10.1 10.1

Acids and alkalis in Acids and alkalis in

daily lifedaily life

The orange juice tastes

sour!

The orange juice tastes

sour!

The baking powder tastes

bitter!

The baking powder tastes

bitter!

Why do some substances taste sour

and some substances taste bitter? Why do some substances taste sour

and some substances taste bitter?

p.2p.2The detergent works well in removing dirt.

The detergent works well in removing dirt.

Tony! Be careful! The detergent is

very slippery!

Tony! Be careful! The detergent is

very slippery!

Why does the dish become slippery after cleaned with

the detergent?

Why does the dish become slippery after cleaned with

the detergent?

In Unit 6, we have learnt how to classify substances as solids, liquids and gases.

In fact, there are many different ways to classify substances.

In fact, there are many different ways to classify substances.

p.3p.3

Scientists can classify substances as below:

acid(酸 )

acid(酸 )

alkali(鹼 )

alkali(鹼 )

neutral substance(not an acid or alkali)

neutral substance(not an acid or alkali)

SubstancesSubstances

- Many substances in daily life contain acids or alkalis.

- Some foodstuffs taste sour because they contain acids.

p.4p.4

orangeorange

lemonlemontangerinetangerine

grapefruitgrapefruit

contain citric acidcontain

citric acidCitrus Citrus

fruitsfruits

AppleApple contain containss

malic acid malic acid (( 蘋果酸蘋果酸 ))

GrapeGrape contains containstartaric acidtartaric acid

(( 酒石酸酒石酸 ))

whitewhitevinegarvinegar

ketchupketchupred vinegarred vinegar

containcontain etha ethanoic acidnoic acid((or called or called aceacetic acidtic acid))

tomatotomato

Some soft Some soft

drinksdrinks contain containcarbonic acidcarbonic acid

Some Some

drinksdrinks and and

candiescandies

containcontainascorbic acidascorbic acid((vitamin Cvitamin C))

containcontainascorbic acidascorbic acid((vitamin Cvitamin C))

Milk Milk and and yogyoghurthurt

containlactic acidcontain

lactic acid

SpinachSpinach and and

somesome

shampoosshampoos

contain oxalic acid

( 草酸 )

contain oxalic acid

( 草酸 )

Toilet cleanserToilet cleanser containscontains

hydrochloric hydrochloric acidacid

AspirinAspirin also also contains contains acidacid

- Acids taste soursour, but alkalis generally taste bitter.

Did you taste the following alkaline drugs or foodstuff before?Did you taste the following alkaline drugs or foodstuff before?

almondalmond

antacidantacid

How do they taste?

p.4p.4

- Many household cleansers contain alkalis.

Alkalis are good for removing greases. They react with the greases so that the resulting substances can be washed away with water easily.

Alkalis are good for removing greases. They react with the greases so that the resulting substances can be washed away with water easily.

Slippery! Be careful!

After the toilet has just been cleaned, there is often a cautionary notice. Why?

After the toilet has just been cleaned, there is often a cautionary notice. Why?

- Alkalis are generally slippery.p.4p.4

Acids and alkalis can also be found in our body.

stomastomachch

stomastomachch

produceshydrochloric acidthat helps break

down the foodjuicesjuices

inside arealkaline

small small intestinintestin

ee

small small intestinintestin

ee

salivsalivaasalivsalivaa

p.4p.4

- Many household cleansers contain alkalis.

but this toilet cleanser toilet cleanser contains acid instead.

=>=> cannot distinguish between acids and alkalis by their uses

Should we distinguish an acid from an alkali by tasting by tasting it?

Natural acid-alkali indicatorNatural acid-alkali indicator p.5p.5

- We should distinguish between acids and alkalis by using indicators ( 指示劑 ).

- Indicators Indicators contain pigmentspigments

=>=> change colour in acids and alkalis.

- Some natural materials contain pigments and can be used as natural indicators.

Using red cabbage extract as indicator

p.5p.5Experiment Centre 10.1Experiment Centre 10.1

1 Cut the red cabbage into pieces and boil them in hot water to get the red cabbage extract.

2 Examine the red cabbage extract. What colour is it?

______________________________Reddish purple/purple.

3 Add a few drops of the extract into two test-tubes containing lemon juice lemon juice and soap solution soap solution respectively.

4 Write down the colour of the mixtures in the following table.

Substance Colour of the mixture

lemon juice

soap solution

red

green

The colour of red cabbage extract The colour of red cabbage extract is ____________ i

n acids acids and ______________ in alkalis.

redred

greengreen

Indicator magicIndicator magicIndicator magicIndicator magic

You can also make your own indicator in your kitchen by using the red cabbage.

Try to use the indicator to test whether the baking powder solution is acidic or alkaline. baking

powder

p.6p.6Little ScientistLittle Scientist

You can also use the extracts from the following plants as acid-alkali indicators!

onion

grape

Boyle found that pigments in some plants can be used as indicators.

Boyle found that pigments in some plants can be used as indicators.

p.6p.6

a British scientist (1627–1691) One day, when Boyle was working in the laboratory, he put some violet

into a beaker…

The flower is beautiful!

When he did some experiments on acids…

Oh! The acid splashes on the petals.

Boyle washed the flower with water immediately…

The colour of the flower changes from

purple to redred.

Boyle discovered that some flower or plant extracts showed different colours in acids and alkalis.

Boyle’s discovery demonstrates that both luck and careful

observationare required for scientific

discovery.

Boyle’s discovery demonstrates that both luck and careful

observationare required for scientific

discovery.

Making your own acid-alkali indicator

p.7p.7Experiment Centre 10.2Experiment Centre 10.2

1 Put some petals into the mortar.

2 Crush the petals with the pestle.

mortar

mortar

pestle

pestle

4 Add some distilled water so that the volume is enough for three tests.

3 Add a few drops of ethanol, mix it with the petals.

petals mixed with ethanol and distilled

water

petals mixed with ethanol and distilled

water

5 Pour the coloured extract into three test-tubes.

6 Add 2 to 3 drops of lemon juice to one test-tube, soap solution to the second one. The last test-tube contains the extract in distilled water only.

7 Shake the test-tubes and observe the colour of the mixtures.

8 Record your results in the table below.

9 Wash the apparatus. Repeat steps 1 to 8 with petals of other flowers.

1 Why do we need to have a test-tube containing the extract in distilled water?

______________________________ ______________________________

QuestionsQuestions

2 What extract gives the most clear-cut colour changes?

______________________________

p.8p.8

For comparison of colour when there

is no acid or alkali.