reaction in which a substance loses electrons is termed as _______ reaction.
  4. A combination of oxidation and reduction reactions is called a _______ reaction.
  5. A reaction in which a reactant breaks down to form two or more products is called a ______ reaction.

Answer Key

  1. Exothermic
  2. Neutralization
  3. Oxidation
  4. Redox
  5. Decomposition

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