Acid and alkalis

TargetTo know about acid base indicators

Wednesday 3 May 2023

What is an acid?

Here are some facts about acids. Strong acids are corrosive “eating away” at things like metal,

stone and flesh! Weak acids, like lemon juice and vinegar, taste sour. Acids turn litmus red. Acids have a pH below 7. Acids contain hydrogen (but not all things that contain

hydrogen are acids!). Acids can be neutralised with alkalis.

Some common acids

Three acids are particularly common in the laboratory .

These are strong acids that should be treated with the greatest respect.

Acid Formula Salts formed

hydrochloric HCl chlorides

sulphuric H2SO4 sulphates

nitric HNO3 nitrates

Indicators: acid or alkali?

An indicator is a dye which changes colour according to whether it is in an acidic or alkaline solution.

Litmus is available as a liquid that can be added to the solution.

For example, litmus is an indicator that is red in acid and blue in alkali.

It is also available as strips of paper so that you can add a drop of the solution to the paper.

Litmus Test

Indicators: how acid or alkali?

Litmus will tell you whether a solution is acid or alkali: what it won’t tell you is howhow acid or alkali.

Universal indicator has a whole range of colours that tell us how strong an acid or alkali is.

Strong acid Neutral Strong alkaliWeak acid Weak alkali

More alkaliMore acid

Indicators: the pH scaleThis attaches a number called the pH value to each universal indicator colour.

This means we can quickly say how acid or alkali a substance is by quoting a single number.

1 2 14131211109876543

Strong acid Neutral Strong alkaliWeak acid Weak alkali

•pH7 is neutral

•pH 1 is strongly acid

•pH14 is strongly alkali

Indicators: the pH scale

Dimple tileAdd a drop of liquid to be tested – then add a drop of Universal indicator.Write down the colour seen.Say if it is acid, base or neutral.Key:Green = neutralYellow = weak acidRed = strong acidBlue = weak alkaliViolet = strong alkali

Substance Colour with UI Acid, alkali or neutral?

TCP

Lemon juice

Bicarbonate of sodaTalc

Soap

Toothpaste

Acids and alkalis Acids Weak acids have a

sour taste. Acids turn

Universal indicator yellow or red.

Acids have a low pH – less than 7 – the lower the pH the stronger the acid.

Alkalis Alkalis have a

soapy feel – they dissolve grease.

Alkalis turn Universal indicator blue or violet

Alkalis have a high pH – more than 7 – the higher the pH the stronger the alkali

Questions Page 68 and 69

Question – pages 35 to 37 1a = hydrochloric acid b = they have citric acid in them c= sulphuric acid in car batteries d =lactic acid in sour milk e = ant bites have formic acid in them. 2a = acetic, lactic and citric acid in the kitchen b = alkalis dissolve grease so they feel slimy c = alkalis from the Arabic for ash 4. All contain citric acid except vinegar which has

acetic acid.