33

Acids andAlkalis

Sour taste

The opposite of an acid

Telling them apart

Cancelling out

you will learnWhat

4

34

Acids and

Alkalis

Sour taste

Do you know?An acid called methanoic acid isproduced naturally by both stingingants and stinging nettles.

4a

In grade 6, you have studied that there are many ways that you could classify andsort things into different groups or categories.

Is there a way you could put these things into two groups?

For thousands of years peoplehave known that vinegar,lemon juice and many otherfoods taste sour. However, itwas not until a few hundredyears ago that it wasdiscovered why these thingstaste sour - because they are allacids. The term acid, in fact,comes from the Latin termacere, which means sour.

Some common examplesof acids, like lemon juiceand vinegar are weakacids. But some of theacids that you use in thelaboratory are strongacids. These includehydrochloric acid andsulphuric acid. Acids arecorrosive.Strong acids can rapidlyburn your skin.

Vinegar and Lemon

Sulphuric acid and hydrochloric acid

35

?

2 Give two different uses of citric acid at home.

3 Vinegar has a sour taste. What is it?

1 Copy and complete the following sentences.Use the words below to fill the gaps.

Lemon juice contains a chemical called ______This has a ________ taste. Stronger acids suchas ________ acid are very __________ and willdissolve some metals.

corrosive sour sulphuric citric acid IdeasK!Acids have a sour taste.

!Strong acids arecorrosive.

!Acids can be both usefuland dangerous.

Sometimes layers ofhard white limescaleform in kettles andaround taps. Acids canbe used to dissolve thislimescale away. Thelimescale fizzes as itdissolves in the acid.

Kettle cleaners use fairlyweak acids such as citricacid. This is strong enough toremove the limescale, but will not harm the metal of thekettle.

Some toilet cleaners use strong acids such as hydrochloricacid. The porcelain toilet bowl is resistant to the acid, so itis not harmed. You have to be very careful not to spill anyacid.

Using acids

36

Acids and

Alkalis The opposite ofan acid4b

Like acids there is another family of chemicalsthat are often used for cleaning. Soap is used toremove dirt from your skin. Detergents are usedto remove grease and dirt from clothes and todo the washing. Washing soda is used in somekitchen and bathroom cleaners.

These chemicals are called alkalis. The wordalkali is Arabic and means the ‘ashes of aplant’. Alkalis used to be made by burningwood and other plants such as sea plants tomake sodium carbonate and land plants tomake potassium carbonate.

Do you know?Concentrated solutions of alkaliare corrosive and can causesevere burns. Because of this,the containers in which they arestored and transported alwayscarry a hazard warning sign.

Strong alkalisSome alkalis, such as sodium hydroxide and potassium hydroxide, are called strong alkalis.Oven cleaners contain strong, corrosive alkali sodium hydroxide. This reacts with the burnt,fatty deposits that form on the oven walls during cooking.

Detergent

washingUP

Liquid

ovencleaner

soap

washingsoda

37

Alkalis feel soapy when rubbed betweenthe fingers. This is because they reactwith the oils in our skin and start todissolve them. Soap is made by boilinganimal fats or vegetable oils with thestrong alkali sodium hydroxide.

Just add waterAcids and alkalis only show their properties when they are dissolved in water.If you sprinkle dry citric acid crystals onto limescale, nothing happens. It onlystarts to fizz if you add water. Alkalis must also dissolve in water before theycan react.

? 1 Why do alkali feel soapy?

2 What do strong acids and strong alkalis have incommon?

3 Why is washing soda dissolved in water before it isused to clean floors?

IdeasK!Alkalis feel soapy.

!Alkalis are used toremove grease and dirt.

!Acids and alkali showtheir properties when theyare dissolved in water.

Soap

38

Acids and

Alkalis

Telling them apart

Litmus is another indicator dye. Litmusturns red when it is an acid and blue whenit is in an alkali. To test a liquid, you diplitmus paper in it.

An acid will turn blue litmus paper red.

An alkali will turn red litmus paper blue.

4c

How can you tell acids and alkalis apart? You can not taste them to see ifthey are sour, because some are poisonous and others would burn you.They can both be corrosive.

Do you know?A very accurate measure of the pHof a solution can be found by usinga pH meter.

The Litmus test

Do you like red cabage? If you eat it raw, it’s a purple-red colour. If you like itpickled, it’s a much brighter red. The dye in the cabbage tells you whether it is anacid or an alkali. Dyes like this are called indicators.

