Acids, Bases and Salt – JSS2 Intermediate Science Lesson Note (First Term)

Acids, bases and salts are three groups of chemicals: acids taste sour, bases feel soapy, and an acid reacting with a base makes a salt and water.

Behavioural objectives

1. Define an acid, a base, an alkali, an indicator and a salt.
2. Give examples of organic and inorganic acids and of common bases.
3. State the properties of acids and bases.
4. Use litmus paper, the pH scale and natural indicators to identify acids and bases.
5. Write the word equation for neutralisation and give its uses in daily life.

What is an acid?

An acid is a substance that tastes sour and turns blue litmus paper red.

• Everyday example: the sharp taste of lime, lemon or unripe mango comes from acid.

What are the types of acid?

There are two types of acid: organic (natural) acids and inorganic (mineral) acids.

• Organic (natural) acids: found in plants and animals, usually weak and safe to eat in food.
• Inorganic (mineral) acids: made from minerals in the laboratory, very strong, and able to burn skin.

Examples of organic acids

• Citric acid: found in oranges and lemons.
• Acetic acid: found in vinegar.
• Lactic acid: found in sour milk and yoghurt.

Examples of inorganic acids

• Hydrochloric acid (HCl): used in industries. It is also found in the stomach, where it helps digestion.
• Sulphuric acid (H₂SO₄): used in car batteries.

What are the properties of acids?

• Taste: acids taste sour.
• Corrosive action: strong acids burn through cloth, metal and skin.
• Litmus test: acids turn blue litmus paper red (Acid = Red).

What is a base?

A base is a substance that feels soapy or slippery and turns red litmus paper blue.

• Alkali: a base that dissolves in water. Sodium hydroxide is an alkali.
• Insoluble bases: bases that do not dissolve in water, such as magnesium hydroxide, are not alkalis.

What are common examples of bases?

• Sodium hydroxide (caustic soda): used to make soap.
• Magnesium hydroxide: found in “Milk of Magnesia”, a medicine for stomach ache.
• Calcium hydroxide: known as slaked lime, used by builders and farmers.
• Potash: used in local cooking, for example to make abacha or soften beans.
• Baking soda: a weak base with a slightly bitter taste.

What are the properties of bases?

• Taste: bases taste bitter.
• Feel: bases feel slippery or soapy.
• Litmus test: bases turn red litmus paper blue (Base = Blue).

What is the difference between an acid and a base?

An acid is sour and turns blue litmus red. A base is bitter and slippery and turns red litmus blue.

What is an indicator?

An indicator is a substance that changes colour when it is dipped into an acid or a base.

• Why we need it: strong acids and bases are dangerous, so we must never taste them to identify them.

What are the types of indicator?

Three common indicators are litmus paper, universal indicator and natural indicators.

• Litmus paper: the most common laboratory indicator. Blue turns red in acid. Red turns blue in base.
• Universal indicator: a liquid or paper that shows how strong an acid or base is, using the pH scale.
• Natural indicators: made at home from hibiscus (zobo) or red cabbage. Their colour changes when lemon juice or soapy water is added.

What is the pH scale?

The pH scale is a number scale from 0 to 14 that shows how acidic or basic a substance is.

• 0 to 6: acidic. 0 is a very strong acid.
• 7: neutral, as in pure water.
• 8 to 14: basic or alkaline. 14 is a very strong base.

What is neutralisation?

Neutralisation is the reaction between an acid and a base that produces a salt and water.

1. An acid and a base are mixed.
2. They react and cancel each other’s effects.
3. A salt and water are formed.

• Word equation: Acid + Base → Salt + Water
• Example: hydrochloric acid + sodium hydroxide → sodium chloride + water

What is a salt?

A salt is a compound formed when the hydrogen of an acid is replaced by a metal.

• Sodium chloride (NaCl): common table salt.
• Calcium carbonate: a salt found in chalk and eggshells.
• Naming: hydrochloric acid gives chlorides, and sulphuric acid gives sulphates.

What is the importance of acids, bases and neutralisation in daily life?

1. In the body: an antacid is a base that neutralises excess stomach acid and relieves heartburn.
2. In farming: farmers add lime (a base) to soil that is too acidic for crops.
3. Tooth decay: bacteria in the mouth make acid that damages teeth. Toothpaste is a weak base that neutralises it.
4. Bee stings: a bee sting is acidic. A little baking soda (a base) can ease the pain.
5. Soap making: sodium hydroxide is used to make soap.

What are the safety rules for acids and bases?

• Never taste: do not taste any chemical in the laboratory.
• Never touch: strong acids and strong bases burn the skin.
• Spills: wash the skin with plenty of water and tell your teacher at once.
• Labels: read the label before you use any bottle.

Evaluation

1. What is the difference between the taste of an acid and a base?
2. A mystery liquid turns blue litmus paper red. Is it an acid or a base?
3. Name one organic acid found in fruits.
4. Complete this equation: Acid + Base → ______ + ______
5. A liquid has a pH of 3. State whether it is acidic, neutral or basic.

Assignment

Draw a table with two columns, “Acids” and “Bases”. List three examples of each from this note. Then explain how an antacid relieves heartburn.