GSEB Class 9 Science Solutions Chapter 1 Matter in Our Surroundings

 

Question 1. Which of the following are matter? Chair, air, love, smell, hate, almonds, thought, cold, cold drink, smell of perfume.
Answer: Anything that occupies space and has mass is called matter. Matter can exist in three physical states - solid, liquid, and gas.

• Solid state → Chair and almond
• Liquid state → Cold drink
• Gaseous state → Air and smell of perfume.

Note: The sense of smell is not matter. However, the smell or odour of a substance is classified as matter. The smell of any substance (say, perfume) is the gaseous form of that substance which our olfactory system can detect (even of very low concentrations). Hence, smell of perfume is matter.
In simple words: Things that take up room and have weight are called matter. Matter can be solid, liquid, or gas. From the list, chair, air, almonds, cold drink, and the smell of perfume are matter. The feeling of smell itself is not matter, but the tiny particles causing the smell are.

Exam Tip: Remember the basic definition of matter: anything that has mass and occupies space. This helps differentiate between physical substances and abstract concepts like love or thought.

 

Question 2. Give reasons for the following observations. The smell of hot sizzling food reaches you several metres away, but to get the smell from the cold food you have to go close.
Answer: The particles of matter possess kinetic energy and thus are constantly moving. At low temperatures, the kinetic energy is low and hence the particles move slowly. But as the temperature rises, the kinetic energy increases accordingly and hence the particles move faster.
Now since the particles of hot vapours coming out of hot sizzling food move faster, therefore, they easily reach you even when you are several metres away. On the other hand, the particles of vapours coming out of cold food travel only slowly and hence do not reach you. Therefore, to get the smell from cold food you have to go close to the food.
In simple words: Matter particles always move. Hot food has more energy, so its smell particles move quickly and spread far away. Cold food has less energy, so its smell particles move slowly, meaning you need to get closer to smell them.

Exam Tip: The key concept here is kinetic energy and its direct relationship with temperature. Higher temperature means higher kinetic energy, leading to faster particle movement and quicker diffusion.

 

Question 3. A diver is able to cut through water in a swimming pool. Which property of matter does this observation show?
Answer: The ability of a diver to cut through water in a swimming pool shows that matter is made up of particles.
In simple words: When a diver pushes through water, it shows that water is made of tiny particles that have small gaps between them.

Exam Tip: This observation demonstrates the particulate nature of matter and that there are spaces between particles, which allows for movement through them.

 

Question 4. What are the characteristics of particles of matter?
Answer: The characteristics of particles of matter are:

• Particles of matter have spaces between them.
• Particles of matter are continuously moving.
• Particles of matter attract each other.

In simple words: Particles that make up matter have gaps between them, they are always moving, and they pull each other towards them.

Exam Tip: These three characteristics are fundamental to understanding the behavior and properties of matter in its various states (solid, liquid, gas).

 

Question 5. 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.
Answer: The given substances in the increasing order of their densities can be represented as: Air < exhaust from chimney < cotton < water < honey < chalk < iron
In simple words: Density means how much stuff is packed into a certain space. From lightest to heaviest, the order is air, then chimney smoke, then cotton, water, honey, chalk, and finally iron.

Exam Tip: Density is an intensive property and helps to distinguish between different substances. Remember that gases are generally less dense than liquids, which are less dense than solids, but there are exceptions (like ice floating on water).

 

Question 6. (a) Tabulate the difference in characteristics of states of matter.
(b) Comment upon the following: rigidity, compressibility, fluidity, filling a gas container, shape, kinetic energy and density
Answer:
(a)

(b)

Rigidity: It is the property due to which an object retains its shape and size. Solids are rigid while liquids and gases are not.

Compressibility: It is the property due to which a substance is reduced to a smaller volume when force is applied on it. Gases are highly compressible while solids and liquids are not.

Fluidity: It is the property due to which a substance tends to flow. Gases and liquids can flow, hence they are known as fluids.

