Class 9

JAC Board Class 9th Science Solutions Chapter 3 Atoms and Molecules

JAC Class 9th Science Atoms and Molecules InText Questions and Answers

Page 32

Question 1.
In a reaction, 5.3g of sodium carbonate reacted with 6g of ethanoic acid. The products were 2.2g of carbon dioxide, 0.9g water and 8.2g of sodium ethanoate. Show that these observations are in agreement with the law of conservation of mass. Sodium carbonate + Acetic acid → Sodium acetate + Carbon dioxide + Water
Answer:

This shows that during a chemical reaction, mass of reactants mass of products. Hence the given observation are in agreement with the law of conservation of mass.

Question 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 3g of hydrogen gas?
Answer:
Ratio of H : O by mass in water is:
Hydrogen : Oxygen → H₂O ⇒ 1 : 8 = 3 : x or x = 24g
24 g of oxygen gas would be rcquircd to react completely with 3g of hydrogen.

Question 3.
Which postulate of Dalton’s atomic theory is the result of the Law of consersatlon of mass?
Answer:
Awms are indivisible particles, which can neither be created nor destroyed in a chemical rcacuon.

Question 4.
Which postulate of Dalton’s atomic theory can explain the law of definite proportions?
Answer:
“The relative number and kind of atoms are constant in a given compound”

Page 35

Question 1.
Define the atonik mass unit.
Answer:
A unit of mass used in express atomic and molecular weights, equal to one twelfth (1/12th.) of the mass of an stom of carbon – 12 The relative atomic masses of all elements have been found with respect to an atom of carbon – 12.

Question 2.
Why Is it not possible to see an atom with naked eyes?
Answer:
The size of an atom is very small, Further, atoms of most elements do not exist independently. The radius of an atom is of the order of 10^-10m.

Page 39

Question 1.
Write down the formulae of
(a) sodium oxide
(b) aluminium chloride
(c) sodium sulphide
(d) magnesium hydroxide
Answer:
(a) Sodium oxide — Na₂O
(b) Aluminium chloride — AlCl₃
(c) Sodium sulphide — Na₂S
(d) Magnesium hydroxide — Mg(OH)₂

Question 2.
Write down the names of compounds represented by the following formulae:
(a) Al₂(SO₄)₃
(b) CaCl₂
(c) K₂SO₄
(d) KNO₃
(e) CaCO₃
Answer:
(a) Al₂(SO₄)₃ : Aluminium sulphate
(b) CaCl₂ : Calcium chloride
(c) K₂SO₂ : Potassium sulphate
(d) KNO₃ : Potassium nitrate
(e) CaCO₂ : Calcium carbonate

Question 3.
What Is meant by the term chemical formula?
Answer:
The chemical formula of a compound is the symbolic representation of its composition. It gives the number and kinds of atoms which are chemically united in a given compound. For example, chemical formula of sodium chloride is NaCl.

Question 4.
How many atoms are present in a
(a) H₂S molecule and
(b) \(\mathrm{PO}_{4}^{3-}\) ion?
Answer:
(a) H₂S → 3 atoms are present: 2 atoms of hydrogen and 1atom of sulphur.
(b) \(\mathrm{PO}_{4}^{3-}\) → 5 atoms are present: 1 atom of phosphorus and 4 atoms of oxygen.

Page 40

Question 1.
Calculate the molecular masses of H₂, O₂ C₁₂, CO₂, CH₄, C₂H₆, C₂H₄, NH₃, CH₃OH.
Answer:
The molecular masses are:
H₂ → 1 + 1 = 2u
O₂ → 16 + 16 = 32u
Cl₂ → 35.5 + 35.5 = 71u
CO₂ → p 12 + 32 = 44u
CH₄ → 12 + 4 = 16u
C₂H₆ → (12 × 2) + (1 × 6) = 30u
C₂H₄ → (12 × 2) + (1 × 4) = 28u
NH₃ → (1 × 14) + (1 × 3) = 17u
CH₃OH → (1 × 12) + (1 × 3) + (1 × 16)(1 × 1) = 32u.

Question 2.
Calculate the formula unit of masses of ZnO, Na₂O, K₂CO₃, given atomic masses of Zn = 65u, Na = 23u, K = 39u, C = 12u and O = 16u.
Answer:
The formula Unit mass of:
(a) ZnO = 65u + 16u = 81u
(b) Na₂O = (23u × 2) + 16u = 46u + 16u = 62u
(c) K₂CO₃ = (39u × 2) + 12u + (16u × 3)
= 78u + 12u + 48u
= 138u.

