The Dead Sea contains so much saltthat salt crystals build up along itsshores. Is there a limit to how muchsalt can dissolve in a given amountof water?

Learning Outcomes, . ,

III this chapter, you' ....;ililearn:. .• to distinguish. aflU)flg ~q..tl!t~..:~~Il1/en/.and s,'olFti(('r; ~• to distinguish between a saii.tJipted solution dild'cifi

unsaturated solution; ':"'~ .• - """~y'"

to explain. wluu is ir.te.artt.~yq 'c?n,(:el1/!",(ed,soJ~IJ:QP anda dilute solution; .. . "',{:'0. ;..,. ',. \ ~~,_-':: 1-' .:: .-:" '

• to recognise that lhe'amount pf s()hi[t:'~l-.~hicf~,dt:s$ol~esina given sobient. h(1:\B&l,;,uRPel:.ii~2it;' ..:'u>,." .~...~'~~:,,'..

• ftbout .fh~'"f{lt:U.,i··,H'VJit0.Ji:(l/J:~c~th~~:olul;.iliI5;' (>-f'i,;-§Ql1tle'"itl a solve~t'(llut recilgn_is'etlt'(!jmpor.uiiiJ.;e of th~si~,

: .j~~"tOr,!ri!'!1.0me.s()J~d:h4uJtrie~:~ ,,: ". ':~ ...,.,>, ,

". : dlttffit the'properties (Jj·a;;i(1jc'~t1.1dalki~li5utsdJuti(.)ji.'L;.to explain that indicators cihj·substances·.I1Jhid(c.Il8.ngecolour when an acid-or a/,J/qt~is add.ed'to. ihem;~ .,'

• to investigate t~e effec: (~,p\1~~tie6;:4~4S!:diii,/;t;K.ilifi~and neutral solutions o'?- 71,iike:r'sallndif!afor pai)-er.andnauiral tndicators; . ,",~-.C::.' .\~:; . :~.:",t{i.: [,'LVecitig~i;rr.ii!t.qt~t,~f::qi1;~{(~~~f~S:C;l~~.((iiCa(?~~f!f.~i'when acidic qna aJ10/!ne sot(J{I(1!!s·are,ml:i'K.;ed. d~~"",

" ,~..

5.1 Solutes, Solvents andSo utions

Sugar dissolves in water to form sugar solution. The substancehich dissolves is called the solute and the substance which the

solute dissolves in is called the solvent. The solvent forms theculk or the solution. In the case of sugar solution, sugar is thesolute while water is the solvent.

A solution is a mixture formed when one 01' more solutesc..issolvein a solvent.

Wale.r is a common solvent. A solution in which water is thesolvent is known as an aqueous solution. Salt solution, sugarsolution and mineral water are all aqueous solutions. Can you.=-']\ e examples of others? Not all substance, can dissolve in water.For example. super glue dissolves in the solvent acetone...nd some types of paint dissolve in turpentine.

=, drink is madegar, colourings,

- ..rings and carbonae dissolved in water.

Tincture of iodine isan antiseptic madeby dissolving iodinein alcohol.

Air consists of oxygen, carbon dioxide andnoble gt1ses dissolved in nitrogen.

Fig- 5.1 Can you identify the solutets) and solvent in each of the above solutions?

ature of solutions. ~t happens to the sugar when we add 5 g of sugar to 100 g of..:.er'? Will the mass of the sugar solution be less than, equal tomore than 105 g (5 g + 100 g)?

Plan and carry out an investigation to find out whether the mass of a solution is equal to themasses of its constituents added together. In your investigation, find out whether:a. the mass of a glass stopper and a beaker of water is equal to the mass when the glass stopper

is placed inside the beaker of water;

Mass of Mass of = Mass of+

b. the mClSS of a small piece of wood and a beaker of water is equal to the mass when the woodis floating in the beaker of water;

c. the mass of powdered chalk and a beaker ot water is equal to the mass of the beaker ofwater mixed with the powdered chalk;

d. the mass of suqar and a beaker of water is equal to the mass of the beaker of water mixedwith the sugar.