Filling a gas container: Particles of a gas move freely in all directions and occupy all the space available to them. Hence, gas fills a container completely.

Shape: The structure of an object is called its shape. Solids have a definite shape while gases and liquids do not.

Kinetic energy: The energy of the particles of matter due to their movement is called their kinetic energy. Gases have maximum kinetic energy among the three states of matter. Kinetic energy increases with the rise in temperature and vice-versa.

Density: The mass per unit volume of a substance is called its density. Unit of density are kg m\(^{-3}\) or g cm\(^{-3}\). Generally, a substance has maximum density in its solid state as compared to liquid or gaseous state.

In simple words: (a) Solids have a set shape and volume, liquids take the shape of their container, and gases spread everywhere. Solids are hard to squish, liquids are a bit squishy, and gases are very squishy. Particles in solids are tightly held, less so in liquids, and very loose in gases. (b) Rigidity means keeping its shape, compressibility means being squished, fluidity means flowing, gases fill up any container, shape is how something looks, kinetic energy is movement energy (more heat = more movement), and density is how much stuff is in a certain space.

Exam Tip: Understanding these properties helps distinguish between the states of matter. For the table, focus on key differences like shape, volume, particle arrangement, and forces of attraction. For the terms, define each clearly and state how it applies to solids, liquids, and gases.

 

Question 7. 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 particles of a gas are constantly moving in all directions with different speeds. Therefore, they do not have a fixed volume and hence completely fill the vessel in which they are kept.
(b) The particles of a gas are constantly moving in all directions with different speeds. As a result of this random motion, the particles of a gas collide with one another and also against the walls of the container. As a result of these collisions, the gas exerts some force on the walls of the container. This force per unit area is called pressure of the gas. Thus, gases exert pressure due to the collisions of the particles of the gas on the walls of the containing vessel.
(c) A wooden table has a definite shape, distinct boundaries and a fixed volume, therefore, it should be called a solid.
(d) We can easily move our hand in air since the forces of attraction between the particles of a gas, i.e., air are very weak and hence can be easily overcome for the movement of the hand. In contrast, the particles of a solid are closely packed and hence the interparticle forces of attraction are very strong. As a result, they cannot be easily overcome for movement of the hand. In other words, we need a karate expert with almost incredible power to separate these particles.
In simple words: (a) Gas particles move freely everywhere, so they spread out and fill their container completely. (b) Gas particles are always moving and bumping into each other and the container walls, which creates pressure. (c) A wooden table has a set shape and size, so it's a solid. (d) Air particles are far apart and don't pull each other strongly, making it easy to move your hand. Wood particles are tightly packed and pull strongly, making it very hard to push through.

Exam Tip: For each part, relate the observation back to the fundamental properties of matter particles: their movement, spacing, and forces of attraction. This will help you provide clear and logical explanations.

 

Question 8. Liquids generally have lower density as compared to solids. But you must have observed that ice floats on water. Find out why?
Answer: The mass per unit volume of a substance is called density (density = mass/volume). As the volume of a substance increases, its density decreases. Though ice is a solid, it has large number of empty spaces between its particles. These spaces are larger as compared to the spaces present between the particles of water. Thus, the volume of ice is greater than that of water. Hence, the density of ice is less than that of water. A substance with lower density than water can float on water. Therefore, ice floats on water.
In simple words: Ice floats because it's less dense than water. Even though it's a solid, ice has more empty spaces between its particles than liquid water, so the same amount of ice takes up more room. This larger volume for the same mass makes ice lighter than water, allowing it to float.

Exam Tip: This is a common exception to the rule that solids are denser than liquids. Remember that the structure of ice (hydrogen bonding) creates an open, cage-like structure with more empty spaces, making it less dense than liquid water.

 

Question 9. Convert the following temperatures to the Celsius scale:
(a) 300 K
(b) 573 K.
Answer:
(a) \( 300 \, \text{K} = (300 - 273)^\circ \text{C} = 27^\circ \text{C} \)
(b) \( 573 \, \text{K} = (573 - 273)^\circ \text{C} = 300^\circ \text{C} \)
In simple words: To change Kelvin to Celsius, you simply subtract 273 from the Kelvin temperature.

