10.1 Acids and alkalis in daily life

The orange juice tastes sour! 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.2 The detergent works well in removing dirt. Tony! Be careful! The detergent is very slippery! 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. p.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) Substances

- Many substances in daily life contain acids or alkalis. - Some foodstuffs taste sour because they contain acids. p.4

orange lemon tangerine grapefruit contain citric acid Citrus fruits

Apple contains malic acid ( 蘋果酸 ) Grape contains tartaric acid ( 酒石酸 )

white vinegar ketchup red vinegar contain ethanoic acid (or called acetic acid) tomato

Some soft drinks contain carbonic acid

Some drinks and candies contain ascorbic acid (vitamin C)

Milk and yoghurt contain lactic acid

Spinach and some shampoos contain oxalic acid ( 草酸 ) contain oxalic acid ( 草酸 )

Toilet cleanser contains hydrochloric acid Aspirin also contains acid

- Acids taste s ss sour, but alkalis generally taste bitter. Did you taste the following alkaline drugs or foodstuff before? almond antacid How do they taste? p.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.

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

- Alkalis are generally slippery. p.4

Acids and alkalis can also be found in our body. stomachstomach produces hydrochloric acid that helps break down the food juices juices inside are alkaline small intestine salivasaliva p.4

- Many household cleansers contain alkalis. toilet cleanser but this toilet cleanser contains acid instead. => => cannot distinguish between acids and alkalis by their uses

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

Natural acid-alkali indicator p.5 - We should distinguish between acids and alkalis by using indicators ( 指示劑 ). Indicators pigments - Indicators contain pigments => => 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.5 Experiment Centre 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.

lemon juice soap solution 3 Add a few drops of the extract into two test-tubes containing lemon juice and 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 ______ acids _______ The colour of red cabbage extract is ______ in acids and _______ in alkalis. red green

Indicator 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.6 Little 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. p.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… red The colour of the flower changes from purple to red.

Boyle discovered that some flower or plant extracts showed different colours in acids and alkalis. Boyle’s discovery demonstrates that both luck and careful observation are required for scientific discovery. Boyle’s discovery demonstrates that both luck and careful observation are required for scientific discovery.

Making your own acid-alkali indicator p.7 Experiment Centre Put some petals into the mortar. 2 Crush the petals with the pestle. mortar 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 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? ______________________________ Questions 2 What extract gives the most clear-cut colour changes? ______________________________ p.8 For comparison of colour when there is no acid or alkali.