Class 9 Science Chapter 1 Matter In Our Surrounding

Key Concept: Energy Absorption

b) It reduces the humidity around the cloth

[Solution Description] Wind increases the rate of evaporation by removing the saturated air layer around the wet cloth, allowing more air to come into contact with the water molecules. This increased airflow facilitates faster evaporation as the particles at the surface absorb energy from their surroundings.

Key Concept: Dry Ice Application

b) It sublimates directly into gas

[Solution Description]

Dry ice is solid carbon dioxide that sublimates directly from solid to gas without passing through a liquid phase. This property makes it useful for refrigeration as it avoids any wet residue, unlike water ice which melts into water. Hence, the sublimation aspect of dry ice is advantageous in maintaining dryness.

Key Concept: Application-Based

b) A windy, dry day

[Solution Description] Clothes dry faster when the evaporation rate is high. A windy and dry environment promotes rapid removal of moisture from clothes because wind speeds up the removal of evaporated water vapour, while low humidity allows more evaporation.

Key Concept: Real World Application, Phase Change Energy Dynamics

b) 450 kJ

[Solution Description] The latent heat of fusion is the amount of energy needed to change the state of a certain mass of a substance from solid to liquid without changing its temperature. The formula for calculating the energy ($Q$) required is: $Q = m \times L_f$

where $m$ is the mass and $L_f$ is the latent heat of fusion.

Given $m = 3 \, \text{kg}$ and $L_f = 150 \, \text{kJ/kg}$,

substituting these values gives: $Q = 3 \, \text{kg} \times 150 \, \text{kJ/kg} = 450 \, \text{kJ}$

Therefore, the energy required to completely melt 3 kg of the substance is 450 kJ.

Key Concept: Real-World Application

d) Dissolved oxygen in water allows them to breathe.

[Solution Description] Aquatic animals can breathe underwater because oxygen dissolves in water. This dissolved oxygen is what fish and other aquatic organisms use for respiration.

Key Concept: Volume and Color

b) Decreases

[Solution Description] When water volume increases during dilution, the concentration of potassium permanganate decreases. The increase in volume causes the color intensity of the solution to decrease because the particles are spread out over a larger area.

Key Concept: Advanced Experiment Analysis, Complex Real-World Application

a) Diffusion is slower at 25°C because particles have less kinetic energy.

[Solution Description] The rate of diffusion is influenced by the temperature because higher temperatures increase the kinetic energy of particles. This increased energy results in faster movement of water molecules as well as the solute particles (potassium permanganate), leading to quicker mixing at 50°C than at 25°C. Therefore, in the beaker with water at 50°C, the particles will diffuse more rapidly due to increased molecular motion compared to 25°C.

Key Concept: Diffusion Rate Analysis, Real-world Scenario

c) Low altitude, warm conditions

[Solution Description] The rate of diffusion of gases increases with higher temperatures due to increased kinetic energy and particle speed, following Graham’s law of effusion. For $CO_2$, the highest diffusion rate occurs at high temperature and low pressure because particles move faster and have greater space. Therefore, option involving highest temperature will result in highest diffusion rate.

Key Concept: Latent Heat

a) Melting of ice

[Solution Description] The latent heat of fusion is the heat required to convert a solid into a liquid at the same temperature. Therefore, it plays an essential role during the melting process.

Key Concept: Advanced Gas Laws, Pressure-Temperature Relationship

b) 2 atm

[Solution Description]

According to Boyle’s Law, which states $P_1 V_1 = P_2 V_2$ for constant temperature conditions, we can calculate the final pressure $P_2$.

Given:

– Initial pressure, $P_1 = 5$ atm

– Initial volume, $V_1 = 10$ liters

– Final volume, $V_2 = 25$ liters

We need to find $P_2$. Using the formula: $P_2 = \frac{P_1 V_1}{V_2} = \frac{5 \times 10}{25} = 2 \text{ atm}$

Therefore, the final pressure is 2 atm.