Exam Tip: Always remember the conversion factor: \( \text{Celsius} = \text{Kelvin} - 273 \). Ensure you include the correct unit symbol (\(^\circ \text{C}\) for Celsius and K for Kelvin) in your final answer.

 

Question 10. What is the physical state of water at:
(a) 250°C
(b) 100°C
Answer:
(a) Water at \( 250^\circ \text{C} \) exists in gaseous state.
(b) At \( 100^\circ \text{C} \), the boiling point of water, water exists both as a liquid as well as a gas.
In simple words: At 250°C, water is always a gas (steam). At 100°C, water is changing from liquid to gas, so you will find both liquid water and water vapour at this temperature.

Exam Tip: Remember that \( 0^\circ \text{C} \) is the melting/freezing point (liquid and solid can coexist), and \( 100^\circ \text{C} \) is the boiling point (liquid and gas can coexist). Above \( 100^\circ \text{C} \), water is entirely gaseous.

 

Question 11. For any substance, why does the temperature remain constant during the change of state?
Answer: During a change of state, the temperature remains constant. This is because all the heat supplied to increase the temperature is utilised in changing the state by overcoming the forces of attraction between the particles. Therefore, this heat does not contribute in increasing the temperature of the substance.
In simple words: When a substance changes its state (like melting or boiling), its temperature stays the same. This is because the added heat energy is used to break the bonds between particles, not to make them move faster and increase temperature.

Exam Tip: The heat absorbed or released during a change of state without a change in temperature is called latent heat. This concept is crucial for explaining phase transitions.

 

Question 12. Suggest a method to liquefy atmospheric gases.
Answer: The gases can be converted into liquids by bringing its particle closer, so atmospheric gases can be liquefied either by decreasing temperature or by increasing pressure.
In simple words: To turn atmospheric gases into liquid, you need to get their particles closer together. This can be done by making them very cold or by squeezing them with high pressure.

Exam Tip: Liquefaction of gases relies on reducing the kinetic energy of particles (cooling) and/or reducing the intermolecular distances (increasing pressure) to allow attractive forces to dominate.

 

Question 13. Why does a desert cooler cool better on a hot dry day?
Answer: A desert cooler increases the humidity of the surrounding air. The water particles in the air take the heat from the surrounding objects and evaporates. In hot and dry days, the moisture level is very low in the atmosphere which increases the rate of evaporation. Because of faster evaporation, the cooler works well. That's why a desert cooler cools better on a hot dry day.
In simple words: Desert coolers work by evaporating water, which takes heat from the air, making it cooler. On a hot, dry day, the air has very little moisture, so water evaporates much faster. This faster evaporation means more heat is removed from the air, making the cooler work much better.

Exam Tip: The rate of evaporation is higher when the humidity is low and the temperature is high, which is why desert coolers are most effective in dry climates.

 

Question 14. How does the water kept in an earthen pot (matha) become cool during summer?
Answer: There are some pores in an earthen pot through which the liquid inside the pot evaporates. The heat required for evaporation is taken from the water in the pot. This makes the water inside the pot cool. In this way, water kept in an earthen pot becomes cool during summer.
In simple words: Earthen pots have tiny holes. Water inside these pots slowly escapes through the holes and evaporates. This process takes heat from the remaining water, making it feel cool, especially during hot summer days.

Exam Tip: This is another example of cooling due to evaporation. The porous nature of earthen pots facilitates continuous evaporation from the surface, leading to a cooling effect.

 

Question 15. 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.
In simple words: Acetone, petrol, and perfume evaporate very quickly, even when it's not hot. When you put them on your palm, they quickly absorb heat from your skin to turn into gas, making your palm feel cold.

Exam Tip: Substances with high volatility (evaporate easily at room temperature) produce a noticeable cooling effect as they absorb latent heat of vaporization from the surface they are on.