From your investigation, did you find out that the mass of asolution j<; equal to the total mass of the-solute and the solvent?

Mass of solution = Mass of solvent + Mass of solute

When a solute dissolves in a solvent, it does not disappear. Ttremains inside the solvent and no mass is lost. Hence, mass isconserved .

....;/( 'Try It Out

,~ ... "'"T'IlI& .....

a. Predict whether the volume is conserved when a solutedissolves in a solvent (that is, is the volume of the solution =volume of solute + volume of solvent?).

b. Explain your prediction.c. How can you put your prediction to the test?d. Test your prediction and make observations. Is your

prediction correct?

96

When a solute dissolves ill a solvent, the solute breaks apart intosmaller particles that mix completely and evenly with the paniclesof the solvent.

,/'Oo'?-.i Cj ~, .----Q~Q~ ... ~ Q G" °r;;;;O @ ~ '0 GOO~I'" . .', Q! e .-..QgoO°Q;Qf)" O() oaQ. 0 00 oOoi,g" <)

Q Q O@Q~~ Q _.K .. 0 o OQ10 ,,~O Q'o<) " 0

SolutionI 0 <)

Q . 0 0 CD 0 Q g 0I Q \ Soll¢t& . o e o!o 0, 0

{i> (i'; 0 0 ; .0 <)

Solvent bre~ks Q ! 0\~,..__Solvent 0/ G 0°. .d'1wnJolute <) ~Q

',GQ Q "",/.- 0

'-~"""- Q -- ~-_Q~_"'" ~~,

Fig. 5.2 Whut happens when a solute dissolves in a solvent

This explains why a solution is homogeneous, that is, its colour,density, appearance and other physical and chemical propertiesare the same in every part of the solution.

Uses of solvents and solutionsThe water we drink and the air we breathe are examples of solventsand solutions which keep us alive. Plants absorb mineral saltswhich are dissolved in water through their roots (Fig. 5.3).'

Most substances react more readily in their solution form than intheir solid form.

When copper sulphate solution isadded to sodium hydroxide solution,

a cloudy suspension of pale blue copperhydroxide forms almost immediately.

Copper sulphate crystalsand sodium hydroxide

pellets do not react witheach other when dry.

Fig. 5.4 Most substances arc more ready to react in theirsolution form than in their solid form.

Fig. 5.3 Plants absorb mineralsalts in solution.

What kinds of solvents andsolutions do you use in YOLlr

daily life? How do you usethem?

97

Here are some other useful applications of solvents and solutionsin homes, agriculture, medicine and the industry.

Water is used for cooking,drinking, washing, bathingand cleaninq.

.... ". ,j - • "'.,, .... ,',::. ,~ ~':." •• , s , :-,

Water is used to dissolvedetergents and many types ofstains,

Mineral salts and fertilisers are dissolvedinwater so that plants can absorb them.

98

Many beverages, such asmineral water, fizzy drinksand tea arc solutions.

In hydroponics, the roots of plants areimmersed in sol!lt.iQr1sc orrtaininq dissolvedmineral salts,

-. -.,-'"

·. ;,,:,

Water is,used as a rawma1~f.ial i[1 'the manufactureQUoad, drinks.,'textiles, soapsarncl,detergents as vllell ;;rsforciGlanir'Yg and GooJi:n,g ..

Turp~ntin~, arechotandother types of solvents areused in trye,ma'huf a cture ofPtlints, inks and dyes.

Car, machine, aerospace andconstruction industries use avariety of alloys which are solid-solid solutions,

Fig. 5.5 Applications of solvents and solutions

. "..,~,.. '.'~~" ..' - ,. ..~ '",:~-_u-, "~,'" '. '''.'' _'_"".,?,;_(_"_

99

• A solution is a homogeneous mixture formed when one or more solutes dissolve(s) in asolvent.

• A solution in which water is the solvent is known as an aqueous solution.

