Identifying acids

We are learning how to:

• distinguish between substances that are acids and alkalis

• explain the use of an indicator

• give the colour change in an indicator when it contacts an acid.

Identifying acids

Activity 10.2.1

Exploring how indicators change colour with different acids

Here is what you need:

• Hydrochloric acid

• Sulfuric acid

• Nitric acid

• Red and blue litmus paper

Here is what you should do:

1. Copy Table 10.2.1.

• Phenolphthalein

• Methyl orange

• Test tubes

• Tweezers

• Droppers.

Safety

Observe the safety icon on the acid bottles. All indicator papers should be held with tweezers. Avoid spillage.

AcidIndicatorColour change hydrochloric (HCl) sulfuric (H2SO4) nitric (HNO3) hydrochloric sulfuric nitric hydrochloric sulfuric nitric hydrochloric sulfuric nitric red litmus paper blue litmus paper phenolphthalein methyl orange

2. Pour a few drops of hydrochloric acid into each of two test tubes.

3. Suck up a dropperful of hydrochloric acid from one of the test tubes.

4. Squeeze one drop of hydrochloric acid onto a piece of each of the coloured indicator papers and record the colour you observe.

5. Pour one drop of the liquid indicators into each of the two test tubes of hydrochloric acid. Record the colour change you observe.

6. Wash out all the test tubes and droppers thoroughly.

7. Repeat steps 2 to 6 for sulfuric acid and then for nitric acid.

8. Compare the results you found for the various acids. Did all acids give the same results for each indicator?

The word acid is from the Latin acidus, meaning sour. An acid is a chemical substance. Solutions formed from these chemical substances usually have a sour taste. In order to identify acids, indicators are used. Indicators are made from special dyes and the results of your experiments should show the colour change.

Activity 10.2.2

Check your breath

10.2

Fun fact

Most fruits contain acids, such as citric acid, but these are weak acids and we can eat them without coming to any harm. They also provide the tangy flavours of fruits.

In this activity you will explore the gas that is exhaled from your body. Here is what you need:

• Two test tubes

• Dropper

• Straw. Here is what you should do:

• Universal indicator

1. Add a little water to two clean test tubes.

2. Put one drop of universal indicator into one of the test tubes and observe its colour.

3. Place a straw into each test tube.

4. Ask a volunteer to gently breathe out a few breaths through the straw and into each test tube.

5. Is there a colour change in the indicator? What conclusions can you draw from this?

Carbon dioxide is a naturally occurring gas that is important for photosynthesis. It is produced during respiration, decay, fermentation, combustion and volcanic eruptions. Carbon dioxide is acidic and forms carbonic acid with water.

Check your understanding

1. What colour changes are observed when the following are added to an acid?

a) Blue litmus paper b) Phenolphthalein c) Methyl orange.

2. Name three common acids.

Key terms

acid a type of chemical substance indicators substances made from special dyes that change colour depending on whether an acid or alkali is present