 

Question 16. Why are we able to sip hot tea or milk faster from a saucer rather than a cup?
Answer: A liquid has a larger surface area in a saucer than in a cup. Thus, the liquid evaporates faster and cools faster in a saucer than in a cup. For this reason, we are able to sip hot tea or milk faster from a saucer than a cup.
In simple words: A saucer has a wider surface than a cup. This larger surface area means the hot tea or milk can evaporate more quickly, which helps it cool down faster. Because it cools faster, you can drink it sooner from a saucer.

Exam Tip: The rate of evaporation increases with an increase in surface area. This principle explains why clothes dry faster when spread out or why hot liquids cool quicker in a wide container.

In-Text Activities Solved

(Textbook Page 1)

 

Activity 1.1
Answer:
Observation: When common salt or sugar is added to water, the particles of salt/sugar get into the empty spaces between particles of water. The salt will dissolve in water. There will be no change in the level of the solution by the addition of a small amount.
Conclusion: This experiment indicates that there are some vacant spaces among the particles of water and the particles of salt occupy these spaces. The particles of matter have spaces between them, as a result.
In simple words: We saw that salt or sugar dissolves in water without making the water level rise much. This shows that water particles have little gaps between them, and the salt or sugar particles fit into those gaps. So, all matter particles have spaces.

Exam Tip: This activity demonstrates the concept of intermolecular spaces and the particulate nature of matter, explaining why substances dissolve and mix without significantly increasing volume.

(Textbook Page 1)

 

Activity 1.2
Answer:
Observation: A crystal of potassium permanganate contains millions of tiny particles which keep on dividing by addition of solvent / water, hence the dark purple colour of the potassium permanganate solution will somewhat decrease.
Conclusion: When this experiment is repeated 5-8 times, the purple colour of the potassium permanganate solution will not disappear completely. Some colour will always persist even when the solution is very dilute. Matter is made up of very small particles which cannot be observed under a powerful microscope.
In simple words: When we add potassium permanganate to water, it spreads out, making the water purple. Even if we dilute it many times, the purple color doesn't totally disappear. This means matter is made of tiny, tiny particles that are too small to see directly.

Exam Tip: This activity highlights that matter is composed of extremely small particles and that these particles retain their properties even when highly dispersed.

(Textbook Page 2)

 

Activity 1.3
Answer:
Observation: In order to feel the smell of incense stick, we need to go near them. When the incense stick is lighted, the fragrance will immediately spread. Because as the temperature rises the kinetic energy of the incense particles increases. Hence, particles of incense more rapidly and thus intermix with the particles of the air rapidly. Yes we will get the smell sitting at a distance.
Conclusion: It can be concluded that particles of matter are always in motion. The movement of the particles is slow in case of unburnt incense stick. On the other hand, the movement of particles of matter is very fast when you supply heat energy by burning the incense stick.
In simple words: If an incense stick isn't lit, you have to get close to smell it. But once lit, its smell quickly fills the room. This happens because heat makes the tiny scent particles move faster and mix quickly with the air. This shows that matter particles are always moving, and heat makes them move even faster.

Exam Tip: This activity illustrates the kinetic nature of particles and how temperature influences their speed and the rate of diffusion. Higher temperature means higher kinetic energy and faster diffusion.

(Textbook Page 2)

 

Activity 1.4
Answer:
Observation: Due to weak attraction forces between the particles of ink, the colour of the blue or red ink spreads throughout the water in the beaker immediately. The drop of honey also spreads but takes much more time than ink due to strong intermolecular forces. The total time taken by the drop of ink to spread evenly throughout the water is much more less than that of honey.
Conclusion: Thus, we can conclude that particles of liquid diffuse on its own because they are constantly in motion. However, the rate of diffusion of two different liquid particles may vary and depends upon the strength of forces. Stronger the forces of attraction lower is the average speed.
In simple words: Blue or red ink spreads quickly in water because its particles don't strongly attract each other. Honey, however, spreads much slower because its particles have strong attraction forces. This shows that liquid particles move and mix on their own, but how fast they mix depends on how strongly their particles pull each other.

