Science, Grade 9 - Chapter 3: Atoms and Molecules

Questions, Page 32 – 33

1. In a reaction, 5.3 g of sodium carbonate reacted with 6 g of ethanoic acid. The products were 2.2 g of carbon dioxide, 0.9 g water and 8.2 g of sodium ethanoate. Show that these observations are in agreement with the law of conservation of mass. sodium carbonate + ethanoic acid → sodium ethanoate + carbon dioxide + water
Solution:
The reactants in this reaction are sodium carbonate and the ethanoic acid and the products are carbon dioxide, water and sodium ethanoate.
The total mass of the reactants = 5.3 + 6 = 11.3 g
The total mass of the products = 2.2 + 0.9 + 8.2 = 11.3 g
Thus, the total mass of the reactants is same as that of the products. Thus, the mass was neither created nor destroyed during the chemical reaction. Therefore, the mass was conserved during the reaction of sodium carbonate and ethanoic acid.

2. Hydrogen and oxygen combine in the ratio of 1:8 by mass to form water. What mass of oxygen gas would be required to react completely with 3 g of hydrogen gas?
Solution: The law of constant proportions states that in a chemical substance the elements are always present in definite proportions by mass. Hydrogen and oxygen combine in the ratio of 1:8 by mass to form water. Thus, in order to form water, 3 × 8 = 24 g of oxygen will react completely with 3 g of hydrogen gas.

3. Which postulate of Dalton’s atomic theory is the result of the law of conservation of mass? Solution: The second postulate of Dalton’s atomic theory which states that “Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction” is a result of the law of conservation of mass.

4. Which postulate of Dalton’s atomic theory can explain the law of definite proportions? Solution: The sixth postulate of Dalton’s atomic theory which states that “The relative number and kinds of atoms are constant in a given compound” can explain the law of definite proportions.


Questions, Page 34

1. Define the atomic mass unit.
Solution:
Atomic mass unit is a mass unit equal to exactly one-twelfth the mass of a carbon-12 atom. It is used to measure the masses of subatomic particles.

2. Why is it not possible to see an atom with naked eyes?
Solution:
The dimension of atoms is very small of the order of 1 nm. More than a million atoms when staked one over the other will make a layer that is barely as thick as a sheet of paper. Thus, we cannot see an atom with naked eyes.


Questions, Page 39

1. Write down the formulae of
(i) sodium oxide

(ii) aluminium chloride

(iii) sodium suphide
(iv) magnesium hydroxide

Solution:
(i) Na2O
(ii) AlCl3
(iii) Na2S
(iv) Mg(OH)2

2. Write down the names of compounds represented by the following formulae:

(i) Al2(SO4)3
(ii) CaCl2
(iii) K2SO4
(iv) KNO3
(v) CaCO3.
Solution:
(i) Aluminium Sulphate
(ii) Calcium chloride
(iii) Potassium sulphate
(iv) Potassium nitrate
(v) Calcium carbonate

3. What is meant by the term chemical formula?
Solution: The chemical formula of a compound is a symbolic representation of its composition.


4. How many atoms are present in a
(i) H2S molecule and
(ii) PO43– ion?
Solution:
(i) Three (two hydrogen atoms and one sulphur atom)
(ii) Five (One phosphorus and four oxygen)


Questions, Page 40

1. Calculate the molecular masses of H2, O2, Cl2, CO2, CH4, C2H6, C2H4, NH3, CH3OH.
Solution:
Molecular mass of H2 = 2u
Molecular mass of O2 = 16u + 16u = 32u
Molecular mass of Cl2 = 35.5u + 35.5u = 71u
Molecular mass of CO2 = 12u + (2 × 16u) = 44u
Molecular mass of CH4 = 12u + (4 × u) = 16u
Molecular mass of NH3 = 14u + (3 × u) = 17u
Molecular mass of CH3OH = 12u + (3 × u) + 16u + u = 32u

