Creating Neutral Solutions

• In this topic, you will learn how to make a neutral solution and why neutralisation is important.

Mixing Acids and Alkalis

• Acids and alkalis cancel each other out when mixed, reacting to form a neutral solution; this process is called neutralisation.
• Neutrality is a chemical property of a substance.
• Adding too much acid to an alkali makes the liquid acidic.
• Adding too little acid to an alkali leaves the liquid alkaline.
• Gradual addition of acid, drop by drop, helps judge when the solution becomes neutral.

Making a Neutral Solution

• A burette is a special piece of equipment used to accurately neutralise an alkali.
• A universal indicator is added to the alkali in the flask.

Burette Diagrams

• In the initial diagram, the pH in the flask is ~13.
• As acid is added, the pH decreases.
• The flask is shaken after each acid addition.
• After adding 25 cm³ of acid, the pH is 7, indicating a neutral solution.
• The acid reacts with the alkali, neutralising it.
• With a slight excess of acid, the pH drops to ~6, making the liquid weakly acidic.
• This indicates a chemical reaction where new substances form.
• Hydrochloric acid and sodium hydroxide react to produce sodium chloride and water.

Activity 8.2.1: Rainbow Neutralisation

• An activity to demonstrate the different colors shown by universal indicator solution.
• Wear safety glasses.
• A test tube is fixed into a clamp stand.
• A crystal of washing soda is placed at the bottom of the test tube.
• Add water until the tube is two-thirds full.
• Add a few drops of a universal indicator.
• Carefully pour some acid on the top of the test tube, without shaking.
• The tube is left to stand for several days.

Rainbow Neutralisation Results

• At the top, the acid turns the indicator red, showing a strongly acidic environment.
• As the acid particles move down, they mix with water, turning the indicator yellow (weakly acidic).
• In the middle, the acid and washing soda solution react, and the indicator turns yellow (neutral solution).
• At the bottom, the washing soda dissolves, turning the indicator purple/dark blue (strong alkali).
• As washing soda particles move up, they mix with more water, turning the indicator a lighter blue (weakly alkaline).

Neutralisation in Everyday Life: Indigestion

• The stomach produces hydrochloric acid, which aids in digesting food.
• Too much acid leads to indigestion, causing discomfort.
• Medicines, known as antacids, are alkalis that neutralize the excess acid.

Neutralisation in Everyday Life: Toothpaste

• Bacteria in the mouth consume food particles left on teeth.
• This produces acid that damages teeth and causes decay.
• Toothpaste contains alkali to neutralize this acid.

Neutralisation in Everyday Life: Neutralising Lakes

• Harmful chemicals in the air can cause acidic rain.
• The acidic rain damages trees and alters the pH of lakes, rivers, and ponds.
• Acidic conditions are harmful to plants and animals in lakes.
• Some countries add alkalis to lakes to neutralize the acid.

Neutralisation in Everyday Life: Growing Crops

• Soil in some areas can be too acidic for plants to grow well.
• Farmers spread lime on the soil to neutralize the acid and improve plant growth.

Think Like a Scientist: Testing the pH of the Soil

• In this task, you will test a soil sample to find its pH.
• Materials needed: 2 test tubes, a beaker of water, a filter funnel, filter paper, universal indicator, and a soil sample.
• Take a soil sample, add water, and shake.
• Filter the mixture and add a few drops of universal indicator to the filtrate.
• Record your results.

Summary checklist

• Describe neutralisation as a change to a pH of 7.
• Explain how to make a neutral solution.
• Explain why neutralisation is important.