Exam Tip: This activity demonstrates diffusion in liquids and how the strength of intermolecular forces affects the rate of diffusion. Liquids with weaker forces diffuse faster.

(Textbook Page 2)

 

Activity 1.5
Answer:
Observation: As the time passes, the water turns into purple colour solution in both the beakers because the particles of water and potassium permanganate get evenly mixed by the process of diffusion. But the water contained in the hot beaker turns into a purple colour solution at a faster rate as compared to water contained in the cold beaker. Yes, the rate of mixing changes with temperature. This is because, in hot water the particles of water and that of potassium permanganate have more kinetic energy and therefore move faster. Hence, they mix with each other more quickly.
Conclusion: This shows that particles of matter are constantly moving and their rate of diffusion into liquid varies with temperature.
In simple words: When potassium permanganate is added to water, it spreads out, making the water purple. This happens faster in hot water than in cold water. This shows that all matter particles are always moving, and they mix faster when it's warmer because they have more energy.

Exam Tip: This activity reinforces the concept of diffusion and highlights the effect of temperature on the rate of diffusion. Increased temperature provides particles with more kinetic energy, leading to faster movement and mixing.

(Textbook Page 3)

 

Activity 1.6
Answer:
Observation: The third group is easiest to break. This is because; in the third group, the students form a chain by touching each other with only their fingertips. The group in which the particles hold each other with maximum force is the first group, hence such a human chain was most difficult to break.
Conclusion: This shows that particles of matter which are held with each other with maximum force of attraction cannot be separated easily.
In simple words: We saw that the group of students holding hands tightly was the hardest to break apart, but the group barely touching with fingertips was easiest. This proves that matter particles that pull each other strongly are much harder to separate.

Exam Tip: This activity visually represents the varying strengths of intermolecular forces of attraction between particles of matter. Stronger forces require more energy to overcome.

(Textbook Page 3)

 

Activity 1.7
Answer:
Observation: It is very easy to break the piece of chalk into smaller particles due to weakest forces of attraction. On the other hand, the iron ball does not break even with a large force due to strong forces of attraction.
Conclusion: This shows that the force of attraction between the particles of chalk is quite weak; whereas the force of attraction between the particles of iron ball is very strong.
In simple words: We can easily break chalk into smaller pieces because its particles don't hold together very strongly. But an iron ball is very hard to break, even with a lot of force, because its particles pull each other very powerfully. This means chalk has weak particle attraction, and iron has very strong particle attraction.

Exam Tip: This activity reinforces the idea that the strength of attractive forces between particles varies significantly between different substances and determines their physical properties like hardness or brittleness.

(Textbook Page 3)

 

Activity 1.8
Answer:
Observation: Yes, this shows that the interparticle attraction between particles of water (liquid state) is not very strong. The stream of water remains together because the particles of matter attract each other to form uniform layers.
Conclusion: This exhibit that particles of water have force acting between them which keeps them together. However, the strength of this force is not very strong.
In simple words: We observed that water particles in a stream stay together, but their pull on each other is not super strong. This shows that water particles do attract each other, helping the stream stay whole, but the forces are not as powerful as in solids.

Exam Tip: This activity demonstrates the cohesive forces between liquid particles. While strong enough to keep the liquid together, they are weaker than in solids, allowing liquids to flow and change shape.

(Textbook Page 4)

 

Activity 1.9
Answer:
Observation: Yes, all these articles have definite shapes, distinct boundaries and a fixed volume. Pen will break, there will be no effect on book and thread and after a long time needle will break. No, the articles are not capable of diffusing into each other. No, the articles do not get compressed.
Conclusion: Thus, we can conclude that solid articles have a definite shape, distinct boundaries and a fixed volume. They are rigid and non-compressible i.e., they may break under force but their shape cannot be changed.
In simple words: We saw that a pen, book, and needle keep their own shape and size. It's hard to squish them, and they don't mix into each other. You can break them if you apply enough force, but their basic shape won't just change on its own. This proves that solids have a fixed shape and volume and are not easily compressed.