Page 42

Question 1.
If 1 mole of carbon atoms weigh 12grams, what Is the mass (in grams) of 1 atom of carbon?
Answer:
1 mole of carbon atoms = 6.022 × 10²³ atoms
Now, 12/6.022 × 10²³ atoms of carbon weigh = 12g
One atom of carbon weighs = \(\frac{12}{6.023}\) × 10²³
= 1.99 × 10^-23g.

Question 2.
Which has more number of atoms 100 grams of sodium or loo grams of iron (given atomic mass of Na = 23u, Fe 56u)?
Answer:
23 gram atomic unit or 23g sodium (1 mole) = 6.022 × 10²³ atoms
100 gram atomic unit or 100g sodium = \(\frac{6.022 \times 10^{23} \times 100}{23}\)
= 2.617 × 10²⁴ atoms Again 56 gram atomic unit or 56 g iron (1 mole) 6.022 × 10²³
100 gram atomic unit or 100 g iron = \(\frac{6.022 \times 10^{23} \times 100}{56}\)
= 1. 075 × 10²⁴ atoms Thus, 100 g of sodium has more atoms than 100g of iron

JAC Class 9th Science Atoms and Molecules Textbook Questions and Answers

Question 1.
A 0.24g sample of compound of oxygen and boron was found by analysis to contain 0.096g of boron and 0.144g of oxygen. Calculate the percentage composition of the compound by weight.
Answer:
Percentage (%) of boron in the sample
\(\frac{0.096}{0.24}\) × 100 = 40%
Percentage (%) of oxygen in the sample
\(\frac{0.144}{0.24}\) × 100 = 60%
The sample of compound contains 40% boron and 60% oxygen by weight.

Question 2.
When 3.0g of carbon is burnt in 8.00 g oxygen, 11.00g of carbon dioxide is produced. What mass of carbon dioxide will be formed when 3.00g of carbon Is burnt In 50.00g of oxygen? Which law of chemical combination will govern your answer?
Answer:
When 3.0g of carbon is burnt in 8.00g oxygen, 11.00g carbon dioxide is produced. It means all of carbon and oxygen are used up and carbon and oxygen are combined in the ratio of 3 : 8 to form carbon dioxide. Thus when there is 3g carbon and 50.0g oxygen, then also only 8g oxygen will be used and 11.0g carbon dioxide will be formed. The remaining oxygen is not used up. This indicates law of defmite proportions which Say that in compounds, the combining elements are present in definite proportions by mass.

Question 3.
What are polyatomic ions? Give examples.
Answer:
The ions which contain more than one atom (same kind or may be of different kind) and behave as a single unit are called polyatomic ions. For example:

1. Ammonium ion NH is a compound ion which is made up of two types of atoms joined together, viz., nitrogen and hydrogen.
2. Carbonate ion CO is a compound ion which is made up of two types of atoms joined together, viz., carbon and oxygen.

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

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

Question 6.
Calculate the molar mass of the following substances.
(a) Ethvne, C₂H₂
(b) Sulphur molecule, S₈
(c) Phosphorus molecule, P₄ (Atomic mass of phosphorus = 31u)
(d) Hydrochloric acid, HCl
(e) Nitric acid, HNO₃
Answer:
(a) Ethyne, C₂H₂ = (2 × 12) + (2 × 1) = 26g
(b) Sulphur molecule, S₈ = 8 × 32 = 256g
(c) Phosphorus molecules, P₄ = 4 × 31 = 124g
(d) Hydrochloric acid, HCl = (1 × 1) + (1 × 35.5) = 36.5g
(e) Nitric acid, HNO₃ = (1 × 1) + (1 × 14) + (3 × 16) = 63g

Question 7.
What is the muss of
(a) 1 mole of nitrogen atoms?
(b) 4 moles of aluminium atoms (atomic mass of aluminium 27u)?
(c) 10 moles of sodium sulphite (Na₂SO₃)?
Answer:
(a) 1 mole of nitrogen atoms 14u = 14g
(b) 4 moles of aluminium atoms = 4 × 27 = 108u = 108g
(c) 1 mole of sodium sulphite = (2 × 23) + (1 × 32) + (3 × 16) = 126u = 126g
10 moles of sodium sulphite = 10 × 126 = 1260g.

