Indicators, pH and neutralisation
A farmer in Co. Meath spreads crushed lime on a field, and a lake in Co. Wicklow is treated with powdered limestone. Why would anyone put an alkaline powder on soil or into water? To answer that, you need to know how to measure acidity and what happens when an acid meets a base.
Indicators and the pH scale
You cannot taste-test chemicals in a lab, so we use indicators. An indicator is a substance that changes colour depending on whether a solution is acidic or alkaline.
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Litmus is red in acid and blue in alkali. It only tells you which side you are on.
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Universal indicator is a mixture of dyes. It gives a range of colours that match the pH scale.
| Solution | Litmus | Universal indicator (approx.) | pH |
|---|---|---|---|
| Acid | red | red / orange / yellow | below 7 |
| Neutral (pure water) | purple | green | 7 |
| Alkali | blue | blue / purple | above 7 |
The further from 7, the stronger the acid or alkali. A pH of 1 is a strong acid; a pH of 6 is only weakly acidic. Lemon juice is about pH 2, rainwater is slightly acidic (about pH 5.6), pure water is 7, and bleach is about pH 12.
State whether each solution is acidic, neutral or alkaline: a) pH 2 b) pH 7 c) pH 11 d) pH 6
State whether each solution is acidic, neutral or alkaline:
- a) pH 2
- b) pH 7
- c) pH 11
- d) pH 6
State the colour of each indicator in an acidic solution and in an alkaline solution: a) Litmus b) Universal indicator (approximate colours)
State the colour of each indicator in an acidic solution and in an alkaline solution:
- a) Litmus
- b) Universal indicator (approximate colours)
Hydrogen ions and the size of a pH change
Why does pH use numbers? Acids contain hydrogen ions, $\mathrm{H^+}$, when dissolved in water. The more hydrogen ions in a solution, the lower the pH.
The scale is not a simple one. Each step of one pH unit means the hydrogen ion concentration changes by a factor of 10.
| pH | Hydrogen ion concentration compared with pH 7 |
|---|---|
| 7 | 1 (reference) |
| 6 | 10 times more |
| 5 | 100 times more |
| 4 | 1,000 times more |
Solution A has a pH of $3$ and solution B has a pH of $5$. a) State which is more acidic. b) Given that each decrease of one pH unit means a tenfold increase in hydrogen…
Solution A has a pH of $3$ and solution B has a pH of $5$.
- a) State which is more acidic.
- b) Given that each decrease of one pH unit means a tenfold increase in hydrogen ion concentration, calculate how many times greater the hydrogen ion concentration is in A than in B.
- c) A student says "solution B has a pH of $5$, so it is neutral". Explain why the student is wrong.
Neutralisation
When an acid reacts with a base, the properties of both are cancelled out. This is neutralisation. A base that dissolves in water is called an alkali, for example sodium hydroxide.
The salt depends on the acid used:
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hydrochloric acid gives a chloride
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sulfuric acid gives a sulfate
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nitric acid gives a nitrate
Behind the word equation, the $\mathrm{H^+}$ from the acid joins with the hydroxide ion $\mathrm{OH^-}$ from the alkali to make water. That is why the pH moves towards 7.
Bases that are metal carbonates, such as calcium carbonate, also neutralise acids. They give a salt and water, and also carbon dioxide:
$$\text{acid} + \text{metal carbonate} \to \text{salt} + \text{water} + \text{carbon dioxide}$$Name the products formed when an acid reacts with an alkali, and write the general word equation for neutralisation.
Name the products formed when an acid reacts with an alkali, and write the general word equation for neutralisation.
Write word equations and balanced symbol equations for: a) Hydrochloric acid reacting with sodium hydroxide. b) Sulfuric acid reacting with sodium hydroxide, giving…
Write word equations and balanced symbol equations for:
- a) Hydrochloric acid reacting with sodium hydroxide.
- b) Sulfuric acid reacting with sodium hydroxide, giving sodium sulfate and water.
