Science notes, unit 8

Acid and alkali

Textbook Science Year 3 (DLP), pages 105–114

What this chapter covers

You can use litmus paper to tell if a substance is acidic, alkaline or neutral (8.1.1)
You can describe how acidic, alkaline and neutral substances taste and feel (8.1.2)
You can name turmeric, purple cabbage and hibiscus extracts as other indicators (8.1.3)
You can give examples and uses of acidic, alkaline and neutral substances around you (8.1.4)

A boy tasting lemon and a girl drinking coffee without sugar, both pulling faces — Why do they pull funny faces? The lemon is very sour. The coffee without sugar is bitter.

Picture: Science Year 3 (DLP) textbook, p. 105.

8.1Acidic, alkaline and neutral

Things around us can be acidic, alkaline or neutral. This is called the property of a substance. A property is what a thing is like.

Acidic – like lemon juice or vinegar.
Alkaline – like soap or slaked lime.
Neutral – not acidic and not alkaline, like drinking water or sugar solution.

How do we find out? We use litmus paper. Litmus paper is an indicator. An indicator is something that changes colour to show us the property.

Boxes and strips of red litmus paper and blue litmus paper — Litmus paper has two colours: red and blue.

Picture: Science Year 3 (DLP) textbook, p. 106.

How to test

Put the two strips (red and blue) on a filter paper on a tile.
Use a dropper to put one drop of the substance on each strip.
Look at the colours. Write down what you see.

A dropper putting a drop of acidic substance on red and blue litmus strips — A drop of an acidic substance turns the blue strip red.

Picture: Science Year 3 (DLP) textbook, p. 106.

Did you know? We do not dip litmus paper straight into the substance. We drop the liquid on it with a dropper. For a dry thing, like a grape skin or powder, we wet the litmus paper first.

What the colours tell us

Three tiles: acidic turns blue strip red; alkaline turns red strip blue; neutral changes nothing — Look at the middle of each strip, where the drop fell.

Litmus paper	Acidic	Alkaline	Neutral
Blue	Changes to red	No change	No change
Red	No change	Changes to blue	No change

Easy way to remember: Acid makes blue angry red. Alkali makes red cool down to blue. Neutral changes nothing.

Only one strip changes for an acid or an alkali. If no strip changes, the substance is neutral.

Nadia drops some orange juice on litmus paper. The blue strip turns red. The red strip stays red. What is the property of orange juice?

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Acidic. Blue litmus paper turned red.

8.2Taste and touch

We can also use our senses of taste and touch. Our tongue tastes. Our fingers feel.

Three pupils tasting lemon (sour), cabbage (bitter) and water (tasteless) with ice cream sticks — The lemon tastes sour. The cabbage tastes bitter. The water is tasteless.

Picture: Science Year 3 (DLP) textbook, p. 109.

Property	Taste	Touch
Most acidic substances	Sour (like lime, tamarind)	Can feel burning (pedih)
Most alkaline substances	Bitter	Slippery, like soap
Neutral substances	Tasteless, sweet or salty	Slippery or coarse (kasar)

But be careful! Taste and touch are not scientific indicators. They do not always give the right answer.

A girl with coffee and her teacher in the Science room — Coffee tastes bitter, but it turns blue litmus paper red. So coffee is acidic!

Picture: Science Year 3 (DLP) textbook, p. 109.

Example: bitter gourd (peria)

Bitter gourd tastes bitter and feels slippery. You may guess "alkaline". But blue litmus paper turns red. So bitter gourd is really acidic. Litmus paper gives the true answer.

Never taste or touch chemicals in the Science room. In class, we only taste safe food, using a clean ice cream stick, with the teacher.

Can we use taste to find out if a substance is acidic or alkaline? Why?

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No. Bitter things are not always alkaline and sour things are not always acidic. We should use litmus paper.

8.3Litmus paper substitutes

No litmus paper at home? No problem! Some plants can be used as indicators too. We crush them in a little water to make an extract. An extract is the coloured juice we get from a plant.

Turmeric and a beaker of yellow turmeric extract

Turmeric (kunyit)
Yellow extract

Purple cabbage and a beaker of purple extract

Purple cabbage
Purple extract

Hibiscus flower and a beaker of pink extract

Hibiscus (bunga raya)
Pinkish-red extract

Pictures: Science Year 3 (DLP) textbook, p. 112.

In the textbook test, we put a drop of each extract into lime juice (acidic) and sodium bicarbonate solution (alkaline). Then we look for a colour change.

Extract	In lime juice (acidic)	In sodium bicarbonate (alkaline)
Turmeric	Stays yellow	Turns reddish-brown
Purple cabbage	Turns pink or red	Turns green
Hibiscus	Turns bright pink	Turns green

Did you know? Turmeric only changes colour with an alkaline substance. If you spill soap on a curry stain, the yellow stain turns red!

Name three plant extracts that can be used instead of litmus paper.

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Turmeric, purple cabbage and hibiscus.

8.4Acidic, alkaline and neutral substances around us

Acidic, alkaline and neutral substances are everywhere: in the kitchen, in the bathroom, at the pasar and on the farm.

Acidic	Alkaline	Neutral
Lime (limau nipis)	Soap	Salt solution
Tamarind (asam jawa)	Slaked lime (kapur)	Sugar solution
Vinegar	Toothpaste	Drinking water
Orange juice	Sodium bicarbonate	Cooking oil

Vinegar
Acidic

Orange juice
Acidic

Slaked lime
Alkaline

Toothpaste
Alkaline

Sugar solution
Neutral

Drinking water
Neutral

Pictures: Science Year 3 (DLP) textbook, pp. 108 and 110.

How we use them

An acid and an alkali can cancel each other out. That is why we use one to fix a problem caused by the other.

A farmer sprinkles agricultural lime on the soil — **Agriculture:** the soil is too acidic. Farmers sprinkle **agricultural lime** (alkaline) to reduce the acidity.

A father puts vinegar on a wasp sting — **Medicine:** a wasp sting is alkaline. **Vinegar** (acidic) can be put on it as first aid.

A mother helps her child rinse slippery bleach off her hands — **Household:** bleach makes hands slippery. Rinse with **tamarind** (acidic) to stop the slippery feeling.

Situation cards: Science Year 3 (DLP) textbook, p. 111.

Bleach is strong. Ask an adult to help, and wash with lots of water. Never mix bleach with vinegar or other cleaners, because it can make a harmful gas.

Did you know? After you eat nasi lemak or sweets, the germs in your mouth make acid. Acid can damage your teeth. Toothpaste is alkaline, so it cancels out the acid in your mouth.

A bee stung Ahmad. A bee sting is acidic. Should his mother put vinegar or a paste of sodium bicarbonate on it? Why?

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Sodium bicarbonate. It is alkaline, so it cancels out the acidic sting. (Vinegar is for an alkaline wasp sting.)

Summary

Remember	Key fact
Litmus paper	An indicator. Two colours: red and blue.
Acidic	Blue litmus turns red. Most taste sour. Lime, tamarind, vinegar.
Alkaline	Red litmus turns blue. Most taste bitter and feel slippery. Soap, slaked lime, toothpaste.
Neutral	No change in either colour. Tasteless, sweet or salty. Water, salt solution, sugar solution.
Taste and touch	Not scientific indicators. Coffee is bitter but acidic.
Other indicators	Turmeric, purple cabbage and hibiscus extracts.
Uses	Agriculture (lime for acidic soil), medicine (vinegar for wasp sting), home (tamarind for slippery bleach).