• Mass of solution = Mass of solute + Mass of solvent

• When a solute dissolves in a solvent, it breaks up into particles which mix evenly with thesolvent particles.

• Solvents and solutions are widely used in homes, industries, agriculture and medicine.

Fig.5.6 A dilute solution

Fig.5.7 A concentratedsolution

5.2 Solubility

Distinguishing among dilute, concentrated andsaturated solutionsAdd a spoonful of copper sulphate crystals to half a beaker ofwater and stir. The copper sulphate crystals should dissolve in thewater to fo111''1a solution. Observe the colour of the solution. AJdmore copper sulphate to the beaker, one spoonful at a lime, andstir. Note the colour of the solution each time-,

How does the colour of the solution change when more coppersulphate is added? It; there a time when no more copper sulphatewill dissolve in the water no matter how hard or long you stir?

.A dilute solution is f01111edwhen a small amount of solutedissolves in a given amount of solvent. /\ dilute copper sulphatesolution appears light hlue because not much copper sulphate hasdissolved in The water (Fig. 5.6).

A concentrated solution is formed when a large amount of solutedissolves in a given amount of solvent. A concentrated coppersulphate solution appears dark blue because a lot of coppersulphate has dissolved in the water (Fig. 5.7).

As you may have found, a given amount of solvent at a particulartemperature can only dis sol ve Llp to a certain amount of solute.A saturated solution is formed when the maximum amountof solute dissolves in a given amount of solvent at a particulartemperature.

Excess solute does not dissol ve. Do you sec the excess coppersulphate crystals at the bottom of the beaker in the saturatedcopper sulphate solution (Fig. 5.8)?

What does solubility depend on?Solubility is the maximum amount of a solute which candissolve in 100 g of a solvent.at a particular temperature,

Salt is insotuble in alcohol. Thus its solubility in alcohol is zero.On the other hand, up to 22 g of copper sulphate can dissolve inIno g of water at 25°C. Thus, the solubility of copper sulphatein water at 25 "C is 22 g per 100 g of water, ff 25 g of coppersulphate is added to 100 g of water at this temperature, bow muchcopper su lp hate will not dissolve?

Solubility depends on a few factors. Fig. 5.9 shows the solubilityof copper sulphate in water at different temperatures.

Solubilityin water(9/100 g)

100

60

90

80

70

50

40

.30

20

10

o+-------~--~--~--~--+---.---.---.---.-~o 10 20 30 40 50 60 70 80 90 100

Temperature ("C)

Coppersulphatecrystals

Fig. 5.8 A saturated solution

Isthe salt solution in the DeadSea dilute, concentrated orsaturated? Explain.

Fig. 5.9 Graph of solubility ofcopper sulphate in water

!I--------------------------=-----------------------------=-----~--------------------~

Study the graph in Fig. 5.9 and answer the following questions:a. What is the maximum mass of copper sulphate which can

dissolve in water at (i) 0 °C; (ii) 50 HC and (iii) 90°C?b. From the graph, infer a factor which affects solubility.

From the graph of solubility of copper sulphate in water, you cansee that as temperature increases, solubility of copper sulphate inwater also increases. That is, more copper sulphate crystals candissolve in bot water than in the same amount of cold water.

;:( Try It Out '\". ~ ,.~

1. Make a saturated copper sulphate solution at room temperature by mixing copper sulphatecrystals with half a test tube of water until no crystals can dissolve.

2. Heat the test tube of saturated copper sulphate solution gently. What happens to the excesscopper sulphate crystals at the bottom or the test tube as the temperature of the solutionincreases?

3. Stop heating. Add more copper sulphate crystals to the warm copper sulphate solution. Canthe crystals dissolve?

4. Continue adding copper sulphate crystals until no more crystals can dissolve in the warmsolution.

5. Allow the warm saturated copper sulphate solution to cool. What do you observe in the testtube after the solution has cooled? .