Question 8.
Convert Into mole.
(a) 12g of oxygen gas
(b) 20g of water
(c) 22 g of carbon dioxide
Answer:
(a) Given mass of oxygen gas = 12g
Molar mass of oxygen gas (O₂) = 32g
Mole of oxygen gas = \(\frac{12}{32}\)
= 0.375 mole.

(b) Given mass of water = 20g
Molar mass of water(H₂O) (2 × 1) + 16= 18g
Mole of water = \(\frac{20}{18}\)= 1.11 mole.

(c) Given mass of carbon dioxide = 22g
Molar mass of carbon dioxide (CO₂) (1 × 12) + (2 × 16) = 12 + 32 = 44g
Mole of carbon dioxide = \(\frac{22}{44}\)
= 0.5 mole.

Question 9.
What is the mass of
(a) 0.2 mole of oxygen atoms?
(b) 0.5 mole of water molecules?
Answer:
(a) Mole of oxygen atoms = 0.2 mole
Molar mass of oxygen atoms = 16g
Mass of oxygen atoms 16 × 0.2 = 3.2g.

(b) Mole of water molecule 0.5 mole
Molar mass of water molecules = (2 × 1) + 16 = 18g
Mass of H₂O = 18 × 0.5 = 9g.

Question 10.
Calculate the number of molecules of sulphur (S₈) present In 16 g of solid sulphur.
Answer:
Molar mass of sulphur (S₈) = 256 g = 6.022 × 10₂₃ molecules
Given mass of sulphur = 16g
Molecules of sulphur = \(\frac{16 \times 6.022 \times 10^{23}}{256}\)
\(=\frac{96.35 \times 10^{23}}{256}\) = 0.376 × 10²³ molecules.

Question 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 saine element Atomic mass of Al = 27u
Answer:
1 mole of aluminium oxide, Al₂O₃ = (2 × 27) + (3 × 16) = 102u = 102g
102 g of Al₂O₃ has = 6.022 × 10₂₃ Al₂O₃ molecules
0.051 g of Al₂O₃has = \( \frac{6.022 \times 10^{23} \times 0.051}{102}\)
= 3.01 × 10₂₀ molecules
1 molecule of Al₂O₃ gives = 2 Al³⁺ ions
Hence, 0.051g Al₂O₃ gives = 2 × 3.01 × 10²⁰ Al³⁺ ions
= 6.022 × 10²⁰ aluminium ions.

JAC Class 9 Science Solutions

JAC Board Class 9th Science Solutions Chapter 2 Is Matter Around Us Pure

JAC Class 9th Science Is Matter Around Us Pure InText Questions and Answers

Page 15

Question 1.
What is meant by a pure substance?
Answer:
A pure substance consists of a single type of particles and it cannot be separated into other kinds of matter by any separation process.

Question 2.
List the points of difference between ho-mogeneous and heterogeneous mixtures.
Answer:
Differences between homogeneous and heterogeneous mixtures:

Homogeneous mixture	Heterogeneous mixture
It has no visible boundary or boundaries of separation between its constituents.	It has visible boundary boundaries of separation between its constituents.
It has a uniform composition.	It does not have a uniform composition.
They form solutions.	They form suspensions colloids.
The particle size is very small.	The particle size is larger.
For example, sugar + water → sugar solution.	For example, sugar + sand.

Page 18

Question 1.
Differentiate between homogeneous and heterogeneous mixtures with examples.
Answer:
See answer 2 above.

Question 2.
How are sol, solution and suspension different from each other?
Answer:
Comparison among properties of true solution, colloidal solution and suspension:

Property	True solution	Colloidal solution	Suspension
Appearance	Hetero geneous and transparent.	Hetero geneous and translucent.	Hetero geneous and opaque.
Particle size	lnm (10-9 m)	lnm – 1000 nm	1000 lnm (10-6m)
Visibility	Particles are not visible even with a powerful microscope.	Particles can be seen with a high power microscope.	Particles can be seen with naked eyes.
Stability	Stable	Stable	Unstable
Diffusion	Diffuse rapidly	Diffuse slowly	Do not diffuse
Filterability	Passes through filter paper, e.g., sodium chloride dissolved in water.	Passes through filter paper, e.g., blood.	Can be separated by filter paper, e.g., mud water.