Key Concept: Practical Scenario

c) Because it is highly compressible.

[Solution Description] Compressed Natural Gas (CNG) is preferred as a vehicle fuel because it is highly compressible, allowing a large amount of gas to be stored in small cylinders. This property makes CNG an efficient choice for transportation purposes.

Key Concept: Force Comparison

d) Solid

[Solution Description] In solids, the particles are closely packed together because they experience strong forces of attraction.

Key Concept: Critical Thinking, Conceptual Integration

c) Sublimation is solid to gas, deposition is gas to solid

[Solution Description] Sublimation refers to the direct change from a solid state to a gaseous state without passing through a liquid state, whereas deposition is the reverse process where a gas changes directly to a solid. This logical conclusion comes from understanding the fundamental differences in phase transitions between the two processes.

Key Concept: Comparative Analysis, Conceptual Understanding

d) Liquids diffuse faster than solids but slower than gases due to greater space between particles compared to solids.

[Solution Description] The rate of diffusion depends on the space between particles and their kinetic energy. Solids have closely packed particles with minimal movement, resulting in slow diffusion. Liquids have more space and higher kinetic energy, allowing moderate diffusion. Gases, with widely spaced, high-energy particles, diffuse rapidly.

Key Concept: Basic Properties

a) Solid

[Solution Description] Solids are characterized by having a definite shape and volume, distinct boundaries, and rigidity. This makes option A correct.

Key Concept: Movement of Particles

b) Air freshener spreads smell in a room

[Solution Description] The spreading of fragrance from an air freshener across a room is due to the continuous movement of gas particles, demonstrating their ability to move freely and spread out in the air.

Key Concept: Application of Concepts

c) Latent heat of vaporisation absorbs the energy.

[Solution Description] The temperature remains constant during boiling because the energy supplied is used for converting the liquid into vapour instead of increasing the temperature. This energy required for the phase change without a change in temperature is called the latent heat of vaporisation.

Key Concept: Energy Comparison

c) Because water particles have absorbed latent heat of fusion

[Solution Description]

Even though both are at the same temperature, the particles in water have absorbed the latent heat of fusion required to transition from solid to liquid. This additional energy increases the potential energy without raising the kinetic energy.

Key Concept: Pressure Change Effect

b) Gas particles come closer together

[Solution Description] According to the ideal gas laws, increasing the pressure on a gas while keeping the temperature constant will decrease the volume of the gas. As pressure increases, gas particles are forced closer together, reducing the space between them and thus decreasing the volume.

Key Concept: Liquid Shape and Volume

a) Both occupy the same height in the cylinder.

[Solution Description] Both liquids will fill the same volume initially because they are poured in fixed volumes. However, the denser liquid will have more mass per unit volume. The shape within the container remains due to the cylindrical container shape, not the density of the liquids.

Key Concept: Particle Arrangement

d) Solid

[Solution Description] Solids have the strongest force of attraction between particles. This force keeps the particles closely packed in a fixed arrangement, giving solids a definite shape and volume.

Key Concept: Concept Integration, Multi-step Solutions

a) Particles in both states maintain identical spacing.

[Solution Description] In this scenario, a solid changes into a liquid state within a closed container without altering the total volume. Before melting, particles in a solid are closely packed in a fixed arrangement, with only vibrational movement. Upon melting, these particles gain enough energy to overcome the rigid structure, allowing them to flow more freely, characteristic of a liquid. However, since there is no change in the overall volume, it implies that although particles have gained kinetic energy and random motion, the intrinsic properties such as intermolecular spaces and forces transition smoothly without affecting the system volume. Thus, it shows liquids have similar particle arrangements in terms of occupied space but differ in freedom of movement compared to solids.

Key Concept: Basic Concept

b) The spreading of particles from high to low concentration

[Solution Description] Diffusion is the process where particles move from an area of high concentration to an area of low concentration. In this context, when copper sulphate crystals are placed in water, the particles spread and mix with the water molecules, causing the solution to change color uniformly over time.

