Water And Water Pollution

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Chemical Tests for Water

Anhydrous cobalt(II) chloride (blue) turns pink in the presence of water, forming hydrated cobalt(II) chloride.
Anhydrous copper(II) sulfate (white) turns blue in the presence of water, forming hydrated copper(II) sulfate.
Cobalt chloride paper is commonly used to test for water: blue → pink.
These tests are reversible: heating the hydrated salt drives off water, restoring the anhydrous colour.

Cobalt chloride paper changes from blue to pink in the presence of water

Purity of Water

Pure water has a sharp boiling point of 100 °C and melting point of 0 °C.
Impurities raise the boiling point and lower the melting point of water.
Boiling/melting point data can be used to assess water purity.
Distilled water is produced by boiling water and condensing the steam, removing most dissolved impurities.
Distilled water is used in experiments to avoid interference from dissolved ions.

Water Treatment: Sedimentation and Filtration

Untreated water contains insoluble impurities (soil, plant matter) and soluble impurities (dissolved calcium, metal compounds).
First, water is left to stand in sedimentation tanks so that heavy particles settle by gravity.
The water is then filtered through sand and gravel to remove smaller suspended particles.

Water Treatment: Carbon Filtration and Chlorination

Filtration through carbon (charcoal) removes unpleasant tastes and odours.
Chlorination kills bacteria and other microorganisms that are too small to be filtered.
Chlorine is carefully added to the water supply to prevent diseases such as cholera and typhoid.

Summary of Water Treatment Stages

Stage 1: Sedimentation and filtration – solids settle out, then the water is filtered through sand and gravel.
Stage 2: Carbon filtration – removes unpleasant tastes and odours.
Stage 3: Chlorination – kills bacteria and other microbes.

The stages of treating water are sedimentation, filtration with carbon and chlorination

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1.Which of the following is the colour change observed when anhydrous cobalt(II) chloride is added to water?
Easy
- Ablue to pink
- Bpink to blue
- Cwhite to blue
- Dblue to white
2.What colour does anhydrous copper(II) sulfate turn in the presence of water?
Easy
- Awhite to blue
- Bblue to white
- Cwhite to pink
- Dpink to white
3.Pure water boils at exactly 100 °C and melts at exactly 0 °C.
Easy
True or false?
4.State one reason why distilled water is used in practical chemistry rather than tap water.
Easy
5.Complete the sentence about the test for water using anhydrous copper(II) sulfate.
Easy
Anhydrous copper(II) sulfate is ____ and turns ____ in the presence of water.
6.Match each stage of water treatment with its purpose.
Medium
- Sedimentation
- Filtration through carbon
- Chlorination
- Removes unpleasant tastes and odours
- Kills bacteria and microorganisms
- Removes mud, sand and other particles
7.Place the following stages of water treatment in the correct order.
Medium
- Chlorination
- Sedimentation and filtration
- Filtration through carbon
8.Give the chemical equation for the reaction of anhydrous cobalt(II) chloride with water.
Medium