Exam Tip: This activity emphasizes the defining characteristics of solids: definite shape, definite volume, distinct boundaries, rigidity, and incompressibility. They maintain their form unless an external force is applied.

(Textbook Page 4)

 

Activity 1.10
Answer:
Observation: All these liquids will flow when spilt on the floor. Volume of the liquid remains same i.e. 50 mL when it is transferred from one container to the other. The shape of a liquid is not fixed. It takes up the shape of the container in which it is put. Further, when the liquid is poured from one container to the other, it flows.
Conclusion: Thus, we can conclude that liquids do not have a fixed shape but have a fixed volume, i.e., they take up the shape of the container in which they are poured but their volume does not change on changing the shape of the container. Liquids flow and hence they can be called as fluids.
In simple words: When liquids are spilled, they spread out. We also saw that 50 mL of liquid stays 50 mL, even if poured into a different container. Liquids take the shape of whatever they are in and can flow. This shows liquids have a set volume but no fixed shape, and they are fluids.

Exam Tip: This activity demonstrates the properties of liquids: fixed volume, no fixed shape (take the shape of the container), and ability to flow (fluidity). These properties arise from the weaker intermolecular forces compared to solids.

(Textbook Page 5)

 

Activity 1.11
Answer:
Observation: The pistons of the syringes (containing chalk pieces and water) require a large amount of force, while the piston of the third syringe which contains air is comparatively easier to push.
Conclusion: Thus, we can conclude that a solid and a liquid does not get compressed on applying pressure, but a gas can be compressed easily by applying pressure. Therefore, gases are highly compressible liquids are almost incompressible while solids are completely incompressible.
In simple words: We found it hard to push the plungers of syringes filled with chalk or water, but very easy to push the one with air. This tells us that solids and liquids don't squish easily under pressure, but gases are very easy to compress.

Exam Tip: This activity effectively illustrates the property of compressibility, showing that gases are highly compressible due to large intermolecular spaces, while solids and liquids are nearly incompressible.

(Textbook Page 6)

 

Activity 1.12
Answer:
Observation: On increasing the temperature of solids, the kinetic energy of the particles increases. Due to the increase in kinetic energy, the particles start vibrating with greater speed. Hence, heat energy supplied is used up in overcoming the forces of attraction between the liquid water particles to change them from liquid water to steam.
Conclusion: The energy supplied by heat overcomes the forces of attraction between the particles. The particles leave their fixed positions and start moving more freely. A stage is reached when the solid melts and is converted in a liquid. This stage is called as the melting point of the solid.
In simple words: When solids get hotter, their particles gain more energy and shake faster. This added heat energy helps break the ties between particles, letting them move more freely. Eventually, the solid melts into a liquid, and this temperature is called its melting point.

Exam Tip: This activity explains the process of melting and the concept of melting point, highlighting how absorbed heat energy is used to overcome intermolecular forces rather than increasing temperature.

(Textbook Page 8)

 

Activity 1.13
Answer:
Observation: The solid ammonium chloride does not melt on heating. It is directly converted into vapours and gets deposited on the inner wall of the funnel as crystalline solid on coming in contact with the cold inner walls of the funnel.
Conclusion: A change of state directly from solid to gas without changing into liquid state or vice versa is called sublimation.
In simple words: We saw that when solid ammonium chloride is heated, it doesn't melt into a liquid. Instead, it turns straight into a gas, which then cools and turns back into a solid on the cold part of the container. This direct change from solid to gas or gas to solid is called sublimation.

Exam Tip: Sublimation is an important process to recognize, as not all solids melt before turning into a gas. Common examples include camphor, naphthalene, and dry ice (solid carbon dioxide).

