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Every person has hydrochloric acid in their stomach; this is a strong acid with a pH of about 1. This acid is very important, because it helps us to digest food and provides some protection against microbes we might have eaten. Our stomachs have a special lining so that the acid does not burn and cause us pain. However, some people find that their acid leaves their stomach and enters the oesophagus (food pipe); this is very painful and feels like burning in your chest, so it is known as acid indigestion or heartburn. This excess acid must be neutralised with a medium strength base. When the neutralisation occurs, two products are made, water and a metal salt.

This lesson looks at remedies for acid indigestion and their effects on the pH of the stomach.

Task 1: Looking at pH of stomach acid and indigestion

Model 1

  1. Open Yenka file Model 1.
  2. What is the common name for indigestion?
    Heartburn, because you feel a burning sensation in your chest, near your heart
  3. Should all the acid be neutralised? Why or why not?
    No, acid is needed to aid digestion by providing the correct pH for enzymes to work. The acid also provides some protection against microbes we might have eaten.
  4. Gives some examples of chemicals that could be used as indigestion remedies.
    Calcium carbonate, magnesium oxide, sodium carbonate
  5. Predict the pH of the acid at the start of the experiment and after the indigestion remedy is added. Which remedy do you think will be best, and why?
    At the start of the experiment it should be about pH 0-1. As the base is added, pH will increase (acidity will decrease). Sodium carbonate will completely neutralise the acid and terminate as an alkali, calcium carbonate barely increases the pH and magnesium oxide increases the pH to a weak acid.
  6. Add the acid, universal indicator and indigestion remedies to each of the conical flasks and press the pause button on the toolbar. Which is the best remedy and why?
    Magnesium oxide, because it neutralises some of the acid, but leaves some for digestion and disease defence
    Sodium carbonate is too strong a base and overshoots, whereas calcium carbonate doesn't neutralise enough of the acid.
  7. Copy the graph into the space below and label it using the following key words in the appropriate places: acid, base, strong, weak, neutral and neutralisation.



Most indigestion remedies contain a mixture of chemicals, some for a quick reaction with the excess acid, others which will have a longer lasting effect. Additional chemicals make the tablet more palatable, for example sugar and colours. You can find out exactly what is in the tablet, as it is listed on the packaging.

Teacher Summary

  • Extension work could be to look at the temperature that the probes register on the bottom tool bar. Neutralisation is exothermic (gives out energy); the reaction with magnesium oxide produces the greatest temperature rise, then sodium carbonate and finally calcium carbonate. It is also possible to collect and test the gas produced and consider the effects on the patient of excess gas production.