Plus de questions

9.A sample of water boils at 101.5 °C. What is the approximate boiling point of pure water in °C?
Easy
- A90
- B110
- C100
- D120
10.Impurities in water increase its melting point and decrease its boiling point.
Medium
True or false?
11.Which of the following is NOT a step in the treatment of water for drinking?
Easy
- ASedimentation
- BDistillation
- CFiltration through carbon
- DChlorination
12.Name two bacterial diseases that can arise from consuming untreated water.
Medium
13.Anhydrous cobalt(II) chloride is blue and turns pink in the presence of water.
Easy
True or false?
14.Anhydrous copper(II) sulfate is blue and turns white in the presence of water.
Easy
True or false?
15.What is the colour of hydrated cobalt(II) chloride?
Easy
- ABlue
- BPink
- CWhite
- DGreen
16.Which substance is used to test for water by a colour change from white to blue?
Easy
- AAnhydrous cobalt(II) chloride
- BHydrated copper(II) sulfate
- CAnhydrous copper(II) sulfate
- DHydrated cobalt(II) chloride
17.A sample of water boils at 101.5 °C. What can be concluded about its purity?
Medium
- AIt is pure water.
- BIt contains impurities.
- CIt has a lower boiling point than pure water.
- DIt is distilled water.
18.Why is distilled water used in practical chemistry instead of tap water?
Easy
- AIt is cheaper.
- BIt contains fewer chemical impurities.
- CIt has a higher boiling point.
- DIt is easier to obtain.
19.Anhydrous cobalt(II) chloride is blue. What colour does it turn in the presence of water?
Easy
- APink
- BWhite
- CRed
- DGreen
20.Which of the following are correct statements about the test for water? (Select all that apply.)
Medium
- AAnhydrous copper(II) sulfate turns from white to blue in the presence of water.
- BAnhydrous cobalt(II) chloride turns from blue to pink in the presence of water.
- CAnhydrous copper(II) sulfate is blue and turns white when water is added.
- DHydrated cobalt(II) chloride is blue.
- EAnhydrous cobalt(II) chloride is pink.
21.Match each water treatment stage with its purpose.
Medium
- Sedimentation
- Filtration through carbon
- Chlorination
- Removes unpleasant tastes and odours
- Kills bacteria and other microorganisms
- Allows solids to settle
22.Place the following water treatment stages in the correct order from first to last.
Medium
- Filtration through carbon
- Chlorination
- Sedimentation and filtration to remove solids
23.Which of the following are bacterial diseases that can arise from consuming untreated water? (Select all that apply.)
Medium
- ACholera
- BTyphoid
- CMalaria
- DInfluenza
- EDengue fever
24.Arrange the following steps to correctly describe how distilled water is produced:
Medium
- Condense the vapour back to a liquid
- Heat water to form vapour
- Collect distilled water
25.A student measures the boiling point of a water sample and finds it to be 101.3 °C. Which of the following is the most likely conclusion?
Medium
- AThe water is pure.
- BThe water contains dissolved impurities.
- CThe water is distilled water.
- DThe thermometer is faulty.
26.Impurities in water decrease its melting point.
Easy
True or false?
27.Match each water treatment stage with its primary purpose.
Medium
- Sedimentation
- Filtration through carbon
- Chlorination
- Removes unpleasant tastes and odours
- Kills bacteria and other microorganisms
- Removes mud, sand, and other particles
28.Arrange the following stages of water treatment in the correct order, starting with the first stage.
Medium
- Chlorination
- Filtration through carbon
- Sedimentation and filtration
29.A student adds anhydrous cobalt(II) chloride to two different liquids. In liquid X, the cobalt(II) chloride turns pink. In liquid Y, it stays blue. Which of the following is correct?
Medium
- ALiquid X contains water; liquid Y does not.
- BLiquid X does not contain water; liquid Y does.
- CBoth liquids contain water.
- DNeither liquid contains water.
30.Which of the following is the correct equation for the reaction of anhydrous cobalt(II) chloride with water?
Medium
- ACoCl₂ + 6H₂O → CoCl₂·6H₂O
- BCoCl₂ + 5H₂O → CoCl₂·5H₂O
- CCoCl₂ + H₂O → CoCl₂·H₂O
- DCoCl₂ + 2H₂O → CoCl₂·2H₂O
31.A student tests a colourless liquid with anhydrous copper(II) sulfate. The solid turns blue. Which conclusion is correct?
Easy
- AThe liquid contains water
- BThe liquid is pure water
- CThe liquid does not contain water
- DThe liquid contains dissolved ions
32.In an experiment to determine the purity of a water sample, a student measures its boiling point as 101.2 °C. Which of the following is the best conclusion?
Medium
- AThe sample is pure water
- BThe sample contains dissolved impurities
- CThe sample contains insoluble impurities
- DThe sample is distilled water
33.Which of the following are purposes of the stages in water treatment? (Select all that apply.)
Medium
- ASedimentation removes large insoluble particles
- BFiltration through carbon removes dissolved ions
- CChlorination kills bacteria and other microorganisms
- DFiltration through sand removes small insoluble particles
- EChlorination removes unpleasant tastes and odours
34.Anhydrous cobalt(II) chloride turns from blue to pink in the presence of water.
Easy
True or false?
35.Place the following stages of water treatment in the correct order, starting with the first stage.
Medium
- Chlorination
- Sedimentation
- Filtration through carbon
36.A student adds a few drops of a liquid to anhydrous cobalt(II) chloride paper. The paper remains blue. The student then adds the same liquid to anhydrous copper(II) sulfate, which remains white. Which statement is correct?
Hard
- AThe liquid contains pure water
- BThe liquid does not contain water
- CThe liquid contains dissolved impurities but no water
- DThe test results are contradictory
37.What is the colour of anhydrous cobalt(II) chloride?
Easy
- ABlue
- BPink
- CWhite
- DGreen
38.Which of the following is the correct colour change when anhydrous cobalt(II) chloride reacts with water?
Easy
- ABlue to pink
- BPink to blue
- CWhite to blue
- DBlue to white
39.A student adds a colourless liquid to anhydrous copper(II) sulfate. The solid remains white. What can be concluded?
Easy
- AThe liquid is pure water
- BThe liquid contains water
- CThe liquid does not contain water
- DThe liquid is an acid
40.Impurities in water increase its melting point.
Easy
True or false?
41.Which of the following are correct statements about the test for water using anhydrous copper(II) sulfate? (Select all that apply.)
Medium
- AAnhydrous copper(II) sulfate is white.
- BAnhydrous copper(II) sulfate turns blue in the presence of water.
- CHydrated copper(II) sulfate is white.
- DThe test is reversible.
42.A student measures the boiling point of a sample of water and finds it to be 102.3 °C. What does this indicate about the sample?
Medium
- AIt is pure water.
- BIt contains dissolved impurities.
- CIt is distilled water.
- DIt contains only insoluble impurities.
43.Distilled water contains fewer dissolved impurities than tap water.
Easy
True or false?
44.Match each stage of water treatment with its primary purpose.
Medium
- Sedimentation
- Filtration through carbon
- Chlorination
- Removes unpleasant tastes and odours
- Kills microorganisms
- Removes large insoluble particles
45.A student heats a sample of impure water and records its boiling point as 102.4 °C. Pure water boils at 100.0 °C. What is the increase in boiling point in °C?
Easy
46.State one reason why distilled water is used in chemistry experiments instead of tap water.
Medium
47.A sample of water is found to have a boiling point of 100.9 °C. The boiling point of pure water is 100.0 °C. The increase is due to dissolved salt. If each gram of salt per litre raises the boiling point by 0.5 °C, what is the concentration of salt in g/L?
Hard
48.A sample of water is found to have a boiling point of 101.2 °C. What is the increase in boiling point compared to pure water?
Easy
49.Complete the word equation for the reaction of anhydrous cobalt(II) chloride with water.
Medium
anhydrous cobalt(II) chloride + water ⇌ ____
50.The melting point of a water sample is measured as -2.5 °C. Pure water melts at 0 °C. What is the decrease in melting point?
Easy
51.Complete the sentence about the purpose of chlorination in water treatment.
Easy
Chlorine is added to water to kill ____.
52.Describe how you would determine whether a sample of water is pure using melting point data.
Medium
53.Describe how you would determine whether a sample of water is pure using boiling point data.
Medium
54.Explain why distilled water is preferred over tap water when carrying out a chemical test that involves a colour change.
Medium
55.A student heats a sample of impure water and records its boiling point as 102.4 °C. Pure water boils at 100.0 °C. What is the increase in boiling point in °C?
Easy
- A4.4 °C
- B1.4 °C
- C3.4 °C
- D2.4 °C
56.A sample of water is found to have a boiling point of 100.9 °C. The boiling point of pure water is 100.0 °C. The increase is due to dissolved salt. If each gram of salt per litre raises the boiling point by 0.5 °C, what is the concentration of salt in g/L?
Hard
- A2.8 g/L
- B0.8 g/L
- C1.8 g/L
- D3.8 g/L
57.A sample of water is found to have a boiling point of 101.2 °C. What is the increase in boiling point compared to pure water?
Easy
- A3.2 °C
- B2.2 °C
- C1.2 °C
- D0.2 °C
58.The melting point of a water sample is measured as -2.5 °C. Pure water melts at 0 °C. What is the decrease in melting point?
Easy
- A4.5 °C
- B2.5 °C
- C1.5 °C
- D3.5 °C

