Maharashtra State Board
Standard IX – Science
Chapter 14 – Substances in Common Use
1. Fill in the blanks.
a. The number of molecules of water of crystallization in washing soda is _______
Ans: 10
Washing soda is chemically known as sodium carbonate decahydrate. Its formula is Na₂CO₃·10H₂O. This shows that one molecule of sodium carbonate is combined with ten molecules of water of crystallization.
b. The chemical name of baking soda is _______
Ans: Sodium bicarbonate (Sodium hydrogen carbonate).
The chemical formula of baking soda is NaHCO₃. It is a mild basic salt and is commonly used in baking and as an antacid.
c. _______ is used in treatment of hyperthyroidism.
Ans: Iodine-131
Iodine-131 is a radioactive isotope used in the treatment of hyperthyroidism. It helps in reducing the overactivity of the thyroid gland.
d. The chemical name of Teflon is _______
Ans: Polytetrafluoroethylene (PTFE)
Teflon is a synthetic polymer made from tetrafluoroethylene monomers. It is widely used for non-stick coatings.
2. Match the pairs.
| A | B |
|---|---|
| 1. Saturated brine | c. crystallization of salt |
| 2. Fused salt | a. sodium metal freed |
| 3. CaOCl₂ | d. oxidation of colour |
| 4. NaHCO₃ | b. basic salt |
Ans:
1 – c
2 – a
3 – d
4 – b
A saturated brine solution gives salt by crystallization.
Fused salt (molten NaCl) gives sodium metal during electrolysis.
Bleaching powder (CaOCl₂) is used for oxidation of colour.
Sodium bicarbonate (NaHCO₃) is a basic salt.
3. Write answers to the following
a. What is meant by radioactivity?
Ans:
(i) Elements with high atomic numbers such as uranium, thorium, and radium have the property of spontaneously emitting invisible, highly penetrating, and high-energy radiations. This property is called radioactivity.
(ii) A substance that shows this property is called a radioactive substance.
(iii) These radiations are of three types: alpha rays, beta rays, and gamma rays.
b. When is the nucleus said to be unstable?
Ans:
(i) The stability of a nucleus depends on the balance between the number of protons and neutrons present in it.
(ii) If there are too many neutrons or too many protons, this balance is disturbed.
(iii) As a result, the strong nuclear forces are not able to hold the nucleus firmly, and it becomes unstable.
(iv) An unstable nucleus emits radiation to become stable.
c. Which diseases are caused by artificial food colours?
Ans:
(i) Excessive consumption of food containing artificial colours may cause health problems.
(ii) Diseases like ADHD (Attention Deficit Hyperactivity Disorder) may affect children due to excessive consumption of foods with added food colours.
(iii) Some artificial colours may also cause allergies, skin problems, and stomach disorders.
d. Where in the industrial field is radioactivity used?
Ans:
Radioactivity is used in the industrial field in the following areas:
(A) Radiography:
(i) Internal cracks and defects in metal objects can be detected using gamma rays.
(ii) Isotopes such as cobalt-60 and iridium-192 are used for this purpose.
(iii) This method is used to detect flaws in metal work.
(B) Measurement of thickness, density, and level:
(i) It is necessary to maintain the required thickness in aluminium, plastic, and iron sheets during manufacturing.
(ii) A radioactive source is placed on one side of the sheet and a detector on the other side.
(iii) The radiation received varies according to the thickness of the sheet.
(C) Luminescent paint and radioluminescence:
(i) Radioactive substances such as radium, promethium, and tritium are used to make objects glow in the dark.
(ii) These are used in clock hands and certain instruments.
(D) Use in ceramic articles:
(i) Luminous colours are used to decorate ceramic tiles and utensils.
(ii) Earlier, uranium oxide was used in such paints.
e. Write down properties of Teflon.
Ans:
(i) Teflon is not affected by atmospheric conditions or chemicals.
(ii) Water and oil do not stick to Teflon-coated surfaces.
(iii) It can withstand high temperatures as its melting point is about 327°C.
(iv) Teflon-coated articles are easy to clean.
(v) It is a poor conductor of electricity.
f. What type of colours will you use to celebrate ecofriendly Rang Panchami? Why?
Ans:
(i) Artificial colours used during Rang Panchami may contain harmful chemicals such as mercury and lead.
(ii) These chemicals can cause skin irritation, asthma, allergies, and other health problems.
(iii) Therefore, eco-friendly colours made from natural sources should be used.
(iv) Natural colours can be prepared from beetroot, turmeric, spinach, forest flame flowers, and gulmohar flowers.
