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Direct Union or Combination Reactions

Any reaction in which two or more substances combine to form a single product is a direct union or combination reaction.
  1. Metal + Non-metal
  2. Metal oxide + Non-metal oxide
  3. Non-metal + Non-metal

Corrosion is the process of deterioration of a metal by chemical interaction with their environment, e.g. rusting of iron and tarnishing of copper and silver.

Ways to Prevent Corrosion

  • Painting
  • Galvanising
  • Sacrificial protection
  • Coating with oil, grease or tar
  • Electroplating with non-corroding materials such as Ni, Cr, Zn, etc.
  • Enameling
  • Alloying

Factors Accelerating Corrosion

  • Reactivity of metal
  • Presence of impurities
  • Presence of air, moisture, gases like SO2 and CO2
  • Presence of electrolytes

Decomposition Reactions

Decomposition is the reverse of combination. That is, a single reactant is broken down into two or more products, either into elements or compounds. A decomposition reaction will take place because the compound is unstable or due to heating or electrical decomposition (electrolysis).
For example, 


Displacement Reactions (or Oxidation–Reduction Equations)
A displacement reaction involves an element reacting with a compound whereby one element displaces another element from the compound.
  1. Active metal + Acid
    When a metal which is above hydrogen in the activity series reacts with an acid, hydrogen is liberated and a salt is formed.
  2. Metal + Salt
    Each metal in the activity series displaces any metals below it to form a salt in the solution.
  3. Halogen + Halide salt
    A halogen (F, Cl, Br, I, At) will displace any less active halogen from a halide salt. The order of activity decreases on moving down the halogen family in the periodic table.
The activity series of metals is listed in Table


Table The Activity Series of Metals





























Metathesis or Double Displacement Reactions

A metathesis is a double displacement reaction that usually occurs in aqueous solution.
A reaction in which there is a mutual exchange of acid and basic radicals is called double displacement or double decomposition reaction.


Metathesis reactions are generally classified as precipitation reactions or neutralisation reactions.

  1. Precipitation reactions
    The precipitate is often indicated by an arrow pointing downwards () written next to its compound.
  2. Neutralisation reactions (or acid–base reactions)
    Reaction between an acid and a base
  3. Reaction between a metal oxide and an acid
    When oxides of many metals are added to water, bases are formed.
  4. Reaction between a non-metal oxide and a base

Combustion Reactions

Combustion reactions generally apply to organic compounds such as hydrocarbons, which are used as fuels.
General form:
Hydrocarbon + Oxygen Description: 25434.jpg Carbon dioxide + Water


A chemical process in which a substance reacts with oxygen to give heat is called combustion. The substance that undergoes combustion is said to be combustible. It is also called fuel, e.g. petrol, kerosene. The fuel may be solid, liquid or gas. Air is necessary for combustion to take place.

The lowest temperature at which a substance catches fire is called the ignition temperature. As long as the temperature is less than the ignition temperature of the substance, it will not catch fire. Substances which have very low ignition temperature and can easily catch fire are called inflammable substances.

Types of Combustion

  1. Spontaneous combustion: This type of combustion occurs without an external ignition source. It is usually a slow process that can take several hours, e.g. decomposition or oxidation with heat building up to a point of ignition.
  2. Explosion: It is a type of combustion which releases a large amount of energy. Due to the build-up of huge amounts of gases, an explosion occurs, e.g. bursting of fire crackers.
  3. Slow combustion: It is a type of combustion where very small quantity of heat is liberated, e.g. digestion of food.

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