Key Concept: Advanced Real-World Scenario, Critical Thinking

c) Wearing lightweight cotton clothing and staying hydrated

[Solution Description] In a desert environment, temperatures are typically high and humidity is low, making it ideal for effective evaporation. Cotton clothing allows sweat to evaporate more efficiently than other materials like polyester or wool. This is because cotton absorbs moisture and facilitates its transfer to the air, assisting in maintaining a cooler body temperature through increased evaporative cooling.

Therefore, wearing lightweight cotton clothing while ensuring proper hydration to facilitate continuous sweating will enhance the cooling effect during a marathon in such an environment.

Key Concept: Temperature Effect on State

c) Kinetic energy increases

[Solution Description]

As the temperature of a substance is increased, the kinetic energy of its particles also increases because temperature is a measure of the average kinetic energy of the particles. This results in faster movement and potentially changes in physical states.

Key Concept: Liquid vs Gas Diffusion

c) There is more space between gas molecules allowing faster diffusion.

[Solution Description] Gases tend to diffuse faster in water as they possess greater kinetic energy compared to liquids, which allows them to spread out rapidly. Additionally, gases have more space between their particles, facilitating quicker movement and mixing.

Key Concept: Comparative Analysis

c) The earthen pot cools more effectively than the metal container.

[Solution Description] The earthen pot cools more effectively than a metal container because it allows water to seep through its porous surface, facilitating evaporation. This evaporation process absorbs heat from the surroundings due to the latent heat of vaporization, thus cooling the remaining water. In contrast, a metal container does not allow evaporation of water from its surface.

Key Concept: Wind Speed Impact

c) It increases the rate of evaporation

[Solution Description] Increased wind speed enhances evaporation by removing moist air near the surface, allowing drier air to replace it and further evaporation to occur.

Key Concept: Latent Heat

c) The heat required to change 1 kg of liquid to gas

[Solution Description] The latent heat of vaporization is the amount of heat required to change 1 kg of liquid into gas without any change in temperature.

Key Concept: Real World Application, Surface Phenomena

c) Spreading water over a large surface area to maximize evaporation.

[Solution Description] To create a cooling effect through evaporation, one can increase the surface area so that more liquid molecules have the chance to escape into the gas phase, taking away heat and reducing temperature. Therefore, spreading water over a large surface will enhance evaporation due to increased surface exposure.

Key Concept: Comparison with Other Processes

c) Deposition skips the liquid state

[Solution Description] Deposition involves the transition from a gas directly to a solid, skipping the liquid state, while condensation involves the change from gas to liquid.

Key Concept: Diffusion Rate, Intermolecular Forces

c) Difference in their molar mass (or particle mass)

[Solution Description]
The rate of diffusion of gases is influenced primarily by the mass of their particles. According to Graham’s Law of Diffusion, at constant temperature and pressure, lighter gases diffuse faster than heavier ones. This is because particles of lighter gases have higher average speeds, allowing them to spread more quickly. For example, hydrogen diffuses much faster than oxygen due to its significantly lower molar mass. Therefore, when two gases diffuse at noticeably different rates under the same conditions, the most plausible explanation is a difference in the mass of their particles. Properties like color or odor may help identify a gas but do not affect its rate of diffusion.

Key Concept: Comparative Analysis, Kinetic Energy Insight

c) The kinetic energy of molecules in Liquid A is higher, easing the escape to the vapour state.

[Solution Description]

The key factors affecting evaporation rate include surface area, temperature, humidity, wind speed, and the nature of the liquid itself. If all conditions are identical, differences arise mainly from inherent properties such as vapor pressure or intermolecular forces.

Liquid A likely has weaker intermolecular forces compared to Liquid B, allowing particles to escape more readily, thus increasing evaporation rates. This indicates higher kinetic energy distribution among particles in Liquid A, facilitating easier transition to the vapor phase.

