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INTRODUCTION
Acids
An acid is a substance which furnishes hydrogen ions (H+) when dissolved in water. For example, in its aqueous solution hydrochloric HCl (aq) dissociates as:
HCl (aq)→ H+(aq) + Cl–(aq)
Bases
A base is a substance which furnishes hydroxide ions (OH–) when dissolved in water. For example, sodium hydroxide NaOH (aq), in its aqueous solutions, dissociates as:
NaOH (aq) → Na+(aq) + OH–(aq)
The term ‘alkali’ is often used for water soluble bases.
Indicators
Indicators are substances that show one color in an acidic medium and another color in a basic medium. Such substances are called acid-base indicators.
Litmus is a natural dye found in certain lichens. It was the earliest indicator to be used. It shows red colour in acidic solutions and blue colour in basic solutions. Phenolphthalein and methyl orange are some other indicators. The colours of these indicators in acidic, neutral and basic solutions are given below.
Colours of some indicators in acidic and basic solutions
Indicator |
Colour in acidic solutions |
Colour in neutral solutions |
Colour in basic solutions |
Litmus |
red |
purple |
blue |
Phenolphthalein |
colourless |
colourless |
pink |
Methyl orange |
red |
orange |
yellow |