(Textbook Page 9)

 

Activity 1.14
Answer:
Observation: The surface area of water exposed to the atmosphere is minimum in case of a test tube, therefore, it takes a long time (2/3 days) for 5 mL of water to evaporate. Although the surface area of 5 ml. of water taken in two open china dishes is the same, yet water in the china dish placed near the window or under the fan evaporates more quickly than the water in the china dish placed inside a cupboard or on a shelf in your class room. All the three steps of activity will take longer time on a rainy day since the humidity is high.
Conclusion: Evaporation is a surface phenomenon and so, when the surface area of the liquid is greater, the rate of evaporation will be more. The rate of evaporation of a liquid decreases with increase in the humidity of air and it increases with increase in the speed of the wind. Let us now differentiate evaporation and boiling.
In simple words: Water in a test tube evaporates slowly, but water spread out in a dish, especially by a window or fan, evaporates much faster. Also, on a rainy day, evaporation is slower because the air is already damp. This shows that evaporation happens faster with more surface area and wind, but slows down when the air is humid.

Exam Tip: Factors affecting the rate of evaporation are crucial: surface area (more = faster), temperature (higher = faster), humidity (lower = faster), and wind speed (higher = faster).

Gujarat Board Class 9 Science Matter in Our Surroundings Textbook Questions and Answers

 

Question 1. Convert the following temperatures to the Celsius scale:
(a) 300 K
(b) 573 K
Answer:
(a) \( 300 \, \text{K} = (300 - 273)^\circ \text{C} = 27^\circ \text{C} \)
(b) \( 573 \, \text{K} = (573 - 273)^\circ \text{C} = 300^\circ \text{C} \)
In simple words: To change Kelvin temperature to Celsius, you just need to subtract 273 from the Kelvin value.

Exam Tip: Remember the formula for Kelvin to Celsius conversion: \( \text{Temperature in }^\circ\text{C} = \text{Temperature in K} - 273 \). Always include the correct unit symbols.

 

Question 2. Convert the following temperatures to the Kelvin scale:
(a) 25°C
(b) 373°C
Answer:
(a) \( 25^\circ \text{C} = (25 + 273) \, \text{K} = 298 \, \text{K} \)
(b) \( 373^\circ \text{C} = (373 + 273) \, \text{K} = 646 \, \text{K} \)
In simple words: To change Celsius temperature to Kelvin, you just need to add 273 to the Celsius value.

Exam Tip: Remember the formula for Celsius to Kelvin conversion: \( \text{Temperature in K} = \text{Temperature in }^\circ\text{C} + 273 \). Ensure you use the correct unit symbols for your answers.

 

Question 3. Give reasons 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 shows the property of sublimation. Evaporation of naphthalene takes place easily and so it disappears in the course of time without leaving a solid.
(b) Perfume vaporizes very fast and its vapours diffuse into air easily. That is why we can smell perfume sitting several metres away.
In simple words: (a) Naphthalene balls disappear over time because they change directly from a solid to a gas, a process called sublimation, without becoming liquid first. (b) Perfume has particles that quickly turn into a gas and spread out in the air, allowing its scent to travel far.

Exam Tip: For (a), recall the definition of sublimation and common examples. For (b), remember that gases diffuse rapidly due to the high kinetic energy and large spaces between their particles.

 

Question 4. Arrange the following substances in increasing order of forces of attraction between the particles: water, sugar, oxygen.
Answer: Sugar is a solid, the forces of attraction between the particles of sugar are strong. Water is a liquid, the forces of attraction here are weaker than sugar. Oxygen is a gas, the forces of attraction are the weakest in gases. Thus, the increasing order of forces of attraction between the particles of water, sugar and oxygen is - Oxygen < Water < Sugar.
In simple words: We need to order substances by how strongly their particles pull each other. Oxygen is a gas, so its particles pull weakest. Water is a liquid, so its particles pull stronger than oxygen but weaker than solids. Sugar is a solid, so its particles pull the strongest. So, the order from weakest to strongest attraction is Oxygen, then Water, then Sugar.