Key Concept: Diffusion Rate

c) Decreasing the temperature of the water

[Solution Description] The rate of diffusion depends on temperature, as higher temperatures increase kinetic energy and speed up particle movement. Therefore, lowering the temperature of the water will decrease the rate of diffusion.

Key Concept: Energy Transfer

b) Skin to acetone

[Solution Description] During evaporation, acetone molecules absorb energy from their surroundings to overcome intermolecular forces and transition into the gas phase. This energy absorption results in energy being transferred from the skin (or surroundings) to the acetone, causing a cooling sensation.

Key Concept: Physical Nature of Matter

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.

[Solution Description]

The assertion states that particles of matter are continuously moving, which is true based on the kinetic theory of matter. The reason provided is also true because as the temperature increases, the kinetic energy of particles increases, leading to faster movement and thus increasing the rate of diffusion. However, while both statements are true, the reason given does not directly explain why particles of matter are continuously moving; it instead describes an effect of temperature on diffusion.

Key Concept: Complex Application, Advanced Comparison

d) Potassium permanganate particles intermix with water particles utilizing the spaces between them.

[Solution Description] Potassium permanganate particles diffuse through the water because the particles of both substances have space between them. The kinetic energy of the molecules causes them to move and occupy these spaces, resulting in diffusion. In liquids, particles are close but not tightly packed, allowing for diffusion.

Key Concept: Basic Definition

d) Air

[Solution Description] Matter is defined as anything that occupies space and has mass. Therefore, among the options given, only air qualifies as matter because it occupies space and has mass.

Key Concept: Interconversion of States, Pressure Effects

b) It will turn into vapor directly by sublimation.

[Solution Description] Dry ice is solid carbon dioxide which sublimates directly from solid to gas at -78.5°C under 1 atmosphere. Moving it to a room with higher temperature and same pressure will cause direct sublimation due to increase in temperature.

Key Concept: Scenario-Based

c) More water will evaporate in the dry environment.

[Solution Description] In a dry environment, the air has a lower water vapour content, allowing more room for water to evaporate into the air. Hence, the bowl in the dry environment will lose more water compared to the bowl in the humid environment, where the air is already saturated with moisture.

Key Concept: Particle Movement

c) Particles move randomly

[Solution Description] The spreading of ink in water suggests that particles are in constant random motion, which allows them to move and spread from areas of higher concentration to areas of lower concentration.

Key Concept: Real-World Application, Diffusion Rate

c) The rate of diffusion increases as particle movement becomes more rapid.

[Solution Description] The rate of diffusion is directly related to the kinetic energy of particles. Increasing the temperature increases the kinetic energy of the particles, causing them to move faster and thus increasing the rate of diffusion.

Key Concept: Basic Definition

b) Air

[Solution Description] Matter is anything that has mass and takes up space. Air, being composed of particles, occupies space and has mass, therefore it is considered matter.

Key Concept: Real-World Application, Non-Straightforward Path

c) Hydrogen bonds cause ice to expand making it less dense

[Solution Description] Ice floats on water because of the hydrogen bonding between water molecules. When water freezes, it forms a crystalline structure that is less dense than liquid water due to the way hydrogen bonds arrange the molecules further apart compared to when they are in a liquid state. This increase in molecular distance causes ice to have a lower density than liquid water, allowing it to float. Hence, the particles in solid ice are less tightly packed than in liquid water, resulting in buoyancy.

Key Concept: Diffusion Rate

b) The diffusion rate increases

[Solution Description] Increasing the temperature provides more kinetic energy to the particles, which enhances their movement. This increased movement results in a faster rate of diffusion.

Key Concept: Real-world Application

c) Due to increased surface area exposure

[Solution Description] A wider surface area exposed to air leads to an increased rate of evaporation because evaporation is a surface phenomenon. Laying the towel flat increases the exposed surface area compared to hanging it in a narrow space.