2. Calculate the formula unit masses of ZnO, Na2O, K2CO3, given atomic masses of Zn = 65u, Na = 23u, K = 39u, C = 12u, and O = 16u.
Solution:
Formula unit mass of ZnO = 65u + 16u = 81u
Formula unit mass of Na2O = (2 × 23u) + 16u = 66u
Formula unit mass of K2CO3 = (2 × 39u) + 12u + (3 × 16u) = 138u


Questions, Page 42

1. If one mole of carbon atoms weighs 12 gram, what is the mass (in gram) of 1 atom of carbon?
Solution: One mole of carbon atoms weighs 12 gram i.e., 6.022 × 1023 atoms of carbon weigh 12 gram. Therefore, 1 atom of carbon weighs 12 ÷ (6.022 × 1023) = 1.993 × 10–23 gram.

2. Which has more number of atoms, 100 grams of sodium or 100 grams of iron (given, atomic mass of Na = 23 u, Fe = 56 u)?
Solution:







(click solution for a larger image of the solution)



End of Chapter Exercises, Pages 43 - 44

1. A 0.24 g sample of compound of oxygen and boron was found by analysis to contain 0.096 g of boron and 0.144 g of oxygen. Calculate the percentage composition of the compound by weight.
Solution:







(click solution for a larger image of the solution)

2. When 3.0 g of carbon is burnt in 8.00 g oxygen, 11.00 g of carbon dioxide is produced. What mass of carbon dioxide will be formed when 3.00 g of carbon is burnt in 50.00 g of oxygen?
Which law of chemical combination will govern your answer?
Solution:
3.0 g of carbon combines with 8.0 g of oxygen to give 11.0 of carbon dioxide.
Thus, by the law of definite proportions, when 3.00 g of carbon is burnt in 50.00 g of oxygen, only 8.00 g of oxygen will be used to produce 11.00 gram of carbon dioxide. The remaining 42.00 g of oxygen will remain as is.

3. What are polyatomic ions? Give examples.
Solution: Polyatomic ions are those ions that contain more than one atom. These atoms can be of the same type of different type. Some examples of polyatomic ions are NO3-,NH4+,OH-, SO42-, and SO32-.

4. Write the chemical formulae of the following.
(a) Magnesium chloride
(b) Calcium oxide
(c) Copper nitrate
(d) Aluminium chloride
(e) Calcium carbonate.
Solution:
(a) MgCl2
(b) CaO
(c) Cu(NO3)2
(d) AlCl3
(e) CaCO3

5. Give the names of the elements present in the following compounds.
(a) Quick lime
(b) Hydrogen bromide
(c) Baking powder
(d) Potassium sulphate.
Solution:
(a) Calcium and oxygen
(b) Hydrogen and bromine
(c) Sodium, hydrogen, carbon, and oxygen
(d) Potassium, sulphur, and oxygen

6. Calculate the molar mass of the following substances.
(a) Ethyne, C2H2
(b) Sulphur molecule, S8
(c) Phosphorus molecule, P4 (Atomic mass of phosphorus = 31)
(d) Hydrochloric acid, HCl
(e) Nitric acid, HNO3
Solution:
(a) Molecular mass of ethyne, C2H2 = 2×12u + 2×u = 26u
(b) Molecular mass of sulphur molecule, S8 = 8×32u = 256u
(c) Molecular mass of phosphorus molecule, P4 = 4×31u = 124u
(d) Molecular mass of HCl = u + 35.5u = 36.5u
(e) Molecular mass of HNO3 = u + 14u + 3×16u = 63u

7. What is the mass of—
(a) 1 mole of nitrogen atoms?
(b) 4 moles of aluminium atoms (Atomic mass of aluminium = 27)?
(c) 10 moles of sodium sulphite (Na2SO3)?
Solution:
(a) Mass of one mole of nitrogen atoms = molecular mass of nitrogen atoms in grams = 14 g
(b) Mass of 4 moles of aluminium atoms = 4 × molecular mass of aluminum atoms in grams = 4 × 27 = 108
(c) Mass of 10 moles of sodium sulphite = 10 × molecular mass of sodium sulphite in grams
Molecular mass of sodium sulphite, Na2SO3 = 2×23 + 32 + 3×16 = 126 g
Thus, the mass of 10 moles of sodium sulphite = 10 × 126 = 1260 g

