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Karnataka State Syllabus Class 8 Science Important Questions Chapter 13 Water
Question 1.
What is water?
OR
What are the elements which water is made up of? State their composition.
Answer:
Water is a chemical compound made up of hydrogen and oxygen. A molecule of water contains two atoms of hydrogen and one atom of oxygen which are chemically combined.
2 H2 (g) + O2 (g) → 2 H2O (l)
Question 2.
List the physical properties of water.
Answer:
Water is a colourless, tasteless and odourless liquid at room temperature. It exists in solid, liquid and vapour state on earth. It vapourizes at all temperatures. Pure water is a bad conductor of heat and electricity. It evaporates at all temperatures. It is an excellent solvent.
Question 3.
Water is tasteless, but we experience different tastes of water in different places. Why?
Answer:
Pure water is tasteless. However, the water we get from different sources is not pure. It contains dissolved substances which give them their taste.
Question 4.
Explain the importance of water.
Answer:
- Water is a wonderful substance. It is one of our most valuable resources.
- It is indispensable for life.
- Without water, life cannot exist on earth.
- All living beings need water for their survival.
- Bio-chemical reactions in our body cannot take place in the absence of water.
- Plants need water for photosynthesis.
- Most of the mass in our body is made of water.
- Water is a very important component of our food.
- Water is used for different purposes like agriculture, cooking, drinking, washing, etc.
Thus, water is a very important substance which is most essential for life.
Question 5.
Why does water extinguish fire although it contains both oxygen and hydrogen?
Answer:
Hydrogen is a combustible gas while oxygen is a supporter of combustion. But, water extinguishes fire although it is made up of hydrogen and oxygen. This is because water is neither a supporter of combustion nor a combustible substance.
Question 6.
Give two reasons to prove that water is a compound and not an element.
Answer:
- Water is made by the chemical union of two elements namely hydrogen and oxygen.
- Water is made of two kinds of atoms which proves that it is a compound. Elements are made up of only one kind of atoms.
Question 7.
What is the chemical formula of water? What is its molecular mass? Draw the structure of water molecule.
Answer:
The chemical formula of water is H2O. Its molecular mass is 18.
Question 8.
Why is water called a universal solvent?
Answer:
Water is capable of dissolving in it more substances than any other solvent. This is why water is called a universal solvent.
Question 9.
Why is sea water saline?
OR
Sea water is salty. Why?
Answer:
Sea water contains plenty of dissolved salts in it. This is why it is salty or saline.
Question 10.
Dip a piece of blue litmus and a red litmus paper in pure water. What happens? What inference can you draw from your observation?
Answer:
When we dip pieces of blue litmus and red litmus paper in pure water, the red litmus remains red and the blue litmus remains blue. This shows that pure water is neither acidic nor basic. It is neutral.
Question 11.
Describe the reaction between water and sodium metal. Write a balanced chemical equation to represent this reaction.
Answer:
When we drop a small piece of sodium metal into tap water taken in a trough, sodium reacts vigorously with water forming sodium hydroxide and hydrogen.
Question 12.
Sodium metal should be preserved in kerosene. Give reason.
Answer:
Sodium is a highly reactive metal. It can react with oxygen and water vapours present in air. However, kerosene docs not react with sodium chemically. Therefore, sodium is preserved in kerosene.
Question 13.
Describe the reaction between water and potassium metal. Write a balanced chemical equation to represent this reaction.
Answer:
Potassium reacts violently w ith water forming potassium hydroxide and hydrogen gas.
Question 14.
Name some of the metals which do not react with water under normal conditions.
Answer:
Metals like copper, aluminium, silver, gold and tin do not react with water at ordinary’ temperature.
Question 15.
Explain the reaction of water with calcium.
Answer:
Water reacts with calcium forming calcium hydroxide and hydrogen gas.
Question 16.
Explain the reaction of water with magnesium metal. Write a balanced chemical equation to represent the reaction.
Answer:
Boiling water reacts with magnesium metal forming magnesium hydroxide and hydrogen gas.
Question 17.
Explain the reaction of water on metals like zinc, iron and magnesium.
Answer:
When steam is passed over hot metals like zinc, iron and magnesium, we get their respective oxides and hydrogen gas.
Question 18.
What happens when steam is passed over red hot iron? Write a balanced chemical equation to represent this reaction.
Answer:
When steam is passed over red hot iron, we get iron oxide and hydrogen gas.
3 Fe + 4 H2O → Fe3O4+ 4 H2↑
Question 19.
How does water react with a non-metal such as coke? Write a balanced chemical equation to represent the reaction.