Question 3.
To make a saturated solution, 36g of sodium chloride is dissolved in lOOg of water at 293K. Find its concentration at this temperature.
Answer:
Mass of solute (sodium chloride) = 36g
Mass of solvent (water) = 100 g
Mass of solution = Mass of solute + Mass of solvent = 36g + 100g =136g
\(\frac{ Mass of solute}{Mass of solution}\) × 100 ⇒ \(\frac{36 \times 100}{136}\)
= 26.47%

Page 24

Question 1.
How will you separate a mixture containing kerosene and petrol (difference in their boiling points is more than 25°C), which are miscible with each other?
Answer:
Set up the apparatus as shown in the figure. Take the given mixture in a distillation flask. Heat the mixture slowly, keeping a close watch at the thermometer. At a certain point, temperature becomes constant. Petrol vaporises first as it has lower boiling point. It condenses in the condenser and is collected from the condenser outlet. Stop heating when the temperature further starts rising. Kerosene is left behind in the distillation flask.

Question 2.
Name the technique to separate:
(a) Butter from curd
(b) Salt from sea – water
(c) Camphor from salt
Answer:
(a) Butter from curd – Centrifugation
(b)Salt from sea – water – Evaporation
(c) Camphor from salt – Subiimation

Question 3.
What type of mixtures are separated by the technique of crystallisation?
Answer:
Crystallisation technique is used to purify a solid with some impurities in it. For example, purification of salt obtained from sea – water.

Page 25

Question 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. making of fruit salad with raw fruits, dissolving common salt in water, burning of paper and wood
Answer:

1. Physical change:
   • Cutting of trees
   • Melting of butter in a pan
   • Boiling of water to form steam
   • Dissolving common salt in water
   • Making a fruit salad with raw fruits
2. Chemical change:
   • Rusting of almirah
   • Passing of electric current through water and the water breaking down into hydrogen and oxygen gases
   • Burning of paper and wood

Question 2.
Try segregating the things around you as pure substances or mixtures.
Answer:

1. Pure substances: Water, sugar, gold, copper wire, salt, ice, etc.
2. Mixtures: Steel, plastic, paper, talc, milk, air, ink, soda water, lemonade, bread, etc.

JAC Class 9th Science Is Matter Around Us Pure Textbook Questions and Answers

Question 1.
Which separation techniques will you apply for the separation of the following?
(a) Sodium chloride from its solution in water.
(b) Ammonium 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.
Answer:
(a) Crystallisation/Evaporation

(b) Sublimation

(c) Filtration

(d) Chromatography

(e) Centrifugation

(f) Separating funnel

(g) Filtration

(h) Magnetic separation

(i) Winnowing

(j) Decantation/Sedimentation

Question 2.
Write the steps you would use for making tea Use the words solution, solvent, solute, dissolve, soluble, insoluble, filtrate and residue.
Answer:
Take a cup of water in a kettle as solvent and heat it. When the solvent boils, add sugar in it which is the solute. Heat it till entire sugar dissolves. Water and sugar form a solution. Then, add some tea leaves in this solution. Boil the contents, add milk which is also soluble in this mixture, and boil again. Filter the tea with the help of a strainer. The tea collected in cup is the filtrate and the tea leaves collected in the strainer is residue.

Question 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).

	Temperature in K Solubility
Substance Dissolved	283	293	313	333	353
Potassium nitrate	21	32	62	106	167
Sodium chloride	36	36	36	37	37
Potassium chloride	35	35	40	46	54
Ammonium chloride	24	37	41	55	66

(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 353K 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 293K. Which salt has the highest solubility at this temperature?
(d) What is the effect of change of temperature on the solubility of a salt?
Answer:
(a) Mass of potassium nitrate (KNO₃) needed to produce a saturated solution of KNO₃ in 100 grams of
water at 313 K = 62 g
Mass of KNO₃ needed in 50 g of
water at 313 K = \(\frac{62.0 \times 50}{100}\)
= 31.0g

(b) As solution cools, potassium chloride gets crystallised. This is because the solubility of a solid decreases with decrease in temperature.

(c) At 293K, solubility of KNO₃ is 32, NaCl is 36, KCL is 35 andNH₄Cl is 37. Ammonium chloride has the highest solubility at 293 K temperature.

(d) As the temperature increases, solubility increases and vice – versa.

Question 4.
Explain the following giving examples.
(a) saturated solution
(b) pure substance
(c) colloid
(d) suspension
Answer:
(a) Saturated solution: A solution in which no more solute can be dissolved at a given temperature is called a saturated solution.

(b) Pure substance: A pure substance consists of a single type of particles. It always has the same colour, taste or texture at a given temperature and pressure. For example, pure water is always colourless, odourless and tasteless.