Step 1 Steps 3 - 4 Step 5Step 2

1//// /U

1 r, r

cSolubility depends on temperature. In general, solubilities ofsolid solutes increase when temperature increases and vice versa,as in the case of copper sulphate. However, the solubilities ofgases are different. More g~ can dissolve in a cold solvent thanin a hot solvent, which is why you can see air bubbles rising outof some water which is being heated. As the water gets hotter, lessair ic; able to dissolve in it,

102

~~~~~~~~~~~==========~~==========~==========================~========~~~.l--

As temperature increases, solubilities of gases decrease.o. What will happen to aquatic life if the water they live in

becomes too warm?b. In the manufacture of fizzy drinks, should carbon dioxide

be added at a high or low temperature? Explain.

What other Factors besides temperature affect solubility? Look atthe solubility (.;LlYVeS of various solutes in water below:

Solubility in water(g/100 g)

500j -

400-

300-

Potassium nitrate

- Ammonium sulphate100 ..-.--=--~ --20 40 60 80 100

Fig.5.10 Solubility curves of various solutes

a. Which solute is most. soluble in water at (i) 0 °C and (ii)so "C'!

b. Do different solutes have different solubilities in water at aparticular temperature?

Besides temperature, solubility depends on the nature ofthe solute. From the solubility curves above, you can see thatdifferent solutes have different solubilities in the same solvent. ata particular temperature.

__f--- Water

Iodine solutionin alcohol

Will the same solute dissolve differently in different solvents?The photographs below show some iodine mixed with water andalcohol. While iodine dissolves very sparingly in water, it is muchmore soluble in alcohol. This shows that solubility depends onthe nature of tile solvent as well .

..

I:~..-._ ... ?1:!

Undissolved --'45i~1JIJiodine

Fig. S.11 Iodine dissolves differently in different solvents.

The ink of some brands of permanentmarkers is soluble in alcohol but not inwater. Can water be used to remove stains fromthese permanent markers? If not, what can be usedto remove them?

Solubility depends on:- the temperature;- the nature of the while;- the nature of the sol vent,

You have learnt about the factors which allect the rate ofdissolving (how fast a solute dissolves). Which factor affects bothsolubility and the rate of dissolving? Which factors affect the rateof dissolving but do not affect solubility?

• A dilute solution is one which contains a small amount of solute in a given amount of solvent.

• A concentrated solution is one which contains a lot of solute in a given amount of solvent.

-"...~~

#;.."._.-- y'Points~)~0~~2$"j

.".

• A saturated solution is one which contains the maximum amount of solute in a given amountof solvent at a particular temperature.

• Solubility is the maximum mass of a solute which can dissolve in 100 g of a solvent at a particulartemperat.ure.

i"r."'"..!'i'~~f'.~r:'J: .. ,....·: ..tt:_",.I,.~ " r-eN No..P.' ('IAY"''''(to .....t"y.,:#~t:~,..,H"''':,'::'4..,.,!f?'','''~..~_.~~~''''!~,~:~;'"t:'':('~·;t~ "~"",,,, ..~ -~ ..~~ ~..J'I/' ". "," .":t ~v.,j.......1IIt'•• ~".''',,-;~.'''~'' ",/;'$ .. ~ t-',""'!' \">•."..{\ ... og-,.(Ii'>"",,~..mv?·.~.

• The factors which affect solubility are:- temperature

nature of solutenature of solvent

5.3 Acids and AlkalisIIave you ever eaten oranges and lemons? Have you ever usedsoaps and toothpaste? If your answer is yes to both questions, thenyou have used acids and alkalis! Acids and alkalis are substanceswhich dissolve in water to form aqueous solutions with certainproperties. Many mixtures around us contain adds and alkalis.

AcidsAcids have a sour taste. Citrus fruits such as oranges and lemonstaste sour because they contain a type of acid called 'citrus acid'.Vinegar tastes sour because it contains ethanoic (acetic) acid.Apples contain malic: acid, grapes contain tartaric acid and yoghurtcontains lactic acid. Fizzy drinks contain carbonic acid, which isformed when carbon dioxide gas dissolves in water.

