Chapter 8 of 8

All 8 chapters
  1. 1Elements and compounds, readunderstood
  2. 2Pure substances and mixtures, readunderstood
  3. 3Separating mixtures, readunderstood
  4. 4Physical and chemical changes, readunderstood
  5. 5Rusting and other reactions, readunderstood
  6. 6Burning and fire safety, readunderstood
  7. 7Acids and alkalis, readunderstood
  8. 8Neutralisation, readunderstood

S3.6.8 Chapter 8

Neutralisation

Why does a chalky tablet ease a sore stomach, and why do farmers spread white powder on soil?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

  • Explain neutralisation as an acid and an alkali cancelling out
  • Explain how an antacid eases indigestion
  • Explain why farmers add lime to sour soil

An acid and an alkali are opposites. When you mix them, each one weakens the other. This is useful for a sore stomach and for sour soil.

Draw it first

Draw a neutralisation

  1. A word equation built in three steps from top to bottom. First, two boxes in a row joined by a plus sign: acid + alkali. Then an arrow pointing down. Then two boxes in a row joined by a plus sign: salt + water. Labels: acid, +, alkali, salt, +, water.acid+alkali

    1 Write acid and alkali, with a plus sign between them. These are the substances you start with.

  2. A word equation built in three steps from top to bottom. First, two boxes in a row joined by a plus sign: acid + alkali. Then an arrow pointing down. Then two boxes in a row joined by a plus sign: salt + water. Labels: acid, +, alkali, salt, +, water.acid+alkali

    2 Draw an arrow. It means makes.

  3. A word equation built in three steps from top to bottom. First, two boxes in a row joined by a plus sign: acid + alkali. Then an arrow pointing down. Then two boxes in a row joined by a plus sign: salt + water. Labels: acid, +, alkali, salt, +, water.acid+alkalisalt+water

    3 Write what they make: a salt and water. The acid and the alkali are used up.

When an acid and an alkali react, they cancel each other out. We call this neutralisation. The reaction makes a salt and water.

Here a salt means any substance made when an acid and an alkali react. Table salt is one salt, but there are many others.

A word equation. Two boxes in a row, acid and alkali, joined by a plus sign. A down arrow points to two boxes under them, salt and water, joined by a plus sign. Labels: acid, alkali, salt, water, +, +.acid+alkalisalt+water
The word equation for neutralisation.

You can watch it with an indicator. Put red cabbage indicator in vinegar, an acid. It turns pink. Add baking soda a little at a time. At first it stays pink, because some acid is left. With enough soda it turns purple, which is neutral. Add too much, and it turns blue.

Three cups of vinegar with red cabbage indicator in them, and baking soda added. The first cup has a little baking soda and is hatched. It says pink, acid. The second cup has enough baking soda and is dotted. It says purple, neutral. The third cup has too much baking soda and is cross hatched. It says blue, alkali. Labels: a little, soda, enough, soda, too much, soda, pink, purple, blue, acid, neutral, alkali.a littlesodapinkacidenoughsodapurpleneutraltoo muchsodabluealkali
Too little alkali leaves some acid. Enough makes the liquid neutral. Too much leaves an alkali.

Baking soda also makes gas as it reacts with vinegar. That is the fizz. The fizz is carbon dioxide.

A word equation. Two boxes in a row, vinegar and baking soda, joined by a plus sign. A down arrow points to the products. On the first line are two boxes, sodium acetate and water, joined by a plus sign. On the second line a plus sign starts the line, then one box, carbon dioxide. Labels: vinegar, baking soda, sodium acetate, water, carbon dioxide, +, +, +.vinegar+baking sodasodium acetate+water+carbon dioxide
Vinegar is the acid. Sodium acetate is the salt. The gas makes the fizz.

Your stomach makes acid to break down food. Sometimes it makes too much, and your stomach hurts. This is called indigestion. An antacid is a medicine that eases it. An antacid is a weak alkali, often a chalky tablet. It cancels some of the extra acid.

See it three ways

With things

Think of a bath that is too hot. You add cold water, and it cools. An antacid does the same job for acid. It cancels some, so the stomach is less sour.

As a picture

The pH scale as a strip numbered from 0 to 14. The part from 0 to 7 is hatched and labelled acid. The line at 7 is heavy and labelled neutral. The part from 7 to 14 is cross hatched and labelled alkali. stomach acid stands at 1, after antacid stands at 3. Labels: stomach acid, after antacid, acid, neutral, alkali, and the numbers 0 to 14.01234567891011121314acidneutralalkalistomach acidafter antacid

In words

The stomach acid is at about pH 1. An antacid raises the pH to about 3. It does not go all the way to 7, because the stomach still needs some acid.

In symbols

3 − 1 = 2

An antacid must be a weak alkali. A strong alkali would burn the inside of the body. Only use medicine that a grown up or a health worker says is right for you.

Soil can be acid too. Rain, and some plant waste, make it acid. Many crops grow poorly in sour soil. Farmers fix this with lime. Lime is made from limestone, and it is an alkali.

