Neutralization
In this topic you will:
- learn how to make a neutral solution
- learn why neutralisation is important.
Key words
- burette
- decay
- digest
- filtrate
- indigestion
- neutralisation
- neutralised
Mixing acids and alkalis
Acids and alkalis can cancel each other out. When you mix them together they react and make a neutral solution. This is called neutralisation. This is another chemical change. Neutrality is also a chemical property of a substance.
If you add too much acid to an alkali, it makes an acidic liquid.
If you add too little acid to an alkali, it stays as an alkaline liquid.
You can add the acid very slowly, adding a few drops at a time. This makes it easier to judge exactly when it becomes neutral.
Questions
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Universal indicator is green when the solution is neutral.
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A neutralisation reaction happens when an acid and an alkali are mixed.
Making a neutral solution
You can use a special piece of equipment called a burette to neutralise an alkali very accurately. You add universal indicator to the alkali in the flask.
Look at the three diagrams of a burette.
In the first diagram, the pH in the flask is about 13. As the acid is added, the pH becomes lower. The acid is added slowly. The flask is shaken each time some acid is added.
In the second diagram, 25 cm³ of acid has been added to the flask. The pH in the flask is now 7. The liquid is now neutral.
The acid has reacted with the alkali and neutralised it.
In the third diagram, a little more acid has been added to the flask. The pH in the flask is now about 6. The liquid is weakly acidic.
When this happens there is a chemical reaction and new substances are formed. If you use hydrochloric acid and sodium hydroxide (an alkali), these are the reactants. When they react together, the products that form are sodium chloride and water.
Reactants: hydrochloric acid + sodium hydroxide
Products: sodium chloride + water
Definition: Neutralisation
Neutralisation is a chemical reaction where an acid reacts with an alkali to form a neutral solution, producing salt and water as products.
Note
Universal indicator is used in titration because it shows a wide range of pH values. This makes it easier to identify exactly when a solution becomes neutral (pH 7).
Lab Safety Reminder
Always wear eye protection and handle acids and alkalis with care. Even weak solutions can be harmful if spilled or splashed.
Neutralisation in everyday life
Indigestion
Your stomach produces hydrochloric acid. This acid gives the stomach the right conditions to digest your food. When your stomach produces too much acid, you have indigestion. It can be very uncomfortable. There are many medicines that can help. They are all alkalis and they neutralise the acid. Sometimes these medicines are called antacids.
Note
Antacids are alkalis. They work by reacting with excess hydrochloric acid in the stomach, helping to relieve pain caused by indigestion.
Toothpaste
There are millions of bacteria in your mouth. These bacteria feed on the pieces of food left on your teeth. The bacteria produce acid when they feed. This acid damages your teeth and makes them decay. Toothpaste contains alkali and this helps to neutralise the acid.
Neutralising lakes
In some parts of the world there are harmful chemicals in the air that make the rain acidic. This acid rain damages trees and changes the pH of the lakes, rivers and ponds. The plants and animals that live in the lakes cannot live in acid conditions. Some countries drop alkalis into the lakes to neutralise the acid.
Example
In Sweden, powdered limestone is dropped into lakes using helicopters to raise the pH of acidified water and protect wildlife.
Growing crops
In some areas, the soil is very acidic and plants do not grow well. Farmers spread lime on the soil to neutralise the acid so that the plants can grow better.
Questions
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Toothpaste is alkaline to neutralize acids in the mouth that can cause tooth decay.
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The acid in your mouth is produced by bacteria when they break down sugars in food and drink.
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Alkaline substances are added to lakes to neutralize acid rain and restore the natural pH balance.
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Farmers spread lime (an alkaline substance) onto acidic soil to neutralize the acidity and improve crop growth.
Think Like a Scientist
Testing the pH of the Soil
In this task you will test a soil sample to find the pH.
You will need: 2 test tubes • a beaker of water • a filter funnel • filter paper • universal indicator • a sample of soil
Step 1. Take a sample of soil in a test tube and add some water.
Step 2. Shake the tube.
Step 3. Filter the mixture in the tube.
Step 4. Add a few drops of universal indicator to the filtrate. (The filtrate is the liquid that comes through the filter paper.)
Step 5. Record your results.
Question
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Different plants prefer different pH levels. For example, blueberries grow well in acidic soils (pH < 6), while cabbage prefers neutral to alkaline soils (pH 6.5–7.5).