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The formula for hydrochloric acid means that it contains hydrogen and chlorine. Nitric acid is composed of hydrogen, nitrogen and oxygen. Sulfuric acid contains hydrogen, sulfur and oxygen.

5.45.4Acids are a family of chemicals which are very important in our lives. They have many uses, from allowing digestion of food and flavouring lollies to cleaning bricks and pools. This Focus is about the properties and uses of this important group of chemicals.

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Concentration and strengthYou can vary the amount of any acid you dissolve in water. The amount of acid in a particular amount of solution is called the concentration. When you make a cup of tea you might dissolve different numbers of spoonfuls of sugar in the tea. As more sugar is added, the tea tastes sweeter. So the concentration of sugar has increased. When a liquid is all acid or has a large amount of acid it is called concentrated acid. When less acid is dissolved in the water the solution is called dilute.

Acids vary in how easily they break up in the solution to form the hydrogen ion. An acid that releases a lot of hydrogen ion is called a strong acid. One which does not release much is called a weak acid. The acids in the table at left are all strong acids. A common weak acid you would have used is in vinegar. It is called ethanoic acid.

I feel sick, MumThe digestive juice in your

stomach contains quite a high concentration of hydrochloric acid. When

you are sick the acid can

burn your oesophagus and

cause ‘heartburn’. It also

tastes sour because acids are sour.

Common acidsThe main feature of the common acids that you use in the laboratory is that they contain hydrogen. This is released into the water solution in a special form called hydrogen ion. Hydrochloric acid, nitric acid and sulfuric acid are the common laboratory acids you will use. They are usually dissolved in water. The chemical formulas for these acids are shown in the table below.

Acid Formula

Hydrochloric acid HCl

Nitric acid HNO3

Sulfuric acid H2SO4

Fig 5.4.1 Acids are common laboratory chemicals.

Fig 5.4.2Strong acids break up more easily than weak acids.

The properties of acidsTasteAcids have many properties. One property you probably know is that they all taste sour. Lemon juice, ‘acid drop’ lollies and vinegar have that sour taste because they contain acid. However, tasting is a very poor way to test if something is an acid. Some acids

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Many substances contain acid and can taste sour if the concentration of acid is high enough.Fig 5.4.3

can burn your skin because they are strong or because they are concentrated and so will release a lot of hydrogen ion into the water. You should never taste something to see if it is an acid.

A common use of the sour taste of acids is in food flavouring. Vinegar is used to give a sour taste. Lemon juice and other acid food additives are often used in foods. Lollies can be made with acid flavours as well. Some acids found naturally in foods are citric acid in citrus fruits, tartaric acid in grapes, lactic acid in milk and malic acid in apples.

Reaction with indicatorsIndicators are special dyes that change colour when acids are in contact with them. Anyone who has a swimming pool would have seen this happen when the water is tested. A common indicator in the science laboratory is called litmus. It comes in a blue or red colour. When acid is touched on blue litmus,

the litmus turns to the red colour. This is called the chemical test for an acid.

There are dozens of other indicators. Another common one is called universal indicator. It will change colour depending on how much acid is in a solution. Indicators like this use a number scale, called pH. The term ‘pH’ means the ‘power of hydrogen’. The scale is numbered from 0 to 14. Anything that is acid has a number less than 7 but more than 0. Water is called neutral and is given the number 7. Anything that has a number from 7 to 14 is called a base. You can see the colour changes and pH for universal indicator in Figure 5.4.5. A common use for this reaction with indicators is in testing swimming pools to adjust their acidity level to reduce growth of algae. An indicator called phenol red is used in swimming pool testing.

Fig 5.4.4 The litmus test for acid

Prac 1 p. 227

Universal indicator colour shows different amounts of acid in a solution. Fig 5.4.5

1 2 3 4 5 6 7 8 9 10 11 12 13 14deep red

red red redorange

greengreen

green blue

blue blueviolet violetyellow

orange

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>>>Reaction with metalsMany metals react with dilute hydrochloric and sulfuric acids to produce hydrogen gas. Prac 2 in Focus 5.3 was an example of the reaction of magnesium. Zinc, iron, aluminium, tin and several other metals also react. Potassium, sodium and calcium react too fast to be tried by school students in a laboratory. An example of a word equation for a metal reacting with acid is:

magnesium + hydrochloric acid !

hydrogen gas +

magnesium chloride solution

Not all metals or acids react this way. Dilute nitric acid does not produce hydrogen with metals. Copper does not react with any dilute acids but it does react with concentrated nitric acid, dilute nitric acid or concentrated sulfuric acid. These do not produce hydrogen, but the dangerous poisonous gases nitric oxide and sulfur dioxide.