The pH scale as a strip numbered from 0 to 14. The part from 0 to 7 is hatched and labelled acid. The line at 7 is heavy and labelled neutral. The part from 7 to 14 is cross hatched and labelled alkali. sour soil stands at 5, soil with lime stands at 6, pure water stands at 7. Labels: sour soil, soil with lime, pure water, acid, neutral, alkali, and the numbers 0 to 14.01234567891011121314acidneutralalkalisour soilsoil with limepure water
Sour soil is about pH 5. Lime raises it, here to about 6, which is closer to neutral. Most crops grow best near neutral soil.
A table with the columns problem and fix. Indigestion: antacid. Sour soil: lime.problemfixindigestionantacidsour soillime
Two problems of too much acid, and the alkali that fixes each.

Farmers wear gloves and masks when they spread lime. It is an alkali and it can burn skin and eyes. They also test the soil first, so they add only as much as the soil needs.

Try it

Cancel out vinegar

You need: red cabbage indicator from the last chapter, vinegar, baking soda, a large cup, a spoon.

Be safe: Use a large cup, because the fizz can spill over. Keep your face back from the cup. If any splashes in your eyes, rinse them with plenty of clean water. Do not taste anything. Wash your hands afterwards.

  1. Put 2 spoons of vinegar in a large cup. Add 2 spoons of cabbage indicator. It turns pink.
  2. Add a pinch of baking soda. Stir. It fizzes. Watch the colour.
  3. Keep adding one pinch at a time. Stir each time. At first the colour stays pink. Stop when it turns purple. Then add more, and see it turn blue.

What you will see: The colour stays pink at first, because there is still acid. It turns purple when the acid has been cancelled. A little more soda turns it blue, because now there is too much alkali. The fizz is carbon dioxide gas.

If you cannot try it, this is what happens.
Three cups of vinegar with red cabbage indicator in them, and baking soda added. The first cup has a little baking soda and is hatched. It says pink, acid. The second cup has enough baking soda and is dotted. It says purple, neutral. The third cup has too much baking soda and is cross hatched. It says blue, alkali. Labels: a little, soda, enough, soda, too much, soda, pink, purple, blue, acid, neutral, alkali.a littlesodapinkacidenoughsodapurpleneutraltoo muchsodabluealkali

A wrong idea many people have

Adding more alkali always makes an acid safer.

Why it feels right: An alkali cancels an acid. So it seems that more alkali must cancel more.

The test that breaks it: Add baking soda to cabbage indicator in vinegar, a pinch at a time. The colour goes pink, then purple, then blue. After purple, more soda only leaves an alkali.

What is true: Neutralisation needs the right amount. Too little leaves some acid. Too much leaves an alkali. This is why farmers test their soil.

Key words

neutralisation
a reaction where an acid and an alkali cancel each other out.
salt
a substance made when an acid reacts with an alkali.
antacid
a weak alkali that cancels some stomach acid.
lime
an alkali that farmers add to sour soil.

Back to your guess

The answer is C: The tablet is a weak alkali. It cancels some of the extra acid.

An antacid is a weak alkali. It cancels, or neutralises, some of the extra acid. Farmers use lime for the same job in soil.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. What happens in neutralisation?

    Tap your answer.

    1. Not this one. They work against each other. The result is closer to neutral, not a stronger acid. Go to S3.6.8. Try another.

    2. Yes, that is right. The acid and the alkali use each other up. The new substances are a salt and water.

    3. Not this one. It is a chemical change, so new substances are made. They are a salt and water. Go to S3.6.8. Try another.

    See the answer

    An acid and an alkali cancel each other out, and make a salt and water.

    The acid and the alkali use each other up. The new substances are a salt and water.

    • If you chose A: They work against each other. The result is closer to neutral, not a stronger acid. Go to S3.6.8.
    • If you chose C: It is a chemical change, so new substances are made. They are a salt and water. Go to S3.6.8.

  2. Which word equation shows neutralisation?

    Tap your answer.

    1. Yes, that is right. The acid and the alkali are the reactants. They make a salt and water.

    2. Not this one. Nothing changes in this one. A reaction makes new substances. Go to S3.6.8. Try another.

    3. Not this one. The arrow is backwards. The acid and alkali are the reactants, so they come before the arrow. Go to S3.6.8. Try another.

    See the answer

    acid + alkali → salt + water

    The acid and the alkali are the reactants. They make a salt and water.

    • If you chose B: Nothing changes in this one. A reaction makes new substances. Go to S3.6.8.
    • If you chose C: The arrow is backwards. The acid and alkali are the reactants, so they come before the arrow. Go to S3.6.8.

  3. How does an antacid ease indigestion?

    Tap your answer.

    1. Yes, that is right. The stomach has too much acid. The antacid is an alkali that reacts with some of it, so the stomach is less sour.

    2. Not this one. More acid would make it worse. An antacid cancels acid. Go to S3.6.8. Try another.

    3. Not this one. It is a weak alkali, and it only raises the pH a little. A very strong alkali would burn. Go to S3.6.8. Try another.

