Class 10 Science


Strength of Acid and Base

These notes are based on the chapter Acids Bases and Salts from NCERT book based on CBSE syllabus.

  • Complete dissociation of acid and base
  • Incomplete dissociation of acid and base
  • pH scale

Strength of acid shows the degree of dissociation of hydrogen ions. Similarly, strength of base shows the degree of dissociation of hydroxide itons.

Acids in which complete dissociation of hydrogen ion takes place are called strong acid. Similarly, bases in which complete dissociation of hydroxide ion takes place are called strong base.

Strong Vs Weak Acid

Mineral acids are strong acids but organic acids are weak acids. Why?

In mineral acids, such as hydrochloric acid, sulphuric acid, nitric acid, etc. hydrogen ion dissociates completely and hence they are considered as strong acid. Since, in organic acids hydrogen ions do not dissociate completely, so they are weak acid.

Alkalis are water soluble base, thus in alkali, complete dissociation of hydroxide ions takes place and they are considered as strong base.

The complete dissociation of hydrogen ions or hydroxide ions is shown by a single arrow. The incomplete dissociation of hydrogen ions or hydroxide ions is denoted by double arrow.

Example of complete dissociation:

NaOH (aq) ⇨ Na+ + OH

HCl ⇨ H+ + Cl

Example of incomplete dissociation:

Mg(OH)2 >> Mg+ + + OH− −

CH3COOH >> CH3COO + H+

Although acetic acid being an organic acid is a weak acid, but concentrated acetic acid is corrosive and can damage the skin if poured over it.

pH Scale

The strength of acid or base depends upon the hydrogen ion concentration. If the concentration of hydrogen ion is greater than hydroxide ion, the solution is called acidic. If the concentration of hydrogen ion is smaller than the hydroxide ion, the solution is called basic. If the concentration of hydrogen ion is equal to the concentration of hydroxide ion, the solution is called neutral solution.

pH is a scale which quantifies the concentration of hydrogen ion in a solution. The range of pH scale is between 0 to 14.

The pH value decreases with increase in hydrogen ion concentration.

  • If the value of pH is 0, this shows maximum hydrogen ion concentration.
  • pH value equal to 14 shows lowest hydrogen ion concentration.
  • pH value equal to 7 shows the hydrogen ion concentration is equal to hydroxide ion concentration.

A neutral solution, such as distilled water has value of hydrogen ion concentration equal to 7 on pH scale. The acidic solution has value of hydrogen ion concentration less than 7 on pH scale. The basic solution has value of hydrogen ion concentration greater than 7 on pH scale.

In pH scale p stands for potenz. Potenz is a German word which means power or potential. Here, H stands for hydrogen ion. Thus, pH means the potential of hydrogen or power of hydrogen.

pH is defined as the decimal logarithm of the reciprocal of the hydrogen ion activity (a(H+ )), in a solution.

ph logarithm formula
ph scale

pH of Common Substances

  • Apples – 3 (Acidic)
  • Bananas – 4 (Acidic)
  • Butter – 6 (Acidic)
  • Water – 7 (Neutral)
  • Borax – 9 (Basic)
  • Lime water – 12 (Basic)
  • Milk – 6 (Acidic)
  • Vinegar – 2 (Acidic)

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