Class 9 Science Chapter 1 Question Answers - Matter in Our Surroundings

Q1: 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.
Ans: The correct order of increasing densities of the substances is as follows:
Air < exhaust from chimneys< cotton< water< honey< chalk< iron.

Q2: Why does a desert cooler cool better on a hot dry day?
Ans: In a desert cooler, when hot air enters through the straw mates it evaporates the water at a fast rate because the rate of evaporation is faster on a hot dry day.  And because of the faster evaporation rate, it cools the air more conveniently than on a dry hot day.

Q3: Why does our palm feel cold when we put some acetone or petrol or perfume on it?
Ans: As a perfume, petrol and perfume are volatile liquids, if put on our palm they will be absorbing heat from our palm and cause cooling.

Q4: What type of clothes should we wear in summer?
Ans: We must wear the type of clothes which allow easy evaporation since evaporation causes cooling. And as the cotton absorbs sweat well and allows easy evaporation, we must prefer wearing cotton clothes in summer.

Q5: Convert the following temperatures to the Kelvin scale.
(a) The temperature is 25 ⁰C
Ans: When we use: K = ⁰C + 273
= 25 + 273
= 298 K
(b)The temperature is 373 ⁰C 
Ans: When we use: K = ⁰C + 273
= 373 + 273
= 656 K

Q6: Give two reasons to justify -
(a) Water at room temperature is a liquid.
Ans: For a temperature of <0 ⁰Cwater is in solid-state, for 0⁰C → 100⁰C → water is in a liquid state and for temperature >100 ⁰C water is in a gaseous state. Since room temperature always lies between 0 ⁰C and 100 ⁰C and within this range the physical state of water is liquid so water is liquid at room temperature.
(b) An iron almirah is a solid at room temperature.
Ans: Since the room temperature is very less than the melting point of iron hence an almirah made up of iron will be a solid at room temperature.

Q7: What produces more severe burns, boiling water or steam?
Ans: When the steam gets converted into boiling water it releases latent heat of water which results more severe burns when we compare it to boiling water.

Q8: What happens when we apply pressure to the particles of matter?
Ans: Pressure can be defined as the force applied per cross-sectional area.  Therefore when we apply pressure to the particles of matter, the force applied brings the particles closer to each other.

Q9: If the melting point of object A is high then what state do you expect it to be at room temperature?
Ans: The temperature at which a substance changes its state from solid to liquid is called its melting point. At a temperature below melting point, the substance will be in solid-state. Therefore, if the melting point of an object A is high then the object will be in solid-state.

Q10: When heat is being supplied to a solid, then what does the heat energy do  to the particles of the solid?
Ans: The heat energy increases the kinetic energy of the particles which allows the particles to overcome the forces of attraction and start moving more freely and changing the state from solid to liquid.

Q11: Why do we say that evaporation is a surface phenomenon?
Ans: Only the particles at the surface of the liquid absorb energy and get converted into vapours, therefore evaporation is called a surface phenomenon.

Q12: Which of the following matter?
Chair, air, love, smell, hate, almonds, thought, cold, cold drink, the smell of perfume.
Ans: As we can define matter as any ‘physical substance’, hence almonds,  air, chair, the smell of perfume, cold drink and smell can be considered as matter.

Q13: What is the physical state of water at:  
(a) A temperature of 250 ⁰C 
Ans: The boiling point of water is 100 ⁰C, hence the physical state of water at 250⁰C will be gaseous.
(b) A temperature of 100 ⁰C 
Ans: The boiling point of water is 100 ⁰C, hence at 100 ⁰C water is in the gaseous state.

Q14: Suggest a method to liquefy atmospheric gases.
Ans: One method to liquefy atmospheric gases is to decrease the temperature and increase the pressure.

Q15: What is the physical state of water at-
(a) A temperature of 25 ⁰C?
Ans: The physical state of water at 25 ⁰C is liquid.
(b) A temperature of ⁰C?
Ans: The physical state of water at ⁰C is solid.
(c) A temperature of 100⁰C?
Ans: The physical state of water at 100⁰C is gas.