..: .

Citrus fruits

Fig.5.12 Some examples of food which contain acids

'{~.r

Rain water is slightly acidic because carbon dioxide. in the airdissolves in Jain water to form a weak acidic solution. However,when air is polluted with acidic gases like sulphur dioxide andoxides of nitrogen, acid rain forms. Acid rain is corrosive andharms plants and animals as well as damages stone and marble.structures,

Fig.5.13 Acid rain damages plant and animal life.

Fig.5.14 Bee stings are acidic.

Acids are found in living things as well. Ant hires and beestings contain formic acid. Our stomachs contain a strong acid,hydroch loric acid, which helps to kill the germs in the food we cat.Bacteria in our mouths produce acid when they feed on leftoverfood stuck. between our teeth. The acid can corrode or •cat away'the teeth and cause rooth decay.

Tn the school laboratory, you will come across aqueous solutionsof hydrochloric acid, sulphuric acid, nitric acid and cthanoic acid.Some of these acids are strong and corrosive, especially whenthey are concentrated. You should not taste or touch them withoutyour teacher's permission!

Fig.5.15 Some common acids found in a laboratory

1Caution! IAcids are potentially dangerous because they are corrosive and Ican burn your skin. Here are some 'precdl,:lti'G'fls to take when using i<Kids:- Should any acid get into your mouth, spit it out immediately and

rinse your mouth with plenty of water, Inform your teacher.- If any acid comes into contact with otber parts of your body or

clothing, wash them thoroughly with pltimty of water. Informyour teacher.

- Goggles should be worn when handling a-cids in the laboratory.- Add acids dropwise using <l dropper instea~ of pouring.- Concentrated acids should be adqed I;G w"!ter and never the

other way round. Adding cbncentra~e;q acids to water dilutesthe acids and makes them less,ha?ard(9l;l~,

~-.''':'''-;-.';'''';'''''''1'C:,,,;"?::,.,,,,,,,,,,,,,,,.,,,,,,,,,,,,,,,,,,,,,,,

Hydrochloric ;::~;:rt

CORROSIVE

Acid

Hel

Fig.5.16 Hazard symbol of corrosive substances such as strongacids and alkalis

Here arc some general properties of acids:Strong acids are corrosive. They can react with many materials andburn when they C011'l.C into direct contact with the skin. Therefore,they must be. handled with care.

- Acids taste sour.Acids turn blue litmus red. Litmus is an indicator which changescolour in acidic or alkaline solutions. Red litmus remains red inacidic solutions.

Fig.5.17 Acids turn blue litmus red.

Fig. 5.19 Test for hydrogen gas

Fig. 5.20 Test for carbon dioxide gas

108

Acidic solutions are good conductors of electriclty.

Fig.5.18 Sulphuric acid is used in car batteries asit is a good conductor of electricity.

Dilute acids react with reactive metals to Iorm hydrogengas.E.g.

Dilute hydrochloric + zinc _____.ZlDC + hydrogenacid chloride

The colourless and odourless hydrogen gas can be identifiedby using a lighted splint. IIydrogen gas extinguishes alighted splint with a 'pop' sound.

Dilute acids react with carbonates tu produce carbondioxide gas.E.g.

Dilute + calcium _____.calcium + water + carbonnitric carbonate nitrate dioxideadd

The colourless and odourless carbon dioxide gas can beidentified by passing it through some limewater (calciumhydroxide solution). A white precipitate is formed whencarbon dioxide gas is bubbled into limewater.

Dilute adds react with alkalis to form salts and wateronly.~o--be

--+ SOOiUIDchloride

sodiumhydroxidesolution

:Jilute hydroch lorie +acid

+ water

1Try It Out "-~ ~.M:;:

Egg shells are largely madeof calcium carbonate.Predict what will happen ifyou add some vinegar to anegg with its shell on. Explainyour prediction. Carry outthe experiment and observe.Was your prediction correct?

The method above can beused to remove the shell ofa raw egg without breakingit. Just leave the egg fullyimmersed in vinegar for a fewdays. For more effective results.change the vinegar after one day.Alternatively. a stronger acid may be used.

