Question

Write short notes on the following:
(i) pH value
(ii) Oxidation reaction
(iii) Combustion reaction

Hint:

pH measures acidity or basicity; oxidation involves oxygen addition or electron loss; combustion is a rapid oxidation that releases heat and light.

Answer 1

(i) pH Value:
Step 1: Definition.
The pH value of a solution is a measure of its hydrogen ion concentration. It determines whether the solution is acidic, neutral, or basic.
Step 2: Formula.
\[ \text{pH} = -\log [\text{H}^+] \] where \([H^+]\) is the hydrogen ion concentration in moles per litre.
Step 3: Scale of pH.
- pH<7 → Acidic solution
- pH = 7 → Neutral solution
- pH>7 → Basic (alkaline) solution
Step 4: Example.
The pH of pure water is 7 (neutral), lemon juice has a pH around 2 (acidic), and soap solution has a pH around 9 (basic).
(ii) Oxidation Reaction:
Step 1: Definition.
An oxidation reaction is a chemical process in which a substance gains oxygen, loses hydrogen, or loses electrons.
Step 2: General Examples.
1. Addition of oxygen: \[ 2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO} \] 2. Loss of hydrogen: \[ \text{H}_2\text{S} + \text{Cl}_2 \rightarrow 2\text{HCl} + \text{S} \] Step 3: Conclusion.
Oxidation always involves an increase in oxidation number or the addition of oxygen to a substance.
(iii) Combustion Reaction:
Step 1: Definition.
A combustion reaction is a process in which a substance reacts rapidly with oxygen, releasing heat and light (energy).
Step 2: Example.
When methane burns in oxygen: \[ \text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O} + \text{Energy} \] Observation: The reaction produces flame and heat — an exothermic reaction.
Step 3: Conclusion.
Combustion reactions are always exothermic and involve oxidation of the fuel substance.