(v) These colours are safe for health and environment.
g. Why has the use of methods like Teflon coating become more common?
Ans:
(i) Teflon is resistant to chemicals and atmospheric conditions.
(ii) Water and oil do not stick to Teflon-coated surfaces.
(iii) It can withstand high temperatures.
(iv) Teflon-coated articles are easy to clean.
(v) It increases durability and prevents corrosion.
4. Give scientific explanation
a. Bleaching powder has the odour of chlorine.
Ans:
(i) When bleaching powder is exposed to air, it reacts with carbon dioxide present in air.
(ii) This reaction produces calcium carbonate and chlorine gas.
(iii) The released chlorine gas gives bleaching powder its characteristic smell.
Chemical equation:
CaOCl₂ + CO₂ → CaCO₃ + Cl₂↑
b. The hard water of a well becomes soft on adding washing soda to it.
Ans:
(i) Hard water contains chlorides and sulphates of calcium and magnesium.
(ii) When washing soda (Na₂CO₃) is added, it reacts with these salts.
(iii) Insoluble carbonates of calcium and magnesium are formed.
(iv) These settle down, and the water becomes soft.
Chemical equation:
MgCl₂ + Na₂CO₃ → MgCO₃↓ + 2NaCl
c. Soap forms a precipitate in hard water.
Ans:
(i) Soap reacts with calcium and magnesium salts present in hard water.
(ii) Insoluble salts of fatty acids are formed.
(iii) These insoluble salts form a precipitate called scum.
(iv) Therefore, lather is not formed easily.
d. The particles of powder are given an electric charge while spraying them to form the powder coating.
Ans:
(i) Powder coating is used to prevent rusting of metal objects.
(ii) The powder is sprayed using electrostatic spray deposition.
(iii) The powder particles are given an electric charge.
(iv) Due to electrostatic attraction, they stick uniformly to the metal surface.
(v) The object is then heated to form a strong and durable coating.
e. The aluminium article is used as an anode in the anodising process.
Ans:
(i) Aluminium naturally forms a thin protective oxide layer in air.
(ii) In anodising, this layer is made thicker using electrolysis.
(iii) The aluminium article is connected to the positive terminal and acts as the anode.
(iv) Oxygen released at the anode reacts with aluminium.
(v) A thick protective layer of aluminium oxide is formed.
f. When radiation from a radioactive substance is passed through an electric field, marks are found at three places on the photographic plate.
Ans:
(i) Radioactive substances emit three types of radiations: alpha, beta, and gamma rays.
(ii) Alpha rays are positively charged and deviate towards the negative plate.
(iii) Beta rays are negatively charged and deviate towards the positive plate.
(iv) Gamma rays are uncharged and do not deviate.
(v) Hence, three separate marks are observed.
g. A certain type of ceramic tiles are fixed on the outer layer of a space shuttle.
Ans:
(i) Space shuttles are covered with special ceramic tiles.
(ii) These tiles can withstand extremely high temperatures.
(iii) They act as heat insulators and protect the shuttle during re-entry into the atmosphere.
(iv) They prevent the aluminium body of the shuttle from melting.
5. Write answers to the following
a. Write about artificial food colours, the substances used in them and their harmful effects.
Ans:
(A) Artificial food colours and substances used in them:
(i) Artificial food colours are added to many food items available in the market to make them look attractive.
(ii) These colours are available in the form of powders, gels, and pastes.
(iii) They are used in ice creams, cold drinks, fruit juices, sauces, pickles, jams, jellies, sweets, and packaged food items.
(iv) Sometimes colours are also added to chilli powder, turmeric powder, and packaged meat to improve their appearance.
(v) Some commonly used artificial food colours are tartrazine and sunset yellow.
(B) Harmful effects of artificial food colours:
(i) Some artificial food colours may contain small amounts of harmful metals such as lead and mercury.
(ii) Continuous consumption of such food can cause serious health problems.
(iii) Children may suffer from ADHD (Attention Deficit Hyperactivity Disorder).
(iv) Artificial colours may also cause allergies, skin irritation, stomach disorders, and asthma.
(v) Therefore, natural food colours are safer and should be preferred.
b. What is meant by water of crystallization? Give examples of salts with water of crystallization, and their uses.
Ans: The fixed number of water molecules chemically combined with one molecule of a salt in its crystalline form is called water of crystallization. These water molecules give the salt its definite shape and structure.