(c) Colloid: A colloid is a solution in which the size of solute particles is bigger than those of a true solution. These particles cannot be seen with naked eyes as they are stable, e.g., ink, blood, smoke, milk, fog and cloud.

(d) Suspension: Suspension is a heterogeneous mixture. The particles of a suspension are greater than 1000 nm and are visible to naked eyes, e.g., chalk powder in water, paints, etc.

Question 5.
Classify each of the following as a homogeneous or a heterogeneous mixture: soda water, wood, air, soil, vinegar, filtered tea.
Answer:

1. Homogeneous mixtures: Soda water, vinegar, filtered tea.
2. Heterogeneous mixtures: Wood, soil, air.

Question 6.
How would you confirm that a colourless liquid given to you is pure water?
Answer:
By finding the boiling point of the given colourless liquid. If the given colourless liquid boils at exactly 373K at 1 atmosphere pressure, then it is pure water. This is because pure substances have fixed melting and boiling points.

Question 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
Answer:
Pure substances are: ice (a compound), iron (an element), hydrochloric acid (a compound), calcium oxide (a compound) and mercury (an element).

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

Question 9.
Which of the following will show “Tyndall effect”?
(a) Salt solution
(b) Milk
(c) Copper sulphate solution
(d) Starch solution
Answer:
Milk and starch solution are colloids and will show Tyndall effect.

Question 10.
Classify the following into elements, compounds and mixtures.
Sodium, Soil, Sugar solution, Silver, Calcium carbonate, Tin, Silicon, Coal, Air, Soap, Methane, Carbon dioxide, Blood
Answer:

• Elements: Sodium, Silver, Tin, Silicon
• Compounds: Calcium carbonate, Meth – ane, Carbon dioxide
• Mixtures: Sugar solution, Soil, Coal, Air, Blood, Soap

Question 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
Answer:
Chemical changes are growth of a plant, rusting of iron, cooking of food, digestion of food, burning of a candle.

JAC Class 9 Science Solutions

JAC Board Class 9th Science Solutions Chapter 1 Matter in Our Surroundings

JAC Class 9th Science Matter in Our Surroundings InText Questions and Answers

Page 3

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

Question 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.
Answer:
Particles of matter are continuously moving. They possess 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 the smell from a cold food we have to go close.

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

Question 4.
What are the characteristics of the particles of matter?
Answer:
The characteristics of the particles of matter are as follows:
(a) Particles of matter have intermolecular space.
(b) Particles of matter have intermolecular forces of attraction.
(c) Particles of matter are moving continuously.

Page 6

Question 1.
The mass per unit volume of a substance is called density = \(\frac{ mass}{volume}\) . Arrange the following in order of increasing density – air, exhaust from chimneys, honey, water, chalk, cotton and iron.
Answer:
Air, exhaust from chimneys, cotton, water, honey, chalk, and iron

Question 2.
(a) Tabulate the differences in the characteristics of states of matter,
(b) Comment upon the following: rigidity, compressibility, fluidity filling a gas container, shape, kinetic energy and density.
Answer:
(a) The differences in the characteristics of states of matter are

Characteristics	Solid	Liquid	Gas
Shape	Fixed shape	No fixed shape	No fixed shape
Volume	Fixed volume	Fixed volume	No fixed volume
Rigidity fluidity	Are rigid, cannot flow	Can flow, not rigid	Can flow, not rigid
Intermole – cular force	Maximum	Less than solids	Least
Intermole – cular space	Very less	More than solids and less than gases	Maximum
Compressible	Negligible	Compressible	Highly compressible

(b) Rigidity: It is the property of matter to resist change in its shape, e.g. solids are rigid.

• Compressibility: It is a property by which matter can be reduced to lower volume on application of force.
• Fluidity: It is the property of matter to flow.
• Filling a gas container: Gases completely fill the volume of the container in which they are kept, at any volume while solids and liquids do not.
• Shape: It indicates definite boundaries.
• Kinetic energy: It is the energy possessed by the particles of matter due their motion.
• Density: It is mass per unit volume. Higher density means more mass is confined in a lesser volume.

Question 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.
Answer:
(a) The molecules of gases are free to move. Gas molecules have the least amount of attraction between them.Thus, the molecules of gases completely fill the vessel in which they are kept.

(b) The molecules of gas are in continuous motion. They collide with each other and with the walls of container. These collisions of the gas molecules 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 molecules of air have least amount of attraction between them. Thus, one can easily move one’s hand in air and push the molecules of air apart. However, the molecules of solid have maximum amount of attraction between them. A greater amount of force is required to move the molecules of a solid apart. Thus, a much greater force is required to move our hand through a solid block of wood.