8. Convert into mole.
(a) 12 g of oxygen gas
(b) 20 g of water
(c) 22 g of carbon dioxide.
Solution:








(click solution for a larger image of the solution)

9. What is the mass of:
(a) 0.2 mole of oxygen atoms?
(b) 0.5 mole of water molecules?
Solution:
(a) The mass of 1 mole of oxygen atoms = 16 g
Thus, the mass of 0.2 mole of oxygen atoms = 16 × 0.2 = 3.2 g
(b) The mass of 1 mole of water molecules = 18 g
Thus, the mass of 0.5 mole of water molecules = 18 × 0.5 = 9.0 g

10. Calculate the number of molecules of sulphur (S8) present in 16 g of solid sulphur.
Solution:








(click solution for a larger image of the solution)

11. Calculate the number of aluminium ions present in 0.051 g of aluminium oxide.
(Hint: The mass of an ion is the same as that of an atom of the same element. Atomic mass of Al = 27 u)
Solution:













(click solution for a larger image of the solution)

Science, Grade 9 - Chapter 2: Is matter around us pure

Questions, Page 15

1. What is meant by a pure substance?
Solution: A pure substance is one that cannot be separated into different constituents by physical or chemical processes. A pure substance is one that contains particles of only one type of a substance.

2. List the points of differences between homogeneous and heterogeneous mixtures.
Solution:
Homogeneous mixtures:
They have uniform compositions. The components of homogeneous mixtures are not physically distinct. Salt in water, sugar in water are examples of homogeneous mixtures.
Heterogeneous mixtures:
They contain physically distinct parts and have non-uniform compositions. Mixtures of sodium chloride and iron filings, salt and sulphur, and oil and water are examples of heterogeneous mixtures.


Questions, Page 15

1. Differentiate between homogeneous and heterogeneous mixtures with examples.
Solution:

Homogeneous mixtures:

They have uniform compositions. The components of homogeneous mixtures are not physically distinct. Most solutions are homogeneous mixtures. Salt in water, sugar in water are examples of homogeneous mixtures.
Heterogeneous mixtures:
They contain physically distinct parts and have non-uniform compositions. Mixtures of sodium chloride and iron filings, salt and sulphur, and oil and water are examples of heterogeneous mixtures. Suspensions and colloids are also heterogeneous mixtures

2. How are sol, solution and suspension different from each other?
Solution:
(Click the table for a larger view)

3. To make a saturated solution, 36 g of sodium chloride is dissolved in 100 g of water at 293 K. Find its concentration at this temperature.
Solution:











Questions, Page 24

1. How will you separate a mixture containing kerosene and petrol (difference in their boiling points is more than 25ºC), which are miscible with each other?
Solution: The process of distillation is used for the separation of components of a mixture containing two miscible liquids that boil without decomposition and have sufficient difference in their boiling points (that is greater than 25°C).

2. Name the technique to separate
(i) butter from curd,

(ii) salt from sea-water,

(iii) camphor from salt.

Solution:
(i) Centrifugation
(ii) Evaporation
(iii) Sublimation

3. What type of mixtures are separated by the technique of crystallisation?
Solution: The crystallisation method is used to purify solids.


Questions, Page 24

1. Classify the following as chemical or physical changes:
• cutting of trees,

• melting of butter in a pan,

• rusting of almirah,

• boiling of water to form steam,

• passing of electric current, through water and the water breaking down into hydrogen and oxygen gases,

• dissolving common salt in water,
• making a fruit salad with raw fruits, and

• burning of paper and wood.