AlkalisHave you ever Lasted soap lather by accident? The bitter tasteof soap comes from the. alkali it contains. Many householdproducts such as soaps, detergents, toothpastes and indigestiontablets contain alkalis. The alkali sodium hydroxide(caustic soda) is used for making soaps, washing r :powders and drain cleaners. Potassium hydroxide(caustic potash) is used for removing paints and ~.dyes while magnesium hydroxide is used inindigestion tablets (antacids).

Indigestion tablets Detergents and soaps

Fig. 5.2 1 Alkalis <He used in indigestion tablets, soaps and detergents. 109

Sodium hydroxide, potassium hydroxide. calcium hydroxide andaqueous ammonia are some common alkalis you will find in thescience laboratory.

.......~-:. ,

_ .....*~i~-..~:.:. >l:-i,,~..:~

Fig. 5.22 Somealkalis found in a laboratory

The general properties of alkalis are as follows:Alkalis are corrosive. Strong alkalis like sodium hydroxideand potassium hydroxide are labelled as caustic became theyare highly corrosive. Like acids, they should be handled withcare!

Alkalis have a bitter taste andare soapy and slippery tothe touch.

Fig. 5.24 Alkalis turn red litmus blue.

)

Fig. 5.23 What makes soap feel soapy?

Alkalis turn red litmus blue. Blue litmus remains blue inalkaline solutions.

Alkalis are good conductors of electricity,

Fig. 5.25 Alkalis can conduct electricity.

- Alkalis react with dilute acids to form salts and wateronly.E.g.

Potassium + dilute nitric ___. potassium nitrate + wat.erhydroxide acid

.. .~

2!?~tO~Draw a table or a chart to compare the similarities anddifferences of the general properties of acids and alkalis.

Alkalis are used in alkaline batteries.Which property of alkalis makesthem suitable for use in batteries?

111

A B c

IndicatorsLook at the flasks below. Can you tell which flask contains anacidic solution, an alkaline solution or neutral solution withouttasting or feeling the substances?

~ ~~

1 i,

i I I

II iIi ),

1 \I \../ .../

Fig. 5.26 How can you tell the three solutions apart?

One way to find out is to use a suitable indicator. An indicator isa dye or a mixture uf dyes which changes colour according tothe acidity or alkalinity of a substance.

One example of an Indicator is litmus. Litmus contains dyes madefrom lichens. Red litmus paper turns blue inalkaline solutions and blue litmus paper F' 527 L' hIg.. IC ens ontums red in acidic solutions. In neutralsolutions such as pure water and saltsolutions, which are neither acidicnor alkaline, red litmus remainsred. and. blue litmus remainsblue.

When red litmus papers are used to test the substances in theflasks A. Band C above. the red litmus remains red in flasks Aand B and changes to blue in flask C. Can you conclude thatflasks A and B contain acidic solutions and flask C contains analkaline solution? Explain why. What else should you do todifferentiate between the substances in flasks A and B?

112

Some of the coloured dyes found in leaves, flowers and other plant parts can be used as indicators.The table below shows some plant extracts and their colours in acidic and alkaline solutions.

Original colour ofdye extracted fl"dm

plant

Colour when mixedwith alkaline

solution

Brown Yellow

Red Orange

Green Yellowish green

Dark red Re.d

Yellow Yellow

Yellow Pink

Yellow

Yellowish green

Green

Yellow

Yellow

Purple Pink

Red Pink

Green

Yellow

Table 5.1 Colours of plant extracts in acidic and alkaline solutions

Which of the plant extracts above are not suitable for use as indicators? Give a reason. Chooseone plant extract which can be used as an indicator and use it to test whether a substance isacidic or alkaline!

pH scale\Ve can use litmus paper to show that both lemon juice andhydrochloric acid are acidic. Can we lise litmus paper to tel)which is more acidic than the other?

How acidic or alkaline a substance is can be expressed using ascale called a pH scale. The pH scale ranges from 0 to 14. Themidpoint of the scale is 7. Neutral substances have a pH valueof 7. Acidic solutions have pH values less than 7 while alkalinesolutions have pH values greater than 7. The more acidic asubstance, the lower its pH value. The more alkaline a substance,the higher its pH value.