Key Concept: Real-World Scenarios

c) Solids have fixed volume and shape

[Solution Description] Solids have rigidity and negligible compressibility, which means they can withstand external forces without changing their shape or volume significantly. During construction, this rigidity helps buildings maintain their structure.

Key Concept: Temperature Effect

d) It increases particle motion

[Solution Description] When temperature increases, the kinetic energy of particles also increases, causing them to move faster. This enhances diffusion and other processes like expansion.

Key Concept: Latent Heat Application

c) Steam contains latent heat of vaporization

[Solution Description]

Steam causes more severe burns than boiling water at 100°C because, when steam comes into contact with the skin, it releases additional energy as it condenses from gas to liquid. This process involves the release of latent heat of vaporization. The steam first loses its latent heat and then cools down to body temperature, transferring a significant amount of thermal energy in the process.

Key Concept: Detailed Comparison

b) Due to larger particle distances and higher speeds in gases

[Solution Description] The particles in gases like air move randomly at high speeds and have larger spaces between them compared to liquids. This allows the smell of perfume to diffuse much faster in air. In contrast, particles in liquids are closer together and move more slowly, leading to slower diffusion rates.

Key Concept: Compression Mechanism

c) Assertion is true, but Reason is false.

[Solution Description]

The assertion is true as sponges are known for their compressibility due to the air trapped in minute holes, which gets expelled when compressed, allowing them to reduce in size temporarily. However, the reason given is false because it contradicts the actual property that allows compression; it is not the solid structure but the expulsion of air that enables this feature.

Key Concept: Real-World Application

c) Because evaporation of sweat requires heat which is taken from the body, cooling it

[Solution Description] Sweating cools the body through the process of evaporation. When sweat evaporates from the skin surface, it absorbs latent heat of vaporization from the body, thereby removing heat and cooling the body down.

Key Concept: Advanced Application, Multi-step Reasoning

d) Nitrogen diffuses faster due to its lower molecular weight

[Solution Description] The rate of diffusion for gases depends on their molecular weights as well as the temperature. Since the temperature is constant and both gases are at the same initial conditions, the lighter gas will diffuse faster. Oxygen has a molecular weight of 32 g/mol, while nitrogen has a molecular weight of 28 g/mol. Therefore, nitrogen will diffuse slightly faster than oxygen.

Key Concept: Solubility and Diffusion

b) When there are very few impurities in the liquid

[Solution Description] Increased solubility allows more gas to dissolve in the liquid, leading to a higher concentration gradient, which can enhance the rate of diffusion as per Fick’s laws.

Key Concept: Temperature and Pressure Effects, Sublimation

b) Increase pressure and maintain the temperature below -56.6°C

[Solution Description] The process of forcing dry ice solid $CO_2$ to become liquid involves increasing the pressure while maintaining or slightly lowering the temperature. Normally, dry ice sublimates at -78.5°C under 1 atmosphere. However, if we increase the pressure significantly above 5.11 atm, which is the triple point pressure for $CO_2$, and raise the temperature to approximately -56.6°C, solid $CO_2$ can transition into liquid form. Therefore, the correct condition is to increase the pressure and adjust the temperature around the specified point.

Key Concept: Temperature Effect

c) Rate of diffusion increases

[Solution Description] As temperature increases, the kinetic energy of particles also increases. This causes the particles to move more rapidly, thus increasing the rate at which they diffuse.

Key Concept: State Properties

b) Solids have both fixed shape and volume.

[Solution Description] Solids have a definite shape and volume, meaning they do not change their form or occupy more space when moved from one place to another. They are also rigid, which means they can resist force applied without altering their structure unless the force exceeds their breaking point.

Key Concept: Pressure Effect

a) It melts into liquid carbon dioxide.

[Solution Description] Increasing the pressure on dry ice while keeping it below its sublimation point will cause it to turn into a liquid instead of sublimating directly into gas.