Solution:
Physical changes: Cutting of trees, melting of butter in a pan, boiling water to form steam, making a fruit salad with raw fruits
Chemical changes: Rusting of almirah, passing of electric current through water and the water breaking down into hydrogen and oxygen gases, burning of paper and wood

2. Try segregating the things around you as pure substances or mixtures.
Solution:
Pure substances: water, glass, iron nail, pencil lead, etc.
Mixtures: Air, blood, butter, milk, steel, paper, etc.

End of chapter Exercises, page 29 – 30

1. Which separation techniques will you apply for the separation of the following?
(a) Sodium chloride from its solution in water.
(b) Ammoniu
m chloride from a mixture containing sodium chloride and ammonium chloride.
(c) Small pieces of metal in the engine oil of a car.

(d) Different pigments from an extract of flower petals.

(e) Butter from curd.
(f) Oil from water.

(g) Tea leaves from tea.

(h) Iron pins from sand.
(i) Wheat grains from husk.
(j) Fine mud particles suspended in water.
Solution:
(a) Evaporation
(b) Sublimation
(c) Filtration
(d) Chromatography
(e) Centrifugation
(f) Distillation
(g) Sieving
(h) Magnetic separation
(i) Sieving and winnowing
(j) Sedimentation, decantation and filtration

2. Write the steps you would use for making tea. Use the words solution, solvent, solute, dissolve, soluble, insoluble, filtrate and residue.
Solution: Take some water in a pan. Keep the pan over flame. Add sugar and tea leaves, which are solute into the water in the pan, which is the solvent. Heat the water over the pan till the sugar, which is soluble in water dissolves in it. Tea leaves are insoluble in water and will remain suspended. Now add water to the sugar and tea leaves solution and bring the mixture to a boil. Filter the prepared tea through a sieve. Filtrate should be poured in a cup, while the residue can be thrown away.

3. Pragya tested the solubility of three different substances at different temperatures and collected the data as given below (results are given in the following table, as grams of substance dissolved in 100 grams of water to form a saturated solution).(a) What mass of potassium nitrate would be needed to produce a saturated solution of potassium nitrate in 50 grams of water at 313 K?
(b) Pragya makes a saturated solution of potassium chloride in water at 353 K and leaves the solution to cool at room temperature. What would she observe as the solution cools? Explain.
(c) Find the solubility of each salt at 293 K. Which salt has the highest solubility at this temperature?
(d) What is the effect of change of temperature on the solubility of a salt?


Solution:
(a) Since 62 g of potassium nitrate is dissolved in 100g of water to prepare a saturated solution at 313 K, 31 g of potassium nitrate should be dissolved in 50 g of water to prepare a saturated solution at 313 K.
(b) The amount of potassium chloride that should be dissolved in water to make a saturated solution increases with temperature. Thus, as the solution cools some of the potassium chloride will precipitate out of the solution.
(c) The solubility of the salts at 293 K are:
Potassium nitrate – 32 g
Sodium chloride – 36 g
Potassium chloride – 35 g
Ammonium chloride – 37 g
Ammonium chloride has the highest solubility at 293 K.
(d) The solubility of a salt increases with temperature.

4. Explain the following giving examples.
(a) saturated solution
(b) pure substance
(c) colloid
(d) suspension
Solution:
(a) At any particular temperature, a solution that has dissolved as much solute as it is capable of dissolving is said to be a saturated solution. Few examples of saturated solutions are soft drinks and nitrogen in Earth’s soil.
(b) A pure substance is one that cannot be separated into different constituents by physical or chemical processes. A pure substance is one that contains particles of only one type of a substance. Pure substances can be elements or compounds. Some examples of pure substances are iron, water, oxygen, etc.
(c) Colloids are heterogeneous mixtures in which the particle size is too small to be seen with the naked eye, but is big enough to scatter light. Smoke, paint, butter are few examples of colloids.
(d) Materials that are insoluble in a solvent and have particles that are visible to naked eyes form a suspension. A suspension is a heterogeneous mixture. Some examples of suspension are water with chalk particles, sandy water and water with stones

5. Classify each of the following as a homogeneous or heterogeneous mixture.
soda water, wood, air, soil, vinegar, filtered tea.
Solution:
Homogeneous – soda water, air, vinegar, filtered tea
Heterogeneous – wood, soil

6. How would you confirm that a colourless liquid given to you is pure water?
Solution: Perform the electrolysis of water, i.e., pass the electricity through it, so that the water molecules break into hydrogen and oxygen. Hydrogen and oxygen are gases at room temperature. Thus, they will be released in the air. If after the complete electrolysis of water, some residue is left, then the water is not pure. If no residue is obtained, the water is pure.

