Grade 9 Integrated Science Notes: Water Hardness (Hard and Soft Water, Causes and Removal)

Grade 9 Integrated Science · Strand 1.0 Mixtures, Elements and Compounds · Sub-strand 1.3 Water Hardness

These are free study notes for Grade 9 learners. They follow the official KICD curriculum design for Grade 9 Integrated Science (the same topics, in the same order, that your teacher uses) and are taken from our full Grade 9 Integrated Science notes. Many homes in Kenya use borehole water, which is often hard. Notice the kettle scale and soap that will not lather: this topic explains both.

Some water lathers easily with soap; some leaves a grey scum and fills kettles with white scale. The difference is hardness. In this topic we study the physical properties of water, distinguish hard from soft water, learn how to soften hard water, and weigh the advantages and disadvantages of each.

Physical properties of water

Sources of water: rain, rivers, lakes, wells, boreholes, springs and tap water. Pure water is colourless, odourless and tasteless and boils at 100 °C at sea level (a little lower at high altitude, as in much of Kenya). Dissolved substances raise the boiling point slightly.

Hard and soft water

The lather test

1. Put 5 cm³ of each water sample in separate test tubes.
2. Add soap solution drop by drop, shaking after each drop.
3. Stop when a lather lasts about 30 seconds; count the drops.
4. The more drops needed, the harder the water.

Temporary and permanent hardness

The white scale inside kettles is the solid left behind when temporary hardness is removed by boiling.

Softening hard water

Advantages and disadvantages

Hard water

Soft water

Test yourself: multiple choice

1. Which water lathers most easily with soap?

   1. Borehole water
   2. Rain water
   3. Sea water
   4. Spring water in limestone areas
   Show the answer

   B: Rain water is soft.

2. Hardness of water is caused by dissolved salts of

   1. sodium and potassium.
   2. iron and copper.
   3. calcium and magnesium.
   4. zinc and tin.
   Show the answer

   C: Calcium and magnesium salts cause hardness.

3. Which method removes both temporary and permanent hardness?

   1. Boiling
   2. Adding washing soda
   3. Filtering
   4. Cooling
   Show the answer

   B: Washing soda removes both.

4. The white scale inside a kettle comes from

   1. temporary hardness.
   2. permanent hardness.
   3. soap.
   4. rust.
   Show the answer

   A: Boiling removes temporary hardness as scale.

5. One disadvantage of soft water is that it

   1. wastes soap.
   2. forms scale.
   3. blocks pipes.
   4. can dissolve lead from old pipes.
   Show the answer

   D: Soft water dissolves lead more easily.

Practice questions

1. Describe the lather test for hard water. (4 marks)

   Show the answer

   Put equal volumes of samples in test tubes; add soap solution drop by drop, shaking; count drops until a lasting lather forms; more drops means harder water.

2. Distinguish between temporary and permanent hardness. (4 marks)

   Show the answer

   Temporary hardness is caused by hydrogencarbonates and removed by boiling; permanent hardness is caused by sulphates and chlorides and is not removed by boiling.

3. Give two advantages and two disadvantages of hard water. (4 marks)

   Show the answer

   Advantages: calcium for bones; good taste. Disadvantages: wastes soap; forms scale.

4. Why is distilled water not used for washing clothes on a large scale? (2 marks)

   Show the answer

   Distillation uses a lot of energy, so it is too expensive.