CAMBRIDGE SECONDARY ONE SCIENCE

CHAPTER 7: ACIDS AND ALKALIS

7.1 ACIDS AND ALKALIS

Many things contain acid. Foods that contain acid have sour taste. Lemons and limes taste sour because contain citric acid.Common acids in the laboratory are hydrochloric acid, sulfuric acid and nitric acid.

7.1 ACIDS AND ALKALIS

Citrus fruits contain citric acid

7.1 ACIDS AND ALKALIS

Strong acids are corrosive. Acids can be diluted with water to make them less dangerous. Diluted acids are still harmful or irritant. The bottles have hazard warning labels.

7.1 ACIDS AND ALKALIS

Strong acid is labelled corrosive Diluted acid is labelled irritant

7.1 ACIDS AND ALKALIS

Many cleaning products contain alkali. Strong alkalis are corrosive. Common alkalis found in the laboratory are sodium hydroxide, potassium hydroxide and calcium hydroxide. Alkalis can be diluted with water to make them less dangerous.

7.1 ACIDS AND ALKALIS

All these products contain alkalis

7.2 IS IT AN ACID OR AN ALKALI?

An indicator can be used to tell an acid from an alkali because it has different colours in an acid and an alkali.Some plant materials such as red cabbage, blackcurrant and beetroot make good indicators. Litmus can also be used as indicator.

7.2 IS IT AN ACID OR AN ALKALI?

Substance Litmus colour Red cabbage juice Type of substance Hydrochloric

acid Red Red Acid

Sodium hydroxide

Blue Yellow Alkali

Water Purple Purple Neutral Lemon juice Red Red Acid

Calcium hydroxide

Blue Yellow Alkali

7.2 IS IT AN ACID OR AN ALKALI?

Video on red cabbage juice indicator:

7.3 THE pH SCALE

The pH scale measures how acidic or alkaline a substance is. Universal indicator changes to different colours in different pHs.

A colour chart for Universal Indicator showing the pH scale

7.4 NEUTRALISATION

Acids and alkalis can cancel each other out when you mix them together to make a neutral solution. This is called neutralisation. To neutralise an alkali, you must use the right amount of acid. If you add too much acid, it makes an acidic liquid. If you add too little acid, it stays an alkaline liquid.

7.4 NEUTRALISATION

A burette can be used to neutralise an alkali very accurately. Universal indicator is added to the alkali in the conical flask

7.5 NEUTRALISATION IN ACTION

Applications of

neutralisation

Toothpaste - Toothpaste

contains alkali that neutralises acid produced by bacteria that

feed on food pieces left on

your teeth

Neutralising lakes

- Acid rain changes the pH

of the lakes. Some countries

drop alkalis into the lakes to

neutralise the acid

Growing crops- Farmers

spread lime on the soil to

neutralise the acid so that

plants can grow better