7. Which of the following materials fall in the category of a “pure substance”?
(a) Ice
(b) Milk
(c) Iron
(d) Hydrochloric acid
(e) Calcium oxide
(f) Mercury
(g) Brick
(h) Wood
(i) Air.
Solution: Ice, iron, hydrochloric acid, calcium oxide, and mercury are pure substances.

8. Identify the solutions among the following mixtures.
(a) Soil
(b) Sea water
(c) Air
(d) Coal
(e) Soda water.
Solution: Sea water, air, and soda water are solutions.

9. Which of the following will show “Tyndall effect”?
(a) Salt solution
(b) Milk
(c) Copper sulphate solution
(d) Starch solution.
Solution: Colloids show Tyndall effect. Milk is a colloid. Thus, it will show Tyndall effect. Therefore, the correct answer is (b).

10. Classify the following into elements, compounds and mixtures.
(a) Sodium
(b) Soil
(c) Sugar solution
(d) Silver
(e) Calcium carbonate
(f) Tin
(g) Silicon
(h) Coal
(i) Air
(j) Soap
(k) Methane
(l) Carbon dioxide
(m) Blood
Solution:
Elements: sodium, silver, tin, and silicon.
Compounds: calcium carbonate, soap, methane, and carbon dioxide.
Mixtures: soil, sugar solution, coal, air, and blood.

11. Which of the following are chemical changes?
(a) Growth of a plant
(b) Rusting of iron
(c) Mixing of iron filings and sand
(d) Cooking of food
(e) Digestion of food
(f) Freezing of water
(g) Burning of a candle.
Solution: Rusting of iron, cooking of food, digestion of food, and burning of candle are chemical changes.

Science, Grade 9 - Chapter 1: Matter in our surroundings

Questions, Page 3

1. Which of the following are matter? Chair, air, love, smell, hate, almonds, thought, cold, cold drink, smell of perfume.
Solution: Chair, air, almonds and cold drink

2. Give reasons for the following observation: The smell of hot sizzling food reaches you several metres away, but to get the smell from cold food you have to go close.
Solution: Particles of matter are continuously moving. They possess the kinetic energy. As the temperature rises, particles move faster. Thus, particles that carry smell of hot sizzling food move faster than the smell of the cold food. Therefore, the smell of hot sizzling food can reach us several metres away, but to get smell from a cold food you have to go close.

3. A diver is able to cut through water in a swimming pool. Which property of matter does this observation show?
Solution: This shows that the particles of matter have space between them. This space is the maximum in the gases and the minimum in liquid. Thus, one cannot cut through a solid easily but a diver is able to cut easily through water in a swimming pool.

4. What are the characteristics of the particles of matter?
Solution: the characteristics of the particles of matter are:
1. Particles of matter have space between them.
2. Particles of matter are continuously moving.
3. Particles of matter attract each other.