The pH values of some common substances are shown on thepH scale below:

More acidic

3 4 6

pH

r I r T 1TomatoLemon juice

Digestive juice ~ Acid rainjuice in ..~~,,-

stomach ~~- Unpollutcc< rilin

Car battery acid Vinegar Fig. 5.28 pH scale

The Universal Indicator, which is a mixture of different typesof indicators, can be used to determine the acidity or alkalinityof a substance. It changes colour depending on how acidic orhow alkaline a substance is. The colours of Universal Indicatorin solutions with different pH values are shown in Fig. 5.28.

The colour changes of various indicators are shown below:

IndicateI'

Bromothymol blue

Litmus

Methyl orange

Phenolphthalein

Phenol red

Universal Indicator

pH 0 2 3 6 8 9 10 11 12 13 144 5 7

+------ Acidity increases Alkalinity increases ----- ....

Fig. 5.29 Colour changes of various indicators

8

Solutionof baking

soda

Limewater

Householdammonia

Toothpaste

Milk ofmagnesia

The pH value of a solution can be found by dipping a pieceof U ni versal Indicator paper into the substance or solutionbefore comparing the colour of the paper against the pH colourchart.

Fig. 5.30 Comparing the colour ofUniversal Indicator paperwith colour chart

Ovencleaner

Sodium hydroxidesolution

Use some Universal Indicator paper or solution to find outthe pH values of some common substances such as rain water,saliva, fruit Juice, fizzy drinks, tap water, dishwashing liquidand soapy water.

Alternatively, a pH meter can be used to measure the pH valuesof substances quickly and accurately.

Fig. 5.31 pH meter

OMGDF.",,1' 0 STORE;) QCt.EP,f';-

READ e flF.CALl

Mixing adds and alkalisHow will the pH of a solution change when an alkali is addedto an acid? To find out, add a few drops of Universal Indicatorsolution to 50 ml of dilute hydrochloric acid in a beaker. Observethe colour changes as aqueous sodium hydroxide is graduallyadded to the. acid, as shown below:

Syringe use~ ~toaddalkaw

6W Add + indicatorpH = 1

When the alkali isadded to the acid, itreduces the effects ofthe acid, making it lessacidic, The pH of thesolution increases.

Alk\i~

Fhd

pH=4 ~

As more alkali is added to theneutral solution, it turns more.alkaline and the pH of thesolution increases further,

pH =9

1\.8 more alkali is added, there comes apoint when the. alkali completely cancelsout. the effect of the. acid and a neutralsolution of pH 7 is obtained. Thus, whenthe right amount of acid and alkali aremixed, a neutral so] ution of salt. and wateris formed. This is called neutralisation.

Acid + alkali It salt + water

-.«F8 Fig. 5.32 pH changes when

mixingan acid andan alkali

A pH meter can be used instead of the Uni versal Indicator tomonitor the changes in pH values of the mixture of acid andalkali. III what wayrs) is using a pIJ meter better than using theUniversal Indicator?

Is mass conserved when neutralisation takes place? That is, isthe total mass of the acid and alkali equal to the total mass ofthe products, salt and water?

Neutralisation is very useful in many situations. T.irnc (calciumoxide) is added to acidic soils or lakes to neutralise the soils orwater which have become too acidic because of acid rain.

Investigate whether one tablet each of different brands ofantacids neutralises the same amount of acid.

118

• Acids and alkalis are substanceswhich dissolve in water to form aqueous solutions with certainproperties.

• Hydrochloric acid, sulphuric acid, nitric acid, ethanoic acid, citric acid and carbonic <Kid aresome examples of acids.

• The general properties of acids are:Acids have a sour taste.Acids turn blue litmus red.Acids are good electrical conductors.Dilute acids react with reactive metals to produce hydrogen gas.Dilute acids react with carbonates to produce carbon dioxide gas.Acids react with alkalis to form salts and water only.Acids have pH values lessthan 7.