Q16: Which of the following statements is correct?
(a) boiling is a bulk phenomenon and evaporation is a surface phenomenon
(b) boiling is a surface phenomenon and evaporation is a bulk phenomenon
(c) boiling and evaporation both are a surface phenomenon
(d) boiling and surface both are bulk phenomenon 
Ans: The correct option is (a) boiling is a bulk phenomenon and evaporation is a  surface phenomenon.

Evaporation, on the other hand, is a surface phenomenon. It occurs only at the surface of the liquid, where molecules gain enough energy to escape into the gaseous phase.
So, boiling involves the conversion of a liquid into a gas throughout its bulk, while evaporation involves the conversion of liquid into a gas at its surface.

Q17: Which of the following has the least inter atomic spacing?
(a) solid
(b) liquid
(c) gases
(d) plasma  
Ans: The correct option is (a), solid.
In a solid, the atoms or molecules are closely packed together, which results in the least interatomic spacing compared to the other states of matter. In a liquid, the particles have more space between them than in a solid, while gases have even greater intermolecular spacing. Plasma is an ionized gas with very high temperatures, and it can have significant spacing between ions and electrons as well.

Q18: Give reasons for the following observation:
The smell of hot sizzling food reaches you several meters away, but to get the smell from cold food you have to go close.
Ans: When it is a higher temperature the diffusion rate (movement) of particles will be very fast when compared to the diffusion rate of particles at a lower temperature and since the temperature of hot sizzling food is higher than cold food, the smell of hot sizzling food will be reaching us from several meters away.

Q19: Liquids generally have a lower density as compared to solids. But you must have observed that ice floats on water. Find out why.
Ans: Ice floats on the water since there is a large empty space inside the 3D structure of ice due to which it becomes less in weight as compared to water and can float on water.

Q20: How does the water kept in an earthen pot (matka) become cool during summer?
Ans: Evaporation happens through the small pores on it causing a cooling effect, in an earthen pot. Therefore water kept in an earthen pot becomes cool during summer because of continuous evaporation.

Q21: Why are we able to sip hot tea or milk faster from a saucer rather than a  cup?
Ans: When we use a saucer instead of a cup the surface for evaporation to occur will be increased resulting in faster evaporation of particles of tea or milk and allowing it to cool faster and taking a sip becomes easier.

Q22: Convert the following temperatures to the Celsius scale.  
(a) The temperature is 293 K
Ans: When we use: ⁰C = K - 273
= 293 - 273
= 20 ⁰C
(b) The temperature is 470 K
Ans: When we use: ⁰C = K - 273
= 470 - 273
= 197 ⁰C

Q23: Give a reason for the following observations.
(a) Naphthalene balls disappear with time without leaving any solid.
Ans: Sublimation can be defined as the conversion between the solid and the gaseous phases of matter, with no intermediate liquid stage. Naphthalene balls will be having the property of sublimation because of which they directly vary from solid to gaseous state without a conversion into liquid. Hence, naphthalene balls will be vanishing with time leaving no solid.
(b)We can get the smell of perfume sitting several meters away.
Ans: Volatile substances such as perfumes change from liquid state to gaseous state very fast which allows them to diffuse and mix up with the air particles to reach our nostrils. Therefore we get the smell of perfume sitting several metres away.

Q24: Why is ice at 273 K more effective in cooling than water at the same temperature?
Ans: Ice will be producing a more intense cooling effect as compared to water at  273 K because at 273 K ice will be absorbing latent heat of melting from the surroundings and will be getting converted into water. Therefore ice at 273 K is more effective in cooling than water at the same temperature.

Q25: What is evaporation? What are the factors affecting it?
Ans: Evaporation can be defined as the process of conversion of a substance from its liquid state to a gaseous state at any temperature below its boiling point.
Evaporation will be dependent on the factor below:
(a) Surface area
(b) Temperature
(c) Humidity  
(d) Wind speed  

Q26: Define latent heat of vaporization and latent heat of fusion.
Ans: The heat energy required to change 1 kg of a substance from its liquid state to a gaseous state at atmospheric pressure without changing its temperature is known as latent heat of vaporization.