Questions, Page 6

1. The mass per unit volume of a substance is called density.(density = mass/volume). Arrange the following in order of increasing density – air, exhaust from chimneys, honey, water, chalk, cotton and iron.
Solution: Exhaust from chimneys, air, cotton, water, honey, chalk, and iron
2. (a) Tabulate the differences in the characteristics of states of matter.
Solution:
(Click the table for a larger view)

(b) Comment upon the following: rigidity, compressibility, fluidity, filling a gas container, shape, kinetic energy and density.
Solution: Rigidity:
Rigidity is the physical property of being stiff and resistance to bending or flowing. Solids are highly rigid. Liquids and gases are not rigid. Thus, they can easily flow and bend. Thus, liquids and gases are called fluids.
Compressibility:
Compressibility is the property of being able to occupy less space. Solids cannot be compressed easily. Liquids can be compressed but only to a limit. Gases are highly compressible.
Fluidity:
Fluidity is the ability to flow. Liquids and gases can flow. Thus, they are known as fluids.
Filling a gas container:
Gases are highly compressible. Thus, large volumes of gases are compressed and stored in containers so that they are easier to store and transport.
Shape:
Solids have definite shape and retain their shape till an outside force changes its shape. Liquids and gases do not have a definite shape. They take the shape of the container in which they are kept.
Kinetic Energy:
Kinetic energy is the energy possessed by objects by virtue of their motion. The greater the speed of motion, the larger is the kinetic energy. The particles of matter are continuously moving. They possess the kinetic energy. The motion of the particles is the maximum in solid and is the minimum in gases. Thus, the particles of solid have the maximum kinetic energy and the particles of gases have the minimum kinetic energy. The particles of liquid have the kinetic energy that is intermediate that of the solids and gases.
Density:
Density is defined as the mass per unit volume. Density refers to the amount of matter that is present per unit volume. Solids are known to have highest density and gases lowest density. Liquids have a density that is intermediate that of the solids and liquids.

3. Give reasons (a) A gas fills completely the vessel in which it is kept. (b) A gas exerts pressure on the walls of the container. (c) A wooden table should be called a solid. (d) We can easily move our hand in air but to do the same through a solid block of wood we need a karate expert.
Solution: (a) The particles of gases are free to move. Gas particles have least amount of attraction between them. Thus, the particles of gases fill completely the vessel in which they are kept.
(b) The particles of gas are in continuous motion. They collide with each other and with the walls of container. These collisions of the gas particles with the walls of the container exert pressure on the walls of the container.
(c) A wooden table has a definite shape and size. Also, it is a non-compressible rigid body. Thus, a wooden table has all the characteristics of a solid.
(d) The particles of air have least amount of attraction between them. Thus, one can easily move the hand air and push the particles of air apart. However, the particles of solid have maximum amount of attraction between them. A greater amount of force is required to move the particles of solid apart. Thus, a much greater force is required to move hand through a solid block of wood.

4. Liquids generally have lower density as compared to solids. But you must have observed that ice floats on water. Find out why.
Solution: Density of water is the maximum at 4°C. The density of ice is less than that of the water. Thus, ice floats on water.


Questions, Page 9

1. Convert the following temperature to Celsius scale: a. 300 K b. 573 K.
Solution:
Celsius scale = Kelvin scale ¬– 273
(a) Celsius scale = 300 – 273 = 27°C
(b) Celsius scale = 573 ¬– 273 = 300°C

2. What is the physical state of water at: a. 250ºC b. 100ºC?
Solution: The physical state of water at
(a) 250°C is gaseous
(b) 100°C might be gaseous or liquid. Steam and water coexist at 100°C.

3. For any substance, why does the temperature remain constant during the change of state?
Solution: The temperature remains constant during the change of state. The temperature of the substance does not increase (or decrease) as the heat is used (or released) to overcome the force of attraction (or as the particles come closer). This heat energy absorbed (or released) is known as the latent heat. The temperature of the substance remains the same as latent heat is supplied to it or is released by it.

4. Suggest a method to liquefy atmospheric gases
Solution: Atmospheric gases liquefy at very low temperatures. It is not possible to attain such low temperatures. However, we can liquefy atmospheric gases without attaining such low temperatures. Atmospheric gases can be liquefied by compressing them. When the atmospheric gases are compressed, the particles of gases come closer to each other and as we keep on compressing the gases, the particles keep coming closer and closer to each other. When the pressure on the atmospheric gases becomes large enough, then the gases liquefy. Thus, atmospheric gases can be liquefied by compressing them.