Reaction with carbonates and hydrogen carbonatesAcids react with chemicals called carbonates and hydrogen carbonates. An example of a carbonate is calcium carbonate. This is what forms limestone, marble, chalk and seashells. All these reactions ‘fizz’ and produce a gas called carbon dioxide. An example is:

calcium carbonate + hydrochloric

acid !carbon dioxide + water +

calcium chloride solution

Sodium hydrogen carbonate will react with acids as in the following word equation:

sodium hydrogen carbonate

+ hydrochloric acid !

carbon dioxide + water +

sodium chloride solution

The reaction between carbonates or hydrogen carbonates and acids is extremely useful in the cooking of cakes, pastries or bread. A mixture of powdered

tartaric acid (cream of tartar) and baking soda react when they are wet. Baking soda is sodium hydrogen carbonate, also called sodium bicarbonate or ‘bicarb’. This produces carbon dioxide gas, which forms bubbles in the pastry or cake mix and leaves small holes in the cake or bread. This causes it to rise and produces a light ‘fluffy’ cake, bread or pastry. In self-raising flour the acid and the hydrogen carbonate are

already mixed in with the flour—otherwise you have to add the cream of tartar and bicarb soda to the flour. These can be bought premixed as baking powder.

Carbonates are also often used in antacid stomach powders and liquids to ease ‘upset stomachs’ if you have ‘heartburn’. Antacids often contain carbonates, which react with excess hydrochloric acid being produced in your stomach. This eases the burning pain you feel if you burp. When you do burp, some of the gas is the carbon dioxide, which is produced from the antacid reaction.

Crumbling cavesLimestone caves are found

in many areas of Australia.

These are formed by acid in rain and groundwater

seeping down from above and reacting with the calcium carbonate.

One way of showing that carbon dioxide gas is given off in this reaction is to bubble the gas through a limewater solution. Limewater is calcium hydroxide. When carbon dioxide mixes with limewater it turns a white colour due to solid calcium carbonate forming in the liquid. This is a chemical test for carbon dioxide gas. You can see how it is done in Figure 5.4.7.

Prac 2 p. 228

Scones made with bicarb and tartaric acid (top) and scones made without these (bottom)Fig 5.4.6

Fig 5.4.7 Testing a gas given off to see if it is carbon dioxide

Properties and uses of acidsProperties and uses of acids

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Reaction with basesAnother important property of acids is that they react with bases. Bases are substances that have a pH above 7. Most bases are metal hydroxides or metal oxides, like sodium hydroxide or calcium oxide. You will study this reaction in Focus 5.5.

Homework book 5.4 pH levels of common drinks

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[ Questions ]Use your book

Common acids 1 What element is found in all the common acids?

Concentration and strength 2 What do we mean by concentrated acid and dilute acid? 3 What do we mean by a weak acid and a strong acid?

Taste 4 What can we say is common to the taste of all acids?

Indicators 5 Give two examples of indicators and explain how we

use each one to tell if something is an acid.

Reaction with metals 6 Describe what the word equation on page 226 (top left)

tells you about hydrochloric acid and magnesium. 7 What is different about how copper and nitric acid

react compared with how magnesium and hydrochloric acid react?

Reaction with carbonates and hydrogen carbonates 8 How is the reaction between acids and hydrogen

carbonates useful in cooking? 9 How are hydrogen carbonates and carbonates useful in

antacid powders and liquids?

Use your head10 Write a word equation for the reaction of zinc and

hydrochloric acid. 11 One product of the baking of cakes with self-raising

flour is sodium tartrate. Write the word equation that explains why the flour rises.

12 a Why can hydrochloric acid be used to clean oxide from copper without damaging the metal?

b Why would nitric acid not be a good choice for this job?

13 Milk does not taste sour and yet it contains an acid called lactic acid. Yoghurt is formed from milk when bacteria called lactobacillus feed on the sugar called lactose in the milk. Give some possible reasons (hypotheses) why milk does not taste sour, but yoghurt does.

14 Car batteries contain acid. If you splashed some on your skin, you could wash it off with water. Explain what other common household chemical you could add to the water that would probably work better than water to remove the acid.

Investigating questions15 Research how the pH of swimming pools is checked

by home owners, and what is done to adjust the pH if it is incorrect.

16 With a parent’s or guardian’s permission, look around your home for products that contain acid, for example any labelled food packets, medicines, skin and hair care products, cleaning products, or garden chemicals. Write down the name of the product, the name of the acid and the purpose for which you use that product. Present your findings in a table. Try to find about ten different products.

17 Car batteries contain an acid. Find out the name of this acid and describe its function in the battery.

18 Investigate the action of dilute sulfuric and hydrochloric acid on magnesium, zinc and copper. You will need to be able to test any gas formed. DYO

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Prac 1 Focus 5.4

Testing pH with indicatorsPurposeTo use some indicators to test the pH of a variety of substances.

RequirementsSamples of lemon juice, shampoo, milk, toothpaste, lemonade, detergent, soap, distilled water; dropper bottles of 0.1 M HCl and 0.1 M NaOH; white depression tiles (like for blood typing) or ten test tubes and test tube rack; dropper bottles; universal indicator; paper towel; phenol red and phenolphthalein; red and blue litmus indicator paper. >>

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