Following a neutralisation with an indicator
Imagine adding sodium hydroxide a little at a time to $25\ \mathrm{cm^3}$ of hydrochloric acid with universal indicator in the flask. The alkali uses up the $\mathrm{H^+}$ ions, so the pH slowly rises.
Colours seen in the flask:
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Start: red (pH about 1, strong acid).
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As alkali is added: orange, then yellow.
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Neutralisation: green, pH 7.
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Past the point: blue, then purple, as the alkali is now in excess.
Notice the pH changes very quickly near pH 7, so the colour jumps from yellow through green to blue after just a drop or two.
A student adds dilute sodium hydroxide, a little at a time, to $25$ cm³ of dilute hydrochloric acid containing a few drops of universal indicator, and records the…
A student adds dilute sodium hydroxide, a little at a time, to $25$ cm³ of dilute hydrochloric acid containing a few drops of universal indicator, and records the colour.
- a) Describe the colour changes expected from start to finish.
- b) State the pH and colour at the point of neutralisation.
- c) Explain why the pH rises as alkali is added.
Neutralisation in everyday life
Neutralisation is useful when a liquid is too acidic.
Soil. Most crops grow best in soil close to neutral. In wet parts of Ireland, soil can become acidic. A farmer adds lime (calcium hydroxide, or crushed limestone, calcium carbonate). It reacts with the acid in the soil, so the pH rises towards 7.
Indigestion. Your stomach contains hydrochloric acid to help digest food. Too much causes discomfort. Antacid tablets contain a weak base such as magnesium hydroxide:
$$\mathrm{Mg(OH)_2 + 2HCl \to MgCl_2 + 2H_2O}$$A weak base is chosen because a strong alkali such as sodium hydroxide is corrosive and would damage the stomach lining. And the stomach must stay acidic: its acid kills bacteria and gives the right conditions for digestive enzymes, so it should not become too alkaline.
Acid rain. Burning fossil fuels in power stations and petrol or diesel in vehicles releases sulfur dioxide and nitrogen oxides. They dissolve in rainwater and form acids, giving rain with pH below 5.6. Acid rain lowers the pH of lakes, harming fish eggs, insects and plants, and it damages limestone buildings. Adding powdered limestone to a lake neutralises the acid:
$$\text{calcium carbonate + acid} \to \text{salt + water + carbon dioxide}$$Back to the start: the farmer in Co. Meath and the lake in Co. Wicklow both add an alkaline powder to neutralise acid and bring the pH up towards 7.
A farmer in Co. Meath finds that the soil in a field has a pH of $4.5$. Crops grow poorly. a) State whether the soil is acidic or alkaline. b) Name a substance, such as…
A farmer in Co. Meath finds that the soil in a field has a pH of $4.5$. Crops grow poorly.
- a) State whether the soil is acidic or alkaline.
- b) Name a substance, such as lime (calcium hydroxide or calcium carbonate), that can be added to the soil, and explain what happens.
- c) Predict whether the pH after treatment would be higher or lower than $4.5$.
Antacid tablets containing magnesium hydroxide are taken to relieve indigestion caused by excess hydrochloric acid in the stomach. a) Write a balanced symbol equation…
Antacid tablets containing magnesium hydroxide are taken to relieve indigestion caused by excess hydrochloric acid in the stomach.
- a) Write a balanced symbol equation for the reaction.
- b) Explain why the tablet is a better choice than a strong alkali such as sodium hydroxide.
- c) Explain why the pH of the stomach should not become too high.
A lake in Co. Wicklow is affected by acid rain, with a measured pH of $4.8$. a) Describe the sources of the gases that cause acid rain. b) Explain how adding powdered…
A lake in Co. Wicklow is affected by acid rain, with a measured pH of $4.8$.
- a) Describe the sources of the gases that cause acid rain.
- b) Explain how adding powdered limestone, calcium carbonate, to the lake would help, and write a word equation for the reaction between calcium carbonate and an acid.
- c) Explain two effects on aquatic life of the lake's low pH.