Answer:
When steam is passed over red hot coke, we get a mixture of carbon monoxide and hydrogen gas. This mixture is called water gas.
Question 20.
What is water gas? How is it prepared? List any two uses of water gas.
Answer:
Water gas is a mixture of carbon monoxide and hydrogen. It is prepared by passing steam over red hot coke. Water gas is used as an industrial fuel. It is used in the manufacture of hydrogen by Bosch process.
Question 21.
How does water react with silicon? Give an equation for this reaction.
Answer:
Water reacts with silicon when steam is passed over red hot silicon. This reaction produces silicon dioxide and hydrogen gas.
Question 22.
Explain the reaction of water with a non-metallic oxide such as carbon dioxide. Write a balanced equation for this reaction.
Answer:
Carbon dioxide dissolves in water forming an acid called carbonic acid.
Question 23.
What is the acid present in soft drinks?
Answer:
Soft drinks contain carbonic acid which gives them a tangy taste.
Question 24.
What is acid rain? How is it caused?
Answer:
Rainfall made sufficiently acidic by atmospheric pollution is called acid rain. It is caused by excessive release of oxides of nitrogen, carbon and sulphur into the atmosphere. These gases react with water drops and form their respective acids which will come down with rain.
Question 25.
Name two compounds that can cause acid rain.
Answer:
Two compounds that can cause acid rain are nitrogen dioxide and sulphur dioxide.
Question 26.
How does water react with metallic oxides like calcium oxide? Give a balanced chemical equation for the reaction.
Answer:
Metallic oxides are basic in nature. Water reacts with metallic oxides to form their respective hydroxides. For instance, water reacts with calcium oxide to form calcium hydroxide.
Question 27.
Name the products formed when water reacts with the following oxides
- sulphur dioxide
- potassium oxide
- carbon dioxide
- calcium oxide.
Write balanced equations for each of the reactions.
Answer:
1. Water reacts with sulphur dioxide to form sulphurous acid.
SO2 +H2O → H2SO3
2. Water reacts with potassium oxide to form potassium hydroxide.
K2O + H2O → 2 KOH
3. Water reacts with carbon dioxide to form carbonic acid.
CO2 + H2O → H2CO3
4. Water reacts with calcium oxide to form calcium hydroxide.
CaO + H2O → Ca(OH)2
Question 28.
What is deliquescence? Explain with an example.
Answer:
Property of certain water-soluble substances to absorb moisture from the atmosphere and dissolve in it to form a solution is known as deliquescence. Substances which exhibit this properly are called deliquescent substances.
For example, potassium nitrate becomes wet when exposed to air. The property of potassium nitrate to absorb water present in the atmosphere is an example of deliquescence.
Question 29.
Give four examples of deliquescent substances.
Answer:
Magnesium chloride, solid sodium hydroxide, solid potassium hydroxide, anhydrous calcium chloride, calcium nitrate and ferric chloride are some compounds having deliquescent property.
Question 30.
Dry sodium hydroxide is considered to be a deliquescent substance. Give reason.
Answer:
Dry sodium hydroxide absorbs moisture from the atmosphere and becomes wet when exposed to air. This is why dry sodium hydroxide is considered to be a deliquescent compound.
Question 31.
When calcium nitrate crystals are exposed to air, a solution is formed. Give reason.
Answer:
Calcium nitrate is a deliquescent substance. It absorbs moisture present in atmospheric air and dissolves in it forming a solution.
Question 32.
Why is deliquescence more common in coastal areas?
Answer:
The atmospheric air in coastal areas contains high percent of moisture. This is why deliquescent substances become wet more quickly than in dry areas.
Question 33.
What are desiccants? Give examples.
Answer:
Substances which remove moisture from the surrounding air are called desiccants. They are some-times referred to as drying agents. eg. Calcium chloride monohydrate, concentrated sulphuric acid.
Question 34.
Define the term efflorescence. Give an example for efflorescent substances.
Answer:
The property of certain hydrates to lose water when exposed to air is called efflorescence. Such chemical compounds are called efflorescent substances.
eg: Washing soda (sodium carbonate), sodium sulphate, etc.
Question 35.
Washing soda loses its crystal structure quickly. Give reason.
Answer:
Washing soda is an efflorescent substance. It loses water molecules when exposed to dry air. This is why it loses its crystal structure quickly.
Question 36.
What is the difference between deliquescent substances and desiccants?
answer:
A desiccant removes moisture from the surrounding air and keeps it. A deliquescent material pulls moisture out of the air until it becomes moist and ultimately dissolves in it.