• Sodium hydroxide. potassium hydroxide and calcium hydroxide are some examples of alkalis.

• The general properties of alkalis are:- Alkalis have a bitter taste and are soapy and slippery to the touch.

Alkalis turn red litmus blue.Alkalis are good electrical conductors.

- Alkalis have pH values yrl:!ater than 7.

• Indicators are dyes which change colour according to·the acidity or elkalinity of a substance.Litmus and Universal indicator are two examples.

• When acids and alkalis are mixed, their pH values change. When the right amount of an acidis mixed with an alkali, neutralisation takes place and a neutral solution of salt in water with apH value of 7 is formed.

,>

•1

1. Ali adds sugar to some water until the sugar cannot dissolve any more.a. What does this tell you about the solubility of sugar in water?b. Can Ali make more sugar dissolve in the water by stirring the solution? Explain.c. Suggest one way in which Ali can make more suqar dissolve in the same amount of

water.

2. The solubility of common salt in water is 36 g per 100 g of water at 20 "c.a. If 30 9 of common salt dissolves in 100 9 of water, what is the mass of the salt solution

formed?b. What is the maximum mass of common salt which will dissolve in 20 9 of water at 20 °C?c. If 50 9 of common salt is added to 150 9 of water at 20 "'C, how many grams of common

salt will remain undissolved in the water?d. What kind of solution is 'formed when 18 9 of common salt dissolves in 50 9 of water at

20 uC7e. Common salt can dissolve in water but not in alcohol. What does this tell us about the

solubility of a substance?

3. In (In experiment, red litmus remained red in an unknown solution. Can you conclude that thesolution is acidic? How can you find out for sure?

4. Aeaker A contains sulphuric acid and beaker B contains cthanoic acid. Can litmus be used todifferentiate the two? Explain. Suggesl an alternative method if you think that litmus cannotbe used for differentiating the two acids.

5. Complete the following general word equations:a' Dilute acid + ------. salt + hydrogenb. + carbonate -----+ salt + water + carbon dioxidec. Acid + alkali ------. + _

6. Classify the 'following substances into two groups - those with pH values less than 7 and thosewith pH values more than 7:beklnc; soda, ruin wutcr, fizzy drink, limewater, milk of magnesia, orange juice, soapy water,toothpaste, vinegar, housetioki ammonia

7. Some dilute hydrochloric acid is gradually added to half a beaker of aqueous sodium hydroxideuntil the pH of the mixture changes to 7.a. Whilt can you use to find out if the pH of the mixture has changed to 7?b. Is the mixture acidic, alkaline or neutral when the pH is 7?c. What are the products formed when dilute hydrochloric acid is mixed with sodium

hydroxide solution? Are the products formed elements or compounds?d. What will happen to the pH of the mixture if morc dilute hydrochloric acid is added?

8. When vinegar is added to some marble chips (calcium carbonate), bubbles of gas are produced.Whal is the gas formed and how can it be identified?

9. Explain the following:a. Lime (calcium oxide) can be used to treat acidic soils.b. Toothpastes are made to be slightly alkaline in nature.

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1. You are given the following materials:

magnesium ribbons, blue litmus papers, water; a battery connected to a bulb and cerbonelectrodes, solid citric acid. solution of acid in propanone

Design an experiment to show that acids only exhibit the properties of acids when dissolvedin water.

2. I\quatic animals have different levels of toleranceto pH changes in water. For example, a wood'frog will die if the pH of water falls below 4,while a clam will die if the pH of water fallsbelow 6. What can you conclude about theacidity of water in a river if:a. both wood frogs and clams survive;b. only the 'wood 'frogs survive;c. both the wood frogs and clams do not survive?

Wood frog

Clam

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