Acids, Bases and Salts · Lesson 4 of 12
How Do Acids And Bases React With Each Other?
“Acid meets base, both calm down, and salt and water take the credit.”
• Explain neutralisation using an acid-base reaction. • Interpret phenolphthalein colour changes during neutralisation. • Write the general form of a neutralisation reaction. • Explain why metallic oxides are basic in nature. • Explain why non-metallic oxides can show acidic nature.
Acids and bases have opposite chemical effects. When suitable amounts of an acid and a base are mixed, these effects can cancel each other. The products are usually a salt and water.
Neutralisation
Sodium hydroxide solution becomes pink when phenolphthalein is added because phenolphthalein is pink in a basic medium. When dilute hydrochloric acid is added drop by drop, the basicity decreases. At the point where the basic effect is neutralised, the pink colour disappears. Adding sodium hydroxide again restores a basic medium and the pink colour reappears.
A Neutralisation Reaction is a reaction in which an acid and a base react to form a salt and water.
Reaction Of Metallic Oxides With Acids
Copper oxide is a black solid. When dilute hydrochloric acid is added while stirring, the copper oxide gradually dissolves and a blue-green solution forms. The colour is due to copper chloride.
A Basic Oxide is an oxide that reacts with an acid to form salt and water, showing behaviour similar to a base.
Reaction Of A Non-Metallic Oxide With Base
Carbon dioxide is a non-metallic oxide. It reacts with calcium hydroxide, a base, to form calcium carbonate and water. Because its reaction with a base resembles the reaction of an acid with a base, carbon dioxide behaves as an acidic oxide.
An Acidic Oxide is an oxide that reacts with a base to form salt and water, showing behaviour similar to an acid.
| Reaction | Pattern | Interpretation |
|---|---|---|
| Acid + Base | Salt + Water | Neutralisation |
| Metal oxide + Acid | Salt + Water | Metallic oxide behaves as basic |
| Non-metallic oxide + Base | Salt + Water | Non-metallic oxide behaves as acidic |
Problem
Balance CuO + HCl → CuCl₂ + H₂O.
- 1.Copper is one on each side.
- 2.Oxygen is one on each side.
- 3.CuCl₂ contains two chlorine atoms.
- 4.Place a coefficient of two before HCl.
- 5.The two hydrogen atoms now form one H₂O molecule.
- 6.The balanced equation is CuO + 2HCl → CuCl₂ + H₂O.
Quiz
What are the usual products of a neutralisation reaction between an acid and a base?
What colour is phenolphthalein in sodium hydroxide solution?
Which products form when copper oxide reacts with hydrochloric acid?
Why is carbon dioxide described as an acidic oxide?
Practice Problems
- Write the balanced equation for sodium hydroxide reacting with hydrochloric acid.
- Explain why phenolphthalein loses its pink colour when hydrochloric acid is added to sodium hydroxide solution.
- Balance CuO + HCl → CuCl₂ + H₂O.
- Explain how the reaction of copper oxide with hydrochloric acid shows that copper oxide is basic in nature.
- Explain why carbon dioxide is described as an acidic oxide when it reacts with calcium hydroxide.
Key Takeaways
• An acid and a base react during neutralisation to form salt and water. • Phenolphthalein can show the changing basicity of a solution during neutralisation. • Metallic oxides react with acids and therefore generally show basic behaviour. • Non-metallic oxides such as carbon dioxide can react with bases and show acidic behaviour. • Neutralisation can be understood as the combination of hydrogen ions and hydroxide ions.