Last reviewed: 2 November 2026
Updated November 2026. These Grade 9 notes on water hardness cover sub-strand 1.3 of Grade 9 Integrated Science: the physical properties of water, the difference between hard and soft water, the lather test, three ways of softening temporary hard water (boiling, adding washing soda and distillation), and the advantages and disadvantages of each type of water. There is a worked practical, a summary table and exam-style questions with answers at the end.
The scope is taken from the KICD Grade 9 Integrated Science curriculum design, which places Water Hardness under Strand 1.0, Mixtures, Elements and Compounds, with about 14 lessons. The design tells teachers to leave out ion exchange, so these notes do too.
On this page
- Physical Properties of Pure Water
- Grade 9 Notes on Water Hardness: Hard vs Soft Water
- The Lather Test: A Worked Practical
- Methods of Softening Temporary Hard Water
- Advantages and Disadvantages of Hard and Soft Water
- Exam-Style Questions With Answers
- Common Mistakes
- Keep Revising Grade 9 Integrated Science
- Frequently Asked Questions
Physical Properties of Pure Water
| Property | Pure water |
|---|---|
| Colour | Colourless |
| Odour (smell) | Odourless |
| Taste | Tasteless |
| Boiling point | 100 °C at sea level (lower at high altitude, such as in Nairobi) |
Water from rivers, boreholes, rain tanks and taps often differs from pure water because it carries dissolved substances. In class, learners compare samples by looking, smelling, tasting (only water from safe sources) and boiling them. Impurities raise the boiling point slightly, which links back to Grade 8 work on physical changes.
Grade 9 Notes on Water Hardness: Hard vs Soft Water
Soft water forms lather easily with soap. Hard water does not lather easily; instead the soap forms a grey, sticky scum. Hardness comes from dissolved calcium and magnesium compounds that the water picks up as it flows over or through rocks. Rain water is usually soft; borehole and spring water from limestone areas is often hard.
Water whose hardness can be removed by boiling is called temporarily hard. This is the type the Grade 9 design focuses on.
The Lather Test: A Worked Practical
- Put equal volumes (say 20 cm³) of rain water, tap water and borehole or river water into three labelled test tubes or bottles.
- Add the same small amount of soap solution to each, a few drops at a time, shaking after each addition.
- Count the drops needed to form a lather that lasts about 30 seconds.
- Record the results in a table. The sample that needs the fewest drops is the softest; the one that needs the most, or forms scum, is the hardest.
Sample result: rain water 4 drops, tap water 9 drops, borehole water 22 drops with scum. Conclusion: rain water is soft and the borehole water is hard. To make it a fair test, keep the volume of water, the soap solution and the shaking the same.
Methods of Softening Temporary Hard Water
1. Boiling
Boiling removes temporary hardness: the dissolved compounds break down and settle as a solid, which is the white “fur” or scale you see inside kettles and sufurias. Let the water cool and pour off the clear part. Boiling also kills germs, but it uses fuel.
2. Adding washing soda
Washing soda (sodium carbonate) reacts with the dissolved calcium and magnesium so that they settle out as solids and can no longer react with soap. It is cheap and is often added to laundry water, which is why the design links this topic to laundry in Agriculture and Nutrition.
3. Distillation
The water is boiled and the steam is cooled in a condenser; the pure water collected (the distillate) leaves the dissolved substances behind. Distillation gives very pure, soft water but uses a lot of energy, so it is used for laboratories, car batteries and medicine rather than for washing clothes.
Advantages and Disadvantages of Hard and Soft Water
| Hard water | Soft water | |
|---|---|---|
| Advantages | Pleasant taste, so often preferred for drinking; supplies calcium for strong bones and teeth; does not dissolve lead from old pipes as easily | Lathers easily, so saves soap; no scale in kettles, boilers and pipes; better for washing and laundry |
| Disadvantages | Wastes soap and forms scum; leaves scale (fur) that blocks pipes and wastes fuel in kettles and boilers; can leave clothes looking dull | Flat taste; lacks the calcium found in hard water; can dissolve lead from old lead pipes |
The design’s key inquiry question asks why hard water is preferred for drinking: answer with taste and calcium.
Exam-Style Questions With Answers
- State two physical properties of pure water. Answer: any two of colourless, odourless, tasteless, boils at 100 °C at sea level.
- A learner needs 25 drops of soap solution to lather sample A and 5 drops for sample B. Which sample is hard? Explain. Answer: sample A, because hard water needs more soap to form a lather.
- Give two methods of softening temporary hard water. Answer: any two of boiling, adding washing soda, distillation.
- Why do kettles used in some areas develop a white coating inside? Answer: boiling temporary hard water deposits solid scale (fur) on the kettle.
- Give one advantage of soft water in the home. Answer: it lathers easily, so less soap is used for washing.
Common Mistakes
- Saying hard water “has more germs”. Hardness is about dissolved minerals, not germs.
- Saying soft water lathers poorly. It is the other way round.
- Describing ion exchange in detail. It is outside the Grade 9 design.
Keep Revising Grade 9 Integrated Science
Water hardness is examined with the rest of Grade 9 science in KJSEA. Under the KNEC KJSEA regulations, Integrated Science has a 70-mark theory paper (905/1) and a 30-mark paper on practical skills (905/2), so practise describing experiments like the lather test step by step. Next, read the Grade 9 notes on waves, the Grade 9 Integrated Science revision guide and the junior school mixtures, elements and compounds revision.
Test yourself free on the Grade 9 CBC Challenge, sit a timed Grade 9 Integrated Science practice paper, or browse the Grade 9 notes hub.
Frequently Asked Questions
What is hard water in Grade 9 Integrated Science?
Hard water is water that does not form lather easily with soap because it contains dissolved calcium and magnesium compounds. Soap forms scum with it instead.
How can temporary hard water be softened?
By boiling it, by adding washing soda (sodium carbonate), or by distillation. The Grade 9 design covers these three methods and leaves out ion exchange.
Why is hard water preferred for drinking?
It has a pleasant taste and supplies calcium, which helps build strong bones and teeth.
How do you test whether water is hard or soft?
Add soap solution drop by drop to equal volumes of each sample and shake. The sample that needs the most soap to form a lasting lather, or forms scum, is the hardest.
Want marked practice? Every paper starts with free questions. A Day Pass (KSh 20) unlocks the full Grade 9 Integrated Science papers with worked answers for a day, and the Monthly Pass (KSh 500) opens every grade and subject for a month. Compare passes.