4.3 Neutralization
Summary: Have you ever had stomach ache? Did you take an indigestion tablet? Tags: science oxford-science student-book-7 chemistry Created: 2026-05-19 Last Updated: 2026-05-19
Key words After this topic, you will be able to:
- define a base and an alkali
- describe how pH changes in Neutralization Reactions
- give examples of useful neutralization reactions.
Think back
- What is a chemical reaction?
- What is an indicator?
- What is the pH of a neutral solution? Key idea A base is a substance that neutralizes an acid. An alkali is a type of base that dissolves in water. Key words neutralization, base, alkali
Have you ever had stomach ache? Did you take an indigestion tablet?
What is a neutralization reaction?
Excess (extra) stomach acid makes your stomach hurt. If you take an indigestion tablet, there is a chemical reaction in your stomach. The chemical reaction removes the excess stomach acid, making new substances as it does so.
The chemical reaction of stomach acid with an indigestion tablet is an example of a neutralization reaction. In a neutralization reaction, an acid reacts with a substance that cancels it out. The pH gets closer to 7.
Which substances neutralize acids?
A base is a substance that neutralizes an acid. Look at Figure 1 – bases include sodium hydroxide, calcium oxide, and copper oxide. Some bases dissolve in water. A base that dissolves is an alkali.
Figure 1Alkalis are bases that dissolve in water. Full text alternative to the content
A Describe one difference between a base and an alkali.
pH changes in neutralization reactions
Gwil has 10 cm3 of acid. He adds universal indicator. He compares the colour of the mixture to the indicator colour chart. The pH is 1. He writes this result in Table 1. Table 1 Gwil’s data. Volume of sodium hydroxide added in cm3
pH
0
1
1
2
2
2
3
2
4
3
5
4
6
5
7
7
Gwil adds 1 cm3 of sodium hydroxide solution to the acid. The pH increases. Gwil writes the new pH in the table. He continues to add sodium hydroxide solution. The pH gets closer to 7. The alkali is neutralizing the acid. Gwil stops adding alkali when the pH is 7. The mixture is neutral.
B Look at the data in Table 1. Describe how the pH changes in the neutralization reaction.
Maths skills Ralph has a solution. He adds acid to the solution. A pH instrument measures the pH. The instrument is attached to a data logger, which sends the data to a computer. The computer draws a graph of the data. Describe in detail what the graph shows.
Full text alternative to the content
How is neutralization useful?
Some soils are more acidic than others. Every plant has a favourite soil pH. You can see some examples in Table 2.
Anchali lives in Thailand. She has a farm and wants to grow tea. She tests the soil. Its pH is 4.5. The soil is too acidic to grow tea.
Anchali adds a base to the soil. The base neutralizes some of the acid in the soil. The soil pH increases to pH 5.0. It is now suitable for growing tea.
In some places, gases from burning coal make sulfur dioxide gas. The gas dissolves in rainwater to make acid rain. The rain falls in lakes. This makes the lakes more acidic. Some water animals and plants cannot live in these lakes.
Environmental organizations may add bases to acid lakes. This is happening in Figure 3. The pH of the lake water increases.
Figure 2Tea plants grow best in soil of pH 5.0 to 6.0.
Figure 3Adding a base to an acidic lake. Table 2 Different plants grow best in soils of different pH. Plant
Soil pH range that the plant grows best in
apple tree
5.0–6.8
cabbage
6.0–7.0
onion
6.0–6.5
tea
5.0–6.0
tomato
5.5–7.0
C Describe two situations in which neutralization reactions are useful.
Stretch zone
- Copy and complete the sentences. A base is a substance that neutralizes an blank. A base that dissolves in water is called an blank. An alkaline solution has a pH that is greater than blank.
Stretch zone
- Yana has an alkaline solution. Its pH is 14. She adds acid to make a neutral solution. Describe how the pH changes.
- Explain one use of a neutralization reaction.