Chemical Changes in Chemistry
(i) Combination Reactions : Combination reactions occur when two substances unite to form a third substance.
For example, combining magnesium (Mg) and oxygen (O2) result in the production of magnesium oxide (MgO)
2Mg + O2=> 2MgO
This ereaction can be accomplished by burning magnesium in air, which supplied the oxygen.
(ii) Decomposition Reaction : Decomposition reaction occurs when a single compound breaks down into two or more simpler substances.
In the decomposition of mercuric oxide (HgO), the elements mercury (Hg) and oxygen (O2) are produced.
2HgO=> 2Hg + O2
(iii) Displacement Reaction : When one element replaces another in a compound, it is known as a displacement reaction.
For example, iron (Fe) may displace copper (Cu) in a solution of cupric sulphate (CuSO4).
Fe + CuSO4=> FeSO4 + Cu
(iv) Double Decomposition Reactions : When two compounds interact to form two other compounds, it is known as a double decomposition reaction.
For example, Sodium iodide (Nal) and lead nitrate (Pb(NO3)2) react to form lead iodide (Pbl2) and sodium nitrate (2NaNO3).
2NaI + Pb(NO3)2=> Pbl2 + 2NaNO3
(v) Hydrolysis : Hydrolysis is a double decomposition reaction in which water reacts with a second substance.
When ammonium chloride (NH4Cl) is combined with water (H2O), it undergoes hydrolysis, yielding ammonium hydroxide (NH4OH) and hydrochloric acid (HCl).
NH4Cl + H2O=> NH4OH + HCl
(vi) Neutralization Reactions : Neutralization is the interaction of an acid with an equivalent quantity of a base.
If the process is carried out in an aqueous solution (dissolved in water), the products are water and a salt.
For example, hydrochloric acid (HCl) and sodium hydroxide (NaOH) neutralize each other when dissolved in water, forming sodium chloride (NaCl), a salt, and water (H2O).
HCl + NaOH -> NaCl + H2O
(vii) Substitution Reaction : Substitution reactions occur when an element, such as chlorine (Cl), replaces one or more hydrogen atoms in a hydrocarbon, such as methane (CH4).
CH4 + Cl2=> CH3Cl + HCl
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