Question 4.
Liquids generally have lower density as compared to solids. But you must have observed that ice floats on water. Find out why.
Answer:
Ice is a solid but its density is lower than water due to its structure. The molecules in ice make a cage like structure with lot of vacant spaces. This makes ice float on water.

Page 9

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

Question 2.
What is the physical state of water at:
(a) 250°C
(b) 100°C?
Answer:
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.

Question 3.
For any substance, why does the temperature remain constant during the change of state?
Answer:
During the change of state, temperature remains constant because heat given to the matter is used up in changing the state of matter. This is called latent heat.

Question 4.
Suggest a method to liquefy atmospheric gases.
Answer:
Atmospheric gases liquefy at very low temperatures. It is not possible to attain such low temperatures. However, atmospheric gases can be liquefied by compressing them. When the atmospheric gases are compressed, the molecules of gases come closer to each other and as we keep on compressing the gases, the molecules keep coming closer and closer to each other. When the pressure on the atmospheric gases becomes large enough, the gases liquefy. Thus, atmospheric gases can be liquefied by compressing them.

Page 10

Question 1.
Why does a desert cooler cool better on a hot dry day?
Answer:
On a hot dry day, there is high temperature and low humidity. Both these factors are responsible for increasing the rate of evaporation. This also means better cooling.

Question 2.
How does the water kept in an earthen pot (matka) become cool during summer?
Answer:
The earthen pot has lots of tiny pores in it. The water oozes out through these pores and gets evaporated at the surface of the pot, by absorbing heat from the water kept in it, thereby causing cooling effect.

Question 3.
Why does our palm feel cold when we put some acetone or petrol or perfume on it?
Answer:
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.

Question 4.
Why are we able to sip hot tea or milk faster from a saucer rather than a cup?
Answer:
Hot tea or milk in a saucer has larger surface area than in a cup. The rate of evaporation is faster with increased surface area. The cooling of tea or milk in a saucer takes place sooner than in a cup. Hence, we are able to sip hot tea or milk faster from a saucer rather than a cup.

Question 5.
What type of clothes should we wear in summer?
Answer:
During summer, we should wear cotton clothes because we perspire more to maintain the temperature of our body. Cotton is a good absorber of sweat. It allows the sweat to evaporate faster, thereby giving a cooling effect.

JAC Class 9th Science Matter in Our Surroundings Textbook Questions and Answers

Question 1.
Convert the following temperatures to the Celsius scale, (a) 293 K (b) 470 K
Answer:
Celsius scale = Kelvin scale – 273
a. Celsius scale = 293 – 273 = 20°C
b. Celsius scale = 470 – 273 = 197°C

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

Question 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.
Answer:
(a) Naphthalene balls show the property of sublimation. Evaporation of naphthalene takes place easily and so it disappears during the course of time without leaving any residue,

(b) Perfume contains volatile solvent, i.e., gaseous particles, which have high speed and large space between them and diffuse faster and can reach people sitting at a distance.

Question 4.
Arrange the following substances in increasing order of forces of attraction between the particles water, sugar, oxygen.
Answer:
Oxygen < Water < Sugar

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

Question 6.
Give two reasons to justify
(a) Water at room temperature is a liquid.
(b) An iron almirah is a solid at room temperature.
Answer:
(a) Water is liquid at room temperature because:

1. At room temperature water has a definite volume, but it does not have a definite shape.
2. At room teperature water can flow easily.

(b) Iron almirah is solid at room temperature because:

1. At room temperature iron amirah has a definite volume as well as a definite shape.
2. At room temperature, iron almirah is very rigid.

Question 7.
Why is ice at 273 K more effective in cooling than water at the same temperature?
Answer:
Ice at 273 K will absorb heat energy or latent heat from the medium to overcome the intermolecular forces to become water. Hence, it cools more effectively than water at the same temperature.

Question 8.
What produces more severe burns, boiling water or steam?
Answer:
Steam at 100°C will produce more severe bums as extra heat is hidden in the form of latent heat in steam, whereas the boiling water does not have this hidden heat.

Question 9.
Name the processes A, B, C, D, E and F in the following diagram showing change in state of matter. Increase heat and decrease pressure

Answer:
A → Liquefaction/melting/fusion
B → Vaporisation/evaporation
C → Condensation
D → Solidification
E → Sublimation
F → Sublimation

JAC Class 9 Science Solutions