Questions, Page 10

1. Why does a desert cooler cool better on a hot dry day?
Solution: A desert cooler increases the humidity of the surrounding air. The water particles in the air take the heat from the surrounding objects and evaporate, thereby making them cooler.

2. How does the water kept in an earthen pot (matka) become cool during summer?
Solution: An earthen pot (matka) has small pores in it. Water seeps through these pores and reaches the outer surface of the earthen pot. This water then evaporates by taking heat from earthen pot, thereby making it cooler.

3. Why does our palm feel cold when we put some acetone or petrol or perfume on it?
Solution: Acetone, petrol, and perfume evaporate at low temperatures. When some acetone, petrol, or perfume is dropped on the palm, it takes heat from the palm and evaporates, thereby making the palm cooler.
4. Why are we able to sip hot tea or milk faster from a saucer rather than a cup?
Solution: Evaporation is a surface phenomenon. If the surface area is increased, then the rate of evaporation increases. Evaporation has a cooling effect. Thus, when hot tea or milk is poured in a saucer, it evaporates faster. Thus, it is cooled faster.

5. What type of clothes should we wear in summer?
Solution: One should wear cotton clothes in summer as cotton is a good sweat absorber. Sweat is absorbed by the cotton and is exposed to the atmosphere. There the sweat evaporates, thereby making one feel cooler.


End of chapter Exercises, Page 12

1. Convert the following temperatures to the Celsius scale. (a) 300 K (b) 573 K
Solution: Celsius scale = Kelvin scale ¬– 273
(a) Celsius scale = 300 – 273 = 27°C
(b) Celsius scale = 573 ¬– 273 = 300°C

2. Convert the following temperatures to the Kelvin scale. (a) 25°C (b) 373°C.
Solution: Kelvin scale = Celsius scale ¬+ 273
(a) Kelvin scale = 25 + 273 = 298 K
(b) Kelvin scale = 373 ¬+ 273 = 646 K

3. Give reason for the following observations. (a) Naphthalene balls disappear with time without leaving any solid. (b) We can get the smell of perfume sitting several metres away.
Solution:
(a) Naphthalene or moth balls sublimate (change from a solid to a gas). So, they disappear with time without leaving any solid.
(b) Smell of perfume is carried by air particles. Air particles are free to move (they possess kinetic energy). Thus, the air particles carrying the smell of perfume can move to someone sitting several metres away.

4. Arrange the following substances in increasing order of forces of attraction between the particles— water, sugar, oxygen.
Solution: Oxygen, water, sugar

5. What is the physical state of water at— (a) 25°C (b) 0°C (c) 100°C?
Solution:
(a) Liquid
(b) Solid or liquid
(c) Liquid or gas

6. Give two reasons to justify— (a) Water at room temperature is a liquid. (b) An iron almirah is a solid at room temperature.
Solution:
(a) Water at room temperature has a definite volume, but it does not have a definite shape. Thus, water at room temperature is a liquid.
(b) An iron almirah at room temperature has a definite volume as well as a definite shape. Thus, an iron almirah at room temperature is a solid.
7. Why is ice at 273 K more effective in cooling than water at the same temperature?
Solution: Ice at 273 K would absorb latent heat as well as the heat energy in order to attain the room temperature. Water at 273 K would just absorb heat energy in order to attain the room temperature. Thus, the water at 273 K would absorb less amount of heat energy from the surroundings as compared to ice at 273 K. Therefore, ice at 273 K is more effective in cooling than water at the same temperature.

8. What produces more severe burns, boiling water or steam?
Solution: Steam would release heat energy to first liquefy and then would release heat energy to attain the room temperature. Boiling water would just release heat energy to attain the room temperature. Thus, steam would release more heat energy than boiling water. Therefore, steam produces more severe burns than boiling water.
9. Name A,B,C,D,E and F in the following diagram showing change in its state.
Solution:
A. Fusion
B. Vaporisation
C. Condensation
D. Solidification
E. Sublimation
F. Sublimation