Q27: What happens when the temperature of the solids increase?
Ans: When we increase the temperature of the solid, we are giving energy to it.  That energy is utilized in increasing the kinetic energy of the particles and as a  result, the speed of the particles is increased and they vibrate more freely. Once the particles overcome the force of attraction between them they start moving more freely.

Q28: Why is it that on increasing the wind speed the rate of evaporation increases?
Ans: If we increase the speed of the wind, then they will be blowing away with them.  The water vapours in the air are blown away when the speed of wind is increased,  making room for more water vapours and increasing the rate of evaporation.

Q29: What are the characteristics of the particles of matter?  
Ans: The particles of matter have the following characteristics:
(i) The particles of matter are in continuous motion.
(ii) There are gaps between the particles of matter.
(iii) There is a force of attraction between the particles of matter which keeps them together.

Q30: Give reasons
(a) A gas fills completely the vessel in which it is kept.
Ans: The particles of gas have negligible attraction force between them because of which the particles move freely in all directions filling the whole container the gas is kept in.
(b) A gas exerts pressure on the walls of the container.
Ans: The particles of gas move freely due to which they collide with the container walls continuously and randomly. Therefore the collision of particles on the container walls exerts pressure on the walls.
(c) A wooden table should be called a solid.
Ans: Solids have rigid and fixed particles and have a definite shape and clear boundaries. Since a wooden table possesses all the qualities of a solid, it should be called a solid.
(d) We can easily move our hand in the air but to do the same through a solid block of wood we need a karate expert.
Ans: Since air is a gas and the forces of attraction between the particles of gas are very less which makes it easy to separate the particles with the help of an external force and hence we can easily move our hand in the air. Whereas in the case of solids the forces of attraction are very strong and we need a very high force to separate the particles of a solid and hence we need a karate expert for it.

Q31: Are the three states of matter inter-convertible? How can they interconnect?
Ans: Yes, the three states of matter can be converted into each other.

The three states of matter are interconvertible as shown below:
(a) By heating we can convert solids into liquids and by cooling we can convert liquids into solids.
(b) We can convert liquids into gases by vaporization and we can convert gases to liquids by condensation.
(c) Using sublimation we can convert solids into gases and vice versa and using condensation we can convert liquids into solids.

Q32: Why should we wear cotton clothes in summer?
Ans: Since cotton is a good absorbent of water, it absorbs all the sweat from our body and allows easy and fast evaporation. The sweat absorbs heat from our body and evaporates which makes us feel cooler during a hot summer day. That is why we should wear cotton clothes in summer.

Q33: A solution of H₂SO₄ is labeled 40%. The density of the solution is  1.3 gm/l. What is the concentration of the solution in %(m/v)?
Ans: The given concentration of the solution is 40%.
Therefore, 100 gm of the solution contains 40 g of H₂SO₄  
Density = mass / volume
1.3gm/l = 100g / volume
Volume of the solution = 100 / 1.3
= 100 / 1.3ml
So, = 100 / 1.3 of the solution contains 40g of H₂SO₄
Therefore, 100 ml of solution will contain 100 × 40 × 1.3 / 100 g of H₂SO₄
= 52 g of H₂SO₄
Therefore, the concentration is 52% (m/v).

Q34: Why is it that to smell cold food, we have to go close but the smell of hot food reaches us several meters away?
Ans: When the particles are at higher temperature, their movements are fast and therefore they can travel up to several meters. Hence the hot food’s smell will be reaching us several meters away.
At lower temperatures, the movements of particles are not very fast and particles do not have enough kinetic energy to travel a distance of several meters.  Therefore we have to go close to smell cold food.

