Transformation Of Substances, Revision Notes: ICSE Class 8 CHEMISTRY, Chemistry

Transformation of substances

Changes can broadly be classified into two types − physical and chemical.
- The characteristics of physical and chemical changes

Physical Change	Chemical Change
1. The chemical composition of a substance does not change.	1. The chemical composition of a substance changes.
2. Most changes are reversible.	2. Most changes are irreversible.
3. No new substances are formed. For example, Ice → Water → Steam	3. New substances are formed. For example, Paper → Ashes

Types of reactions

• Combination reaction

Two or more reactants combine to form one single product.

Examples

\begin{matrix} \underset{(Quick lime)}{\underset{Calcium oxide}{CaO (s)}} & + & \underset{Water}{H_{2} O (l)} & \to & \underset{(Slaked lime)}{\underset{Calcium hydroxide}{Ca {(OH)}_{2} (aq)}} \end{matrix} \underset{Carbon}{C (s)} + \underset{Oxygen}{O_{2} (g)} \to \underset{Carbon dioxide}{{CO}_{2} (g)} \underset{Hydrogen}{2 H_{2} (g)} + \underset{Oxygen}{O_{2} (g)} \to \underset{Water}{2 H_{2} O (l)}

• Exothermic reaction – Heat gets released in the reaction. Most combination reactions are exothermic. For example,

\begin{matrix} \underset{(Quick lime)}{\underset{Calcium oxide}{CaO (s)}} & + & \underset{Water}{H_{2} O (l)} & \to & \underset{(Slaked lime)}{\underset{Calcium hydroxide}{Ca {(OH)}_{2} (aq)}} \end{matrix}

• Endothermic reaction – Heat is absorbed in the reaction. Very few …

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