© boardworks ltd 2003 what is an acid? here are some facts about acids. strong acids are corrosive...

© Boardworks Ltd 2003

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.

nitratesHNO3nitric

sulfatesH2SO4sulfuric

chloridesHClhydrochloric

Salts formedFormulaAcid


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.


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 alkali

Weak acid Weak alkaliMore alkaliMore acid


Indicators: the pH scale

This 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 alkali

Weak acid Weak alkali

•pH7 is neutral

•pH 1 is strongly acid

•pH14 is strongly alkali


Will it be acidic, basic or neutral and how strong?

1 2 14131211109876543

Substance pH Description of acid / alkali

Soda water 6

Car battery acid 1

Soap 8

Washing soda 10

Stomach acid 2

Oven cleaner 14

Vinegar 4

very weak acid

very strong acid

very weak alkali

weak alkali

strong acid

very strong alkali

weak acid

AlkaliAcid



Bases

Bases are substances that neutralise acids.

Bases are usually:•Metal hydroxides contain OH•Metal oxides contain O•Metal carbonates contain CO3

The following general word equation describes neutralisations:

acid + base a salt + water

In the case of carbonates we also get carbon dioxide.


Alkalis

Bases are substances that neutralise acids.

Alkalis are soluble bases.

Although both can neutralise acids solubility is important when it comes to the pH of solutions. For example, adding sodium hydroxide to water gives a solution with a pH of about 14.

When calcium carbonate is added to water it does not dissolve and so the pH remains close to 7. Even so it can neutralise acid that is added although more slowly than a soluble base might.


Neutralisation - Indigestion

If we have too much acid in our stomachs, we get indigestion.

Acid can move up out of our stomach creating a burning feeling in the chest.

We neutraliseneutralise the excess acid by taking a tablet containing a base.

This is usually a carbonate or an oxide.

Strong soluble bases (like sodium hydroxide) would create too alkaline a solution and cannot be used.

AlkaliAcid 1 2 14131211109876543


Neutralisation - Stings

A bee sting is acidic.

One way to treat a basicbasic wasp’s sting is with an acidacid : vinegar - ethanoic acid.

So one way to treat a an acidicacidic bee sting is to dab on a basebase: bicarbonate of soda more properly known as sodium hydrogen carbonate.

A wasp sting is alkaline.


Neutralisation – Soil pH

Many plants won’t grow well in acid soil and so farmers have to regularly check the pH and adjust it by adding a base.

Plants remove compounds from the soil in a way that tends to leave the soil acidic.

Calcium carbonate or calcium hydroxide are cheap and so are often used for this purpose.


The gases are “scrubbed”, as much as possible, of these acidic oxides by reacting them with a base before releasing them into the air.

Many power stations burn coal containing sulfur.

When this burns it produces acidic sulfur oxides which can cause acid rain.

Calcium oxide or calcium hydroxide are often used for this purpose.

Neutralisation - acid gases


Steps have been taken to reduce emissions of acidic sulfur oxides from power stations and nitrogen oxides from cars.

Even so the atmosphere still contains enough of them to make the rain from industrial areas quite acidic.

Acid rain – living things

Trees and lakes are badly affected in many parts of the world including Northern Germany and Scandinavia which suffers from South-West winds from the UK.

Acid rain damaged tress


Acid rain increases the rate of corrosion of metals.

Acid rain – metals and stone

The metal above the wheel arch of this car is rusting away

It also greatly accelerates the rate of chemical weathering of certain stones used in building such as limestone and marble. (These stones are carbonates. What gas will be given off as they dissolve?)

CO2


Acids and Alkalis

Multiple-Choice Questions


A Sodium bicarbonate

B Vinegar

C Litmus

D Salt

1. Which of these is an indicator?


A Potassium hydroxide

B Sodium chloride

C Sodium hydroxide

D Sodium

2. Which of these is a salt?


A pH 7B pH 1C pH 8D pH 13

3. Which of these is a weak alkali?

A substance with:


A Water - pH 7

B Lemon - pH 3

C Oven cleaner - pH 14

D Bicarbonate of Soda – pH 9

4. Which of these might you put on a ant sting to neutralise the acid in the sting?


A Hydrogen

B Sulfur

C Oxygen

D Nitrogen

5. Which of these do all acids contain?


A Silver nitrate

B Sodium hydroxide

C Calcium hydroxide

D Magnesium hydroxide

6. Which of these substances would not neutralise an acid?

© boardworks ltd 2003 what is an acid? here are some facts about acids. strong acids are corrosive...

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