    See the answer

    It is a weak alkali that cancels some of the extra stomach acid.

    The stomach has too much acid. The antacid is an alkali that reacts with some of it, so the stomach is less sour.

    • If you chose B: More acid would make it worse. An antacid cancels acid. Go to S3.6.8.
    • If you chose C: It is a weak alkali, and it only raises the pH a little. A very strong alkali would burn. Go to S3.6.8.

  4. Why must an antacid be a weak alkali and not a strong one?

    Tap your answer.

    1. Not this one. Strength does not make it faster to neutralise. Strong alkalis burn, so a weak one is used. Go to S3.6.7. Try another.

    2. Not this one. A strong alkali has a higher pH, such as 13 or 14. That is why it burns. Go to S3.6.7. Try another.

    3. Yes, that is right. Strong alkalis burn, as strong acids do. A weak alkali is gentle enough to be safe.

    See the answer

    A strong alkali would burn the inside of the body.

    Strong alkalis burn, as strong acids do. A weak alkali is gentle enough to be safe.

    • If you chose A: Strength does not make it faster to neutralise. Strong alkalis burn, so a weak one is used. Go to S3.6.7.
    • If you chose B: A strong alkali has a higher pH, such as 13 or 14. That is why it burns. Go to S3.6.7.

  5. Why do farmers add lime to sour soil?

    Tap your answer.

    1. Yes, that is right. Sour soil is acid. Lime is an alkali, so it neutralises some of the acid and brings the pH closer to 7.

    2. Not this one. Lime is an alkali. Sour soil is already too acid. Go to S3.6.8. Try another.

    3. Not this one. Lime is an alkali. Sour soil is already too acid, and lime cancels some of that acid. Go to S3.6.8. Try another.

    See the answer

    Lime is an alkali. It cancels some of the acid and raises the soil pH.

    Sour soil is acid. Lime is an alkali, so it neutralises some of the acid and brings the pH closer to 7.

    • If you chose B: Lime is an alkali. Sour soil is already too acid. Go to S3.6.8.
    • If you chose C: Lime is an alkali. Sour soil is already too acid, and lime cancels some of that acid. Go to S3.6.8.

  6. Cabbage indicator in vinegar is pink. You add baking soda until it turns blue. What does the blue show?

    Tap your answer.

    1. Not this one. Blue shows an alkali. Cabbage indicator turns pink in an acid. Go to S3.6.7. Try another.

    2. Not this one. Neutral is purple. Blue means the liquid has gone past neutral, into an alkali. Go to S3.6.7. Try another.

    3. Yes, that is right. The vinegar was cancelled first, at purple. The extra baking soda left an alkali, so the colour went blue.

    See the answer

    There is too much baking soda, so the liquid is now an alkali.

    The vinegar was cancelled first, at purple. The extra baking soda left an alkali, so the colour went blue.

    • If you chose A: Blue shows an alkali. Cabbage indicator turns pink in an acid. Go to S3.6.7.
    • If you chose B: Neutral is purple. Blue means the liquid has gone past neutral, into an alkali. Go to S3.6.7.

  7. From an earlier chapter

    What colour does red cabbage indicator turn in vinegar?

    Tap your answer.

    1. Not this one. Vinegar is sour. It is an acid, and cabbage turns pink in an acid. Go to S3.6.7. Try another.

    2. Yes, that is right. Red cabbage is pink in an acid. It is purple in a neutral liquid and blue or green in an alkali.

    3. Not this one. Purple is the colour in a neutral liquid, such as pure water. Vinegar is an acid. Go to S3.6.7. Try another.

    See the answer

    Pink, because vinegar is an acid.

    Red cabbage is pink in an acid. It is purple in a neutral liquid and blue or green in an alkali.

    • If you chose A: Vinegar is sour. It is an acid, and cabbage turns pink in an acid. Go to S3.6.7.
    • If you chose C: Purple is the colour in a neutral liquid, such as pure water. Vinegar is an acid. Go to S3.6.7.

  8. Stomach acid has a pH of 1. After an antacid, the pH is 3. By how much did the pH rise?

    The pH scale as a strip numbered from 0 to 14. The part from 0 to 7 is hatched and labelled acid. The line at 7 is heavy and labelled neutral. The part from 7 to 14 is cross hatched and labelled alkali. stomach acid stands at 1, after antacid stands at 3. Labels: stomach acid, after antacid, acid, neutral, alkali, and the numbers 0 to 14.01234567891011121314acidneutralalkalistomach acidafter antacid
    See the answer

    2

    The pH went from 1 up to 3. 3 − 1 = 2.

  9. A field has a soil pH of 5. After lime is added, the pH goes up by 1. What is the new pH?

    See the answer

    6

    Lime raises the pH. 5 + 1 = 6.

Keep it

Copy this into your exercise book.

Acid + alkali → salt + water. This is neutralisation. An antacid eases stomach acid. Lime raises the pH of sour soil. Too much alkali leaves an alkali.

Draw: Draw three cups of cabbage indicator: pink for acid, purple for neutral, and blue for too much alkali.

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