Question 37.
How do you classify water based on its action with soap?
Answer:
On the basis of the way water reacts with soap, water is classified as
- Soft water and
- Hard water.
Question 38.
What is soft water? Give an example of soft water.
Answer:
Water that readily gives lather with soap is called soft water. Rain water is an example of soft water.
Question 39.
What is hard water?
Answer:
Water sample that does not give much lather with soap is called hard water.
Question 40.
What causes hardness of water?
OR
State the reasons for hardness of water.
Answer:
The hardness of water is due to certain salts dissolved in it. The main salts that cause hardness of water arc chlorides, sulphates and bicarbonates of calcium and magnesium.
Question 41.
List the salts that cause hardness in water.
Answer:
The salts that cause hardness in water are calcium bicarbonate, calcium sulphate, calcium chloride, magnesium bicarbonate, magnesium sulphate and magnesium chloride.
Question 42.
Name a salt that does not cause hardness in water.
Answer:
Calcium carbonate does not cause hardness because it is insoluble in water.
Question 43.
Why is hard water not suitable to be used in industrial boilers?
Answer:
Hard water contains dissolved salts of calcium and magnesium. These salts on boiling turn into insoluble salts and produce scales on the inner wall of the boilers. This will corrode the boiler, reducing its heating efficiency.
Hard water will narrow down the pipes of the boilers which may result in an explosion. Therefore, hard water is not suitable for industrial boilers.
Question 44.
Name any two methods of softening hard water.
Answer:
There are many methods of removing hardness of water. Two of the methods are
- By boiling
- By treating with washing soda.
Question 45.
When can we use boiling as a method for softening hard water?
Answer:
Hard water can be softened by boiling if the hardness is due to dissolved bicarbonates of calcium and magnesium.
Question 46.
How are certain types of hardness in water removed by boiling? Explain with suitable equations.
Answer:
We can soften water if the hardness is caused by either calcium bicarbonate or magnesium bicarbonate. These salts on boiling are converted into their respective carbonates which are insoluble in water. They float on water which can be separated by simple filtering.
Question 47.
With the help of suitable equations, explain how hardness of water can be removed by using washing soda.
Answer:
Washing soda is sodium carbonate. When washing soda is added to hard water, the dissolved salts that cause hardness are converted into their respective insoluble carbonates.
Similar reactions take place when hardness is caused by dissolved sulphates, chlorides and bicarbonates of magnesium.
The insoluble carbonates can be removed by filtration.
Question 48.
Why is water softened by using washing soda unfit for drinking?
Answer:
Water softened by using washing soda may contain unused washing soda and excess of sodium salts. This is why the water softened by using washing soda is not fit for drinking.
Question 49.
Soft water may be unfit for drinking. Give reason.
Answer:
Soft water may contain dissolved salts which do not cause hardness. The presence of such salts renders water unfit for drinking.
Question 50.
What is desalinisation?
Answer:
Desalinisation is an artificial process by which saline water is converted into fresh water.
Question 51.
Describe a simple activity to remove hardness of water by using washing soda.
Answer:
Take some hard water in a beaker. Add a small amount of washing soda to it. Stir it with a glass rod. Washing soda converts dissolved salts that cause hardness into their respective insoluble carbonates. Filter the contents of the beaker to get soft water.
Question 52.
What is the advantage of using washing soda for softening water as compared to boiling?
Answer:
1. Boiling does not remove hardness caused by dissolved sulphates and chlorides of calcium and magnesium. It only removes hardness caused by bicarbonates of calcium and magnesium. Addition of washing soda removes hardness caused by any of these salts.
2. Boiling is a highly expensive method of softening water. When water in large quantity has to be softened, it is more economical to use washing soda.
Question 53.
Describe an activity to show that water that contains dissolved magnesium sulphate is hard.
Answer:
Prepare a soap solution by dissolving 1 g soap in about 100 ml of distilled water. Now, take pure water in a test tube and dissolve a pinch of magnesium sulphate in it. Test for the hardness of water by using soap solution.
The sample of water containing magnesium sulphate does not give lather readily with soap solution. This shows that magnesium sulphate makes water hard.
Question 54.
How do you test whether a given sample of water is hard or soft?
Answer:
Take about 25 mL of the given sample of water in a test tube. Add about 1 g of soap powder to it and shake well. If the water gives free lather with soap powder readily, it is soft. If lather does not appear readily, then, the given sample of water is hard.
Multiple Choice Questions
Question 1.
