Elements, Compounds and Mixture – JSS2 Intermediate Science Lesson Note (First Term)

Matter is made up of elements, compounds and mixtures: an element has one type of atom, a compound has different elements joined chemically, and a mixture has substances that are not chemically joined.

Behavioural objectives

By the end of the lesson, students should be able to:

1. Define an element, a compound and a mixture.
2. Give at least three examples of each from everyday life.
3. Write the symbols of common elements and the formulae of water and salt.
4. State the differences between elements, compounds and mixtures.
5. Name simple methods of separating mixtures.

What is an element?

An element is a substance that cannot be broken down into anything simpler by chemical means.

• Made of one type of atom: every atom in a pure sample of the element is the same kind.
• Gold example: a bar of pure gold, cut again and again, is still gold down to a single gold atom.
• Building blocks: elements are like the letters of the alphabet, and they build every other substance.

What are some common examples of elements?

• Oxygen: the gas we breathe to stay alive.
• Iron: used to make gates, bridges and car frames.
• Gold: used for jewellery because it stays shiny.
• Carbon: found in charcoal used for cooking and in diamonds.

What are the symbols of some elements?

Each element has a short symbol of one or two letters, and the first letter is always a capital.

What is a compound?

A compound is a substance formed when two or more different elements join together chemically.

• New substance: the compound has properties that are different from the elements that formed it.
• Fixed proportion: the elements always join in the same ratio, for example water is always H₂O.
• Chemical formula: a compound is written with the symbols of its elements, such as H₂O and NaCl.

How does water show that a compound is new?

• Hydrogen: a gas that catches fire easily.
• Oxygen: a gas that helps things burn.
• Water (H₂O): when hydrogen and oxygen join chemically, they form a liquid we use to put out fires.

What are other examples of compounds?

1. Common salt (sodium chloride, NaCl): made from a dangerous metal (sodium) and a poisonous gas (chlorine). Together they make food we eat daily.
2. Sugar: made from carbon, hydrogen and oxygen.
3. Carbon dioxide (CO₂): made from carbon and oxygen.

Can a compound be separated easily?

• Key rule: a compound cannot be separated by filtering or by picking it apart.
• Chemical reaction: only a chemical reaction can break a compound into its elements.

What is a mixture?

A mixture is made of two or more substances that are put together but are not chemically joined.

• Gari and groundnut: you can see both in the bowl, and neither has changed into a new substance.
• Any amount: the parts can be mixed in any proportion.
• Original properties: each part keeps its own properties.

What are the types of mixtures?

There are two types of mixtures: homogeneous and heterogeneous.

• Homogeneous: looks the same throughout, such as sugar dissolved in water.
• Heterogeneous: the different parts can easily be seen, such as sand mixed with beans.

What are examples of mixtures in daily life?

• Air: a mixture of nitrogen, oxygen and other gases.
• Seawater: a mixture of water, salt and other dissolved substances.
• Brass: a mixture of the metals copper and zinc.
• Soup and dough: mixtures of food materials.

How can mixtures be separated?

Mixtures are separated by physical methods, which do not need a chemical reaction.

What is the difference between an element, a compound and a mixture?

An element has one kind of atom, a compound has elements joined chemically, and a mixture has substances only mixed together.

Why are elements, compounds and mixtures important?

They make up the materials we use for health, industry, cooking and cleaning.

1. Health: iron is part of the haemoglobin in blood, which carries oxygen. The compound water keeps the body hydrated.
2. Industry: metals are mixed to make alloys such as brass and stainless steel. Stainless steel resists rusting.
3. Cooking: we use mixtures (soups, dough) and compounds (baking soda, salt) to prepare meals.
4. Cleaning: soap helps water and oil mix, so that oily dirt can be washed away.

Evaluation

1. Is air an element, a compound or a mixture? Give a reason.
2. Why is water a compound while salt water is a mixture?
3. Name one element used in your house and give its symbol.
4. Which method would you use to get salt from seawater?
5. State two differences between a compound and a mixture.

Assignment

Make a table with three columns headed Element, Compound and Mixture. List four items found in your home or kitchen under each heading, and give one reason for each choice.