Q35: Give an experiment to show that ammonium chloride undergoes sublimation.
Ans: Experiment for representing the sublimation of ammonium chloride (NHCl₄):
(a) Take a crystal of ammonium chloride (NHCl₄) inside a china dish and an inverted funnel.
(b) With the help of a burner, heat the ammonium chloride (NHCl₄) crystals.
(c) When the ammonium chloride (NHCl₄) crystal is heated, vapours of (NHCl₄) and the Ammonium chloride (NHCl₄) which is solidified along the walls at the beaker’s upper end is observable.
(d) This experiment shows that solid ammonium chloride (NHCl₄) undergoes solidification. It directly changes to vapour state from a solid state, it does not convert into liquid. 

Q36: What is distillation and fractional distillation? What is the basic property that separates the two methods?
Ans: The process of distillation is used for separating the components of a  mixture containing two liquids, having different boiling points and both liquids boil without decomposition.
The process of fractional distillation is used for separating the components of a  mixture containing more than two liquids having a boiling point difference of less than 25 K.
The basic property that separates these two methods is: 
Using distillation we can separate only those components which have a  significant difference in their boiling points. While fraction distillation is used when the difference in boiling points is less.

Q37: Why is it that a wooden chair should be called a solid and not a liquid?
Ans: A wooden chair is a rigid object, the particles of a wooden chair are tightly packed with each other, the chair has a definite shape and the chair has negligible compressibility. Since a wooden chair possesses all the properties of a solid and not of a liquid, it should be called a solid, not a liquid.

Q38: What is the state of inter particle distance inside a solid, liquid, and gas?
Ans: In a solid, the forces of attraction between the particles are very high and hence the particles of a solid will be very close to each other and the inter particle distance is the least. 

• In a liquid, the forces of attraction between the particles are very weak, and therefore the particles of a liquid will not be closely packed with each other and the inter-particle distance is large.  
• In a gas, the forces of attraction between the particles are almost negligible or extremely weak and therefore the particles of a gas are very loosely packed and are very far from each other and the inter particle distance is largest.

Q39: Differentiate between physical and chemical change?
Ans: The difference physical and chemical change is given below:
Q40: How does evaporation cause cooling?
Ans: During evaporation, the particles of a liquid absorb the heat from the surface and are converted into vapours utilizing the absorbed heat. This absorption of heat from the surface will be producing a cool surface.

Q41: Name A, B, C, D, E, and F in the following diagram showing changes in its state. 

Ans:
A is fusion or heating or melting.  
B is vapourisation.
C is cooling or Condensation.
D is cooling or solidification.
E is sublimation.  
F is solidification.

Q42: (a) Tabulate the differences in the characteristics of states of matter. 
Ans: Tabular differences in characteristics of matters are given below:(b) Comment upon the following: rigidity, compressibility, fluidity, filling a  gas container, shape, kinetic energy, and density.
Ans: 

• Rigidity → The property of matter to maintain its shape when external forces are applied to it is known as rigidity. Solids have this property.
• Compressibility → The property of matter to allow compression when high pressure is applied to it is known as compressibility. Some Liquids and all gases have this property. 
• Fluidity → The property of matter to flow and change in its shape when external forces are applied to it is known as fluidity. Both liquids and gases have this property. 
• Filling a gas container → Gases are fluid in nature and are highly compressible which allows them to be filled within a vessel at high pressure. A large volume of gas can be filled in a container of less volume making it suitable and more cost-efficient for transportation. 
• Shape → Only solid objects have well-defined shapes while liquids can acquire any shape depending on the container they are kept in and gases don’t have any shape. 
• Kinetic energy → The particles of a matter are continuously in motion and thus have kinetic energy. As the particles in solids have the least movement, the kinetic energy of solids is the least. The particles of gases have the freest movements and hence they have the highest kinetic energy. The order of kinetic  energies for different types of matters is: solid < liquid < gas  
• Density → Density of any substance can be explained as Mass per unit volume i.e. density = mass/volume.

Q43: A diver is able to cut through water in a swimming pool. Which property of matter does this observation show?
Ans: It is given that a diver is able to cut through water in a swimming pool. This is representing that the particles of water will be held together by weak forces of attraction between them and when any external force is applied the particles can be separated.