The cabbage test

Notice what happens when you add lemon juice into oneglass and baking soda into the other glass. Acids turncabbage water red and alkalis turn cabbage water green!

Chop some red cabbage and put it into hot water.Let it cool, then pour the liquid into clean glasses.

39

pHew!

IdeasK! Indicators changecolour with acids and alkalis.

!Acids turn blue litmusred.

!Alkalis turn red litmusblue.

!The pH scale is used tomeasure the strength ofacids and alkalis. Lownumbers are strong acids.High numbers are strongalkalis.

!Water is neutral at pH7.

1 Copy and complete the following sentences.Use the words below to fill the gaps.

pH red indicator blue

Indicators change colour in acids or alkalis. Litmus isan __________ which turns ________ in acids andin __________ alkalis. Universal indicator changescolour with the _________ scale.

?

3 Wet cement harms your skin.a. How could you tell if cement is an acid or an

alkali?b. Is it a weak or strong acid or alkali?

Some indicators do morethan show whether asubstance is acidic,alkaline or neutral. Theyalso show how strong theacid or alkali is. The scalethat is used to measurethe strength of acids andalkalis is the pH scale.

The H in pH refers tohydrogen. All acidscontain hydrogen (H+)ions.

pH scale

1 5 7 10 14strong acid weak acid neutral weak alkali strong alkali

2 What colour would litmus turn in:a. lemon juiceb. soap solutionc. vinegard. washing soda solution?

40

Acids and

Alkalis

Cancelling out4d

Sometime we experience a burning feeling in our stomach.The technical term for this condition is acid indigestion.

How can we cure this?

Antacids to the rescue!At the chemist’s you can buy different types of antacids. These help to getrid of some acid in your stomach. ‘Antacid’ means ‘anti-acid’, the oppositeof an acid. When you mix an acid with an alkali they cancel out. They leavea neutral liquid behind. This is called neutralisation.

NeutralisationWhen an acid reacts with an alkali it produces a salt and water.

A bee sting is very painful as it contains an acid.This can be neutralized by an alkali such as soap.

A wasp sting is very painful because it contains analkali. This can be neutralized by an acid such asvinegar.

Do you know?NaOH+ HCI NaCI + H2O

OH-

Na+

OH-CI-

CI-H+

H+ H+

Na-

Na-

Na-

H2OH2O H2O

H2O

H2O

CI-

CI-

Na+

Na+

NaOH HCI NaCI

41

You can make different salts by usingdifferent acids and bases. The pictures showsome of these salts.

Forming salts?

!Acids and bases canceleach other out. This is calledneutralisation.

!Alkalis are bases thatdissolve in water.

!When an acid reacts witha base, a new chemicalcalled a salt is formed.

acid + base= salt + water

IdeasK? 1 What chemicals must you mix to make common

salt?

2 Copy and complete this word equation:acid +__________ = water + _________

3 You could make potassium nitrate by mixingpotassium hydroxide with nitric acid.

a. What other chemical would be formed?

b. Write this reaction as a word equation.

Potassium nitrate is used ingunpowder.

fireworks

Potassium chloride is used inmedicine for diarrhoea.

Calcium sulphate isused in plaster.

child having plaster on

Iron capsules

Iron sulphate is used in ironcapsules. People take theseto help their blood carryoxygen.

Copperchloride isused tocure fishfungus.

42

Acids and

Alkalis

Dmitri Ivanovich Mendeleev was born in Tobolsk, Siberia, onFebruary 7, 1834. The blonde-haired, blue-eyed boy was theyoungest of 14 children born to Maria Dmitrievna Korniliev andIvan Pavlovitch Mendeleev. Maria Korniliev’s family settled inTobolsk in the early 1700’s and introduced paper- and glass-making to Siberia.Unfortunately, Ivan died when Dmitri wasquite young, leaving his wife to support the large family.

Mendeleev is best known for his work on the periodic table.

He arranged the 63 known elements into a Periodic Table basedon Atomic Mass. This was published in Principles of Chemistryin 1869. He predicted the existence and properties of newelements and pointed out accepted atomic weights that were inerror.

Mendeleev anticipated Andrews’ concept (1869) of the criticaltemperature of gases. He also investigated the thermalexpansion of liquids, and studied the nature and origin ofpetroleum. He was considered one of the greatest teachers of histime.

DDDDDMITRIMITRIMITRIMITRIMITRI M M M M MENDELEEVENDELEEVENDELEEVENDELEEVENDELEEV(1(1(1(1(1834-1834-1834-1834-1834-190909090907)7)7)7)7)