The elements present in a molecule of water are
(a) hydrogen and carbon dioxide
(b) electrons and protons
(c) oxygen and hydrogen
(d) nitrogen, oxygen and hydrogen
Answer:
(c) oxygen and hydrogen
Question 2.
Magnesium sulphate crystals are popularly called
(a) Gypsum salt
(b) Epsom salt
(c) Green vitriol
(d) Alum
Answer:
(b) Epsom salt
Question 3.
When you buy washing soda from the market, you will get it in powder form because
(a) the crystals of washing soda quickly lose water molecules
(b) the shop keeper prefers to powder the crystals of washing soda
(c) washing soda does not form crystals
(d) washing soda is a drying agent.
Answer:
(a) the crystals of washing soda quickly lose water molecules
Question 4.
A sample of hard water is heated in a utensil. A white scale of calcium carbonate is formed. The likely salt dissolved in it is
(a) calcium chloride
(b) magnesium sulphate
(c) calcium hydroxide
(d) calcium bicarbonate
Answer:
(d) calcium bicarbonate
Question 5.
A salt that causes hardness in water and that liberates carbon dioxide on boiling is
(a) magnesium bicarbonate
(b) calcium chloride
(c) magnesium sulphate
(d) calcium sulphate
Answer:
(a) magnesium bicarbonate
Question 6.
The products formed when steam is passed over red hot iron are
(a) ferric hydroxide and hydrogen
(b) ferric oxide and hydrogen
(c) ferrous hydroxide and hydrogen
(d) ferrous oxide and hydrogen
Answer:
(b) ferric oxide and hydrogen
Question 7.
Sodium carbonate crystal loses water molecules on exposure to air. The related property is called
(a) deliquescence
(b) Fluorescence
(c) Efflorescence
(d) Luminescence
Answer:
(c) Efflorescence
Question 8.
Drinking water should be
(a) pure
(b) soft
(c) hard
(d) coloured
Answer:
(b) soft
Question 9.
The gas liberated when water reacts with sodium metal is
(a) water gas
(b) oxygen
(c) hydrogen
(d) carbon dioxide
Answer:
(c) hydrogen
Question 10.
Which one of the following is called universal solvent?
(a) kerosene
(b) milk
(c) sulphuric acid
(d) water
Answer:
(d) water
Question 11.
Crystal form of washing soda when exposed to air
(a) becomes hard
(b) becomes powder
(c) decomposes giving carbon dioxide
(d) becomes wet
Answer:
(b) becomes powder
Question 12.
Commonly found hard water contains the salts of the metal
(a) sodium
(b) potassium
(c) calcium
(d) lithium
Answer:
(c) calcium
Question 13.
Iron nails kept in a closed jar containing paper packets of calcium chloride do not rust because calcium chloride absorbs
(a) water vapour
(b) carbon dioxide
(c) air
(d) oxygen
Answer:
(a) water vapour
Question 14.
Which of the following is not an example for drying?
(a) Copper sulphate crystals lose their blue colour in concentrated sulphuric acid.
(b) During the preparation of chlorine gas it is bubbled through concentrated sulphuric acid.
(c) When sugar crystals are immersed in concentrated sulphuric acid, crystals get charred.
(d) During the preparation of sulphur dioxide gas, it is bubbled through concentrated sulphuric acid.
Answer:
(c) When sugar crystals are immersed in concentrated sulphuric acid, crystals get charred.
Question 15.
Though sodium nitrate and potassium nitrate are both oxidising agents, sodium nitrate is not used as oxidising agent because, it
(a) absorbs water vapour
(b) does not absorb water vapour
(c) forms sodium carbonate
(d) is not available easily
Answer:
(a) absorbs water vapour
Fill In The Blanks
(1) Chemical name of Epsom salt is magnesium sulphate
(2) A household chemical commonly used to make water soft is sodium carbonate
(3) The solution of calcium oxide in water turns red litmus to blue
(4) Water that does not give lather with soap readily is called hard water
(5) Hydrated oxide of iron is called rust
(6) A mixture of carbon monoxide and hydrogen is called water gas
(7) The gas produced by the decomposition of calcium bicarbonate is carbon dioxide
(8) Sodium carbonate is commonly known as washing soda
Match The Following
| A | B |
| 1. Causes scaling | (a) Calcium oxide |
| 2. Basic oxide | (b) Soap |
| 3. Test for hardness of water | (c) Sulphur dioxide |
| 4. Acidic oxide | (d) Calcium bicarbonate |
| (e) Neutral oxide | |
| (f) Does not react with water |
Answer:
1 – d, 2 – a, 3 – b, 4 – c