Cartes mémo

What is the chemical test for water using anhydrous cobalt(II) chloride?

Anhydrous cobalt(II) chloride (blue) turns pink in the presence of water, forming hydrated cobalt(II) chloride.

What is the chemical test for water using anhydrous copper(II) sulfate?

Anhydrous copper(II) sulfate (white) turns blue in the presence of water, forming hydrated copper(II) sulfate.

What colour is anhydrous cobalt(II) chloride?

Blue.

What colour is hydrated cobalt(II) chloride?

Pink.

What colour is anhydrous copper(II) sulfate?

White.

What colour is hydrated copper(II) sulfate?

Blue.

How can the purity of water be tested?

By measuring its boiling point or melting point. Pure water boils at 100°C and melts at 0°C; impurities raise the boiling point and lower the melting point.

Why is distilled water used in practical chemistry instead of tap water?

Distilled water contains fewer chemical impurities that could interfere with chemical reactions and affect results.

How is distilled water produced?

By heating water to form vapour and then condensing it back to a liquid, removing most dissolved impurities.

What are the three main stages of water treatment?

1) Sedimentation and filtration to remove solids, 2) Filtration through carbon to remove tastes and odours, 3) Chlorination to kill microbes.

What is the purpose of sedimentation in water treatment?

To allow mud, sand and other particles to settle to the bottom by gravity, forming a layer of sediment.

What is the purpose of filtration through carbon in water treatment?

To remove unpleasant tastes and odours from the water.

What is the purpose of chlorination in water treatment?

To kill bacteria and other microorganisms that are too small to be trapped by filters.

Name two bacterial diseases that can arise from consuming untreated water.

Cholera and typhoid.

What are insoluble impurities in untreated water?

Soil, pieces of plants and other organic matter.

What are soluble impurities in untreated water?

Dissolved calcium, metallic compounds and inorganic pollutants.

What is the colour change when anhydrous cobalt(II) chloride is added to water?

Blue to pink.

What is the colour change when anhydrous copper(II) sulfate is added to water?

White to blue.

What is the boiling point of pure water?

100°C.

What is the melting point of pure water?

0°C.

How do impurities affect the boiling point of water?

Impurities increase the boiling point, so impure water boils above 100°C.

How do impurities affect the melting point of water?

Impurities decrease the melting point, so impure water melts below 0°C.

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