Examples of salts containing water of crystallization:
(i) Alum – K₂SO₄·Al₂(SO₄)₃·24H₂O
(ii) Borax – Na₂B₄O₇·10H₂O
(iii) Epsom salt – MgSO₄·7H₂O
(iv) Barium chloride – BaCl₂·2H₂O
(v) Glauber’s salt – Na₂SO₄·10H₂O
(vi) Blue vitriol – CuSO₄·5H₂O
Uses of these salts:
(i) Alum:
(a) Used in purification of drinking water.
(b) Used in pickles to maintain crispness.
(c) Used in baking powder.
(d) Used as an after-shave.
(ii) Borax:
(a) Used as a cleaning agent.
(b) Used in making detergents.
(c) Used to keep away insects like ants.
(iii) Epsom salt:
(a) Used in medicinal bath soaks.
(b) Used to improve plant growth.
(iv) Barium chloride:
(a) Used in purification of brine.
(b) Used in manufacture of pigments.
(c) Used in heat treatment of metals.
(v) Glauber’s salt:
(a) Used as a drying agent in laboratories.
(b) Used as a mild laxative.
(vi) Blue vitriol:
(a) Used in Bordeaux mixture as a fungicide.
(b) Used in medical tests for anaemia.
c. Write briefly about the three methods of electrolysis of sodium chloride.
Ans: Electrolysis of sodium chloride can be carried out under different conditions.
(i) Electrolysis of molten sodium chloride:
When solid sodium chloride is melted at high temperature (above 800°C) and electrolysed, sodium metal is obtained at the cathode and chlorine gas is released at the anode.
(ii) Electrolysis of concentrated aqueous sodium chloride solution (brine):
Hydrogen gas is released at the cathode and chlorine gas at the anode. Sodium hydroxide remains in the solution.
(iii) Electrolysis of dilute sodium chloride solution:
Hydrogen gas is released at the cathode and oxygen gas at the anode.
Thus, different products are obtained depending on the conditions of electrolysis.
6. Write the uses
a. Anodizing
Ans:
(i) Anodizing is used to protect aluminium surfaces from corrosion.
(ii) It increases the thickness of the protective oxide layer on aluminium.
(iii) It is used in making utensils, window frames, and decorative articles.
b. Powder coating
Ans:
(i) Powder coating protects metal objects from rusting and corrosion.
(ii) It provides a hard, durable, and attractive finish.
(iii) It is widely used for outdoor metal furniture, vehicles, and appliances.
c. Radioactive substances
Ans:
(i) Used as fuel in nuclear reactors to generate electricity.
(ii) Used in detecting cancer and treating diseases like hyperthyroidism.
(iii) Used to measure thickness of metal and plastic sheets.
(iv) Used in luminescent paints to make objects glow in the dark.
(v) Used in food preservation and prevention of sprouting in onions and potatoes.
d. Ceramic
Ans:
(i) Used in making tiles, bricks, pottery, and terracotta items.
(ii) Special ceramics are used in jet engines and ships.
(iii) Used on outer surfaces of space shuttles as heat-resistant tiles.
7. Write the harmful effects
a. Artificial dye
Ans:
(i) Hair dyes may cause hair fall and damage to hair texture.
(ii) They may cause skin irritation and eye problems.
(iii) Some dyes may cause stomach disorders.
(iv) Excessive use of natural plant dyes may also harm the environment.
b. Artificial food colour
Ans:
(i) May contain harmful metals such as lead and mercury.
(ii) May cause ADHD in children.
(iii) May cause allergies and digestive problems.
c. Radioactive substances
Ans:
(i) Radioactive radiation may damage the central nervous system.
(ii) It may cause genetic mutations and hereditary defects.
(iii) It may lead to cancer such as skin cancer and leukemia.
(iv) Radioactive pollution may enter food chains through water and plants.
(v) Long-term exposure can be very harmful to living organisms.
d. Deodorant
Ans:
(i) Some deodorants contain aluminium compounds which may cause skin irritation.
(ii) They may cause headaches, asthma, and respiratory problems.
(iii) Excessive use may lead to skin disorders.
8. Write the chemical formula
Ans:
(a) Bleaching powder – CaOCl₂
(b) Common salt – NaCl
(c) Baking soda – NaHCO₃
(d) Washing soda – Na₂CO₃·10H₂O
9. Explain what you see in the following picture.
Ans:
(i) The picture shows the process of powder coating.
(ii) In this method, polymer resin and pigments are melted, mixed, cooled, and ground into fine powder.
(iii) The powder is sprayed on a metal surface using electrostatic spray deposition.
(iv) The charged powder particles stick uniformly to the surface.
(v) The coated object is heated in an oven.
(vi) A strong, durable, and attractive protective layer is formed.
(vii) This method prevents rusting and increases the life of the object.