Exam Tip: The strength of intermolecular forces generally increases from gases to liquids to solids. Relate this directly to the states of matter for common substances.

 

Question 5. What is the physical state of water at:
(a) 25°C
(b) 0°C
(c) 100°C
Answer:
(a) At \( 25^\circ \text{C} \), the physical state of water is liquid.
(b) At \( 0^\circ \text{C} \), the physical state of water can be either a solid (ice) or a liquid.
(c) At \( 100^\circ \text{C} \), the physical state of water can be either a liquid or a gas (steam).
In simple words: (a) At normal room temperature (25°C), water is liquid. (b) At 0°C, water can be both solid (ice) and liquid, as this is its freezing/melting point. (c) At 100°C, water can be both liquid and gas (steam), as this is its boiling point.

Exam Tip: Critical temperatures for water are \( 0^\circ \text{C} \) (melting/freezing point) and \( 100^\circ \text{C} \) (boiling/condensation point). At these points, both phases (solid/liquid or liquid/gas) can coexist.

 

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) At room temperature (\( 25^\circ \text{C} \)), water is a liquid because it has the following characteristics of a liquid:

• At room temperature, water has no shape but has a fixed volume, that is, it occupies the shape of the container in which it is kept.
• At room temperature, water flows.

(b) An iron almirah is a solid at room temperature (\( 25^\circ \text{C} \)) because:

• It has a definite shape and volume like a solid at room temperature.
• It is rigid as a solid at room temperature.

In simple words: (a) Water is liquid at room temperature because it flows easily and takes the shape of its container, but its volume stays the same. (b) An iron almirah is solid at room temperature because it has a set shape and size, and it's stiff and unyielding.

Exam Tip: When justifying the state of matter, always refer to the defining properties: shape, volume, fluidity, and rigidity. For water, emphasize its variable shape and constant volume; for iron, its definite shape, volume, and rigidity.

 

Question 7. Why is ice at 273 K more effective in cooling than water at the same temperature?
Answer: Ice at \( 273 \, \text{K} \) has less energy than water (although both are at the same temperature). Water possesses the additional latent heat of fusion. Hence, at \( 273 \, \text{K} \), ice is more effective in cooling than water.
In simple words: Ice at 273 K (0°C) is better at cooling than water at the same temperature. This is because ice needs to absorb extra heat, called latent heat of fusion, to turn into water. This extra heat absorption means ice removes more heat from its surroundings, making it more effective for cooling.

Exam Tip: The key concept here is latent heat of fusion. Ice at its melting point absorbs a significant amount of heat from the surroundings to change into liquid water without a temperature increase, providing a stronger cooling effect.

 

Question 8. What produces more severe burns, boiling water or steam?
Answer: Steam produces more severe burns than boiling water. This is because steam has more energy than boiling water, present in it in the form of latent heat of vaporisation,
In simple words: Steam causes worse burns than boiling water. This happens because steam carries extra hidden heat, called latent heat of vaporization, which boiling water does not have. When steam touches your skin, it releases this extra heat as it condenses, causing a more severe burn.

Exam Tip: This question also relates to latent heat. Steam at \( 100^\circ \text{C} \) contains more energy (latent heat of vaporization) than boiling water at \( 100^\circ \text{C} \), making it more dangerous upon contact.

 

Question 9. Name A, B, C, D, E and F in the following diagram showing change in its state.
Answer:
A → Melting/Fusion
B → Boiling/Vaporisation
C → Condensation
D → Solidification
E → Sublimation
F → Sublimation
In simple words: A is melting, when a solid turns liquid. B is boiling, when liquid turns gas. C is condensation, when gas turns liquid. D is solidification, when liquid turns solid. E and F are both sublimation, where a solid directly becomes a gas or vice versa.

Exam Tip: Understand the terms for each phase transition: Melting (solid to liquid), Freezing/Solidification (liquid to solid), Boiling/Vaporization (liquid to gas), Condensation (gas to liquid), and Sublimation (solid to gas or gas to solid).