Acidic, Basic, and Neutral

Overview

Substances around us can be acidic, basic or neutral. We use indicators to tell them apart by colour change. This chapter explains the properties of acids and bases and how to test them safely.

Key concepts

• Acids taste sour (e.g. lemon, vinegar) and turn blue litmus red.
• Bases taste bitter and feel soapy; they turn red litmus blue.
• Neutral substances (like water) do not change litmus.
• Natural indicators include litmus, turmeric and china rose.
• Neutralisation is the reaction between an acid and a base.

Important terms / formulae

• Indicator: a substance that changes colour in acids or bases.
• Acid: sour substance, turns blue litmus red.
• Base: bitter substance, turns red litmus blue.
• Neutralisation: acid + base gives salt and water.

Solved example or key process

Testing lemon juice: dip a strip of blue litmus paper into lemon juice. It turns red, showing lemon juice is acidic. Dip red litmus into soap solution; it turns blue, showing soap is basic.

Important questions

1. How does litmus behave in acids and bases?
2. Name two natural indicators.
3. What is neutralisation? Give an everyday example.
4. Why should we never taste laboratory chemicals?

Quick revision

Acids are sour and turn blue litmus red; bases are bitter and turn red litmus blue; neutral substances do not change litmus. Indicators help identify them, and acids and bases neutralise each other.