Water – JSS2 Intermediate Science Lesson Note (First Term)

Water is a colourless liquid made of hydrogen and oxygen, found in rain, rivers, wells and seas. About 70% of the Earth’s surface is covered by water, but not all of it is easy to use.

Behavioural objectives

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

1. Name the natural and treated sources of water.
2. State the physical properties of pure water.
3. State the chemical properties of water and its formula.
4. Describe the four stages of the water cycle in order.
5. Distinguish between evaporation and condensation.

What are the sources of water?

Water comes from two groups of sources: natural sources and treated water.

Natural sources of water

• Rain water: the purest natural water before it touches the ground, but it picks up dust and smoke as it falls.
• Surface water: water in rivers, lakes and streams, where most animals drink, but it often contains dirt and germs.
• Ground water: water that has soaked into the soil, obtained from wells and boreholes.
• Sea or ocean water: the largest source of water, but it is salty and cannot be used for drinking or farming without special treatment.

Ground water is usually cleaner because the soil acts like a natural filter.

Treated water

• Pipe-borne water: water supplied by the Water Board through pipes to homes.
• Treatment: it is filtered and treated with chemicals such as chlorine to make it safe to drink.

What are the physical properties of water?

Physical properties are the features of water that we can observe or measure without changing it into another substance.

• Appearance: pure water is colourless, odourless and tasteless.
• Impure signs: brown water, a smell like rotten eggs or a sour taste show that water is not pure.
• Boiling point: pure water boils at 100°C at sea level.
• Freezing point: pure water freezes to ice at 0°C.
• Density: water expands when it freezes, so ice is less dense than water and floats.
• Solvent: water dissolves more substances than any other common liquid, such as sugar, salt and soap.

Common mistake

• Taste of bottled water: the taste comes from minerals in it.
• Pure water: truly pure water has no taste at all.

What are the chemical properties of water?

Chemical properties describe how water behaves when it meets other substances.

• Neutral: pure water is neither an acid nor a base, and has a pH of 7.
• Reaction with sodium: sodium reacts violently with water and gives off hydrogen gas.
• Reaction with iron: iron reacts slowly with water and oxygen in the air to form rust.
• Composition: each molecule of water has two atoms of hydrogen and one atom of oxygen.
• Formula: the chemical formula of water is H₂O.

What is the water cycle?

The water cycle is the continuous movement of water between the Earth’s surface and the air, so the same water is used again and again.

What are the stages of the water cycle?

1. Evaporation: the sun heats oceans and rivers, and the water turns into invisible water vapour that rises into the sky.
2. Condensation: higher up the air is colder, so the vapour turns back into tiny water droplets that gather to form clouds.
3. Precipitation: when clouds become heavy, the water falls to Earth as rain, snow or hail.
4. Collection: water flows into rivers and oceans or soaks into the ground as ground water, and the cycle begins again.

What is transpiration?

• Transpiration: the loss of water vapour from the leaves of plants.
• Link to the cycle: it adds more water vapour to the air, like evaporation.

What is the difference between evaporation and condensation?

• Evaporation: the change of liquid water into water vapour by heat.
• Condensation: the change of water vapour into liquid water droplets by cooling.

Evaluation

1. Why is sea water not suitable for drinking?
2. At what temperature does pure water boil at sea level?
3. What is the chemical formula for water, and what does it show?
4. Explain the difference between evaporation and condensation.
5. Why does ice float on liquid water?

Assignment

Draw the water cycle in your notebook. Label the four stages and write one sentence to explain each stage.