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BiologyClass 9CBSE

Why do doctors advise putting strips of wet cloth on the forehead of a person suffering from high fever?

Doctors advise putting strips of wet cloth on the forehead of a person suffering from high fever because the water from the wet cloth evaporates. During evaporation, the water absorbs latent heat from the forehead, which is at a higher temperature. This absorption of heat causes a cooling effect, helping to bring down the body temperature of the feverish person.

BiologyClass 9CBSE

What is sublimation? Give two examples of substances that exhibit sublimation.

Sublimation is the process where a solid changes directly into a gas without passing through the liquid state, and vice versa. Examples of substances that exhibit sublimation include: 1. Dry ice (solid carbon dioxide) 2. Naphthalene balls (used as mothballs)

BiologyClass 9CBSE

Why does the smell of hot sizzling food reach you several metres away, but the smell of cold food does not?

The smell of hot sizzling food reaches you several metres away because the particles of the aromatic substances in hot food have higher kinetic energy due to the increased temperature. This higher kinetic energy causes them to move faster and diffuse more rapidly through the air. Cold food particles have lower kinetic energy, move slower, and thus diffuse much slower, making their smell less noticeable from a distance.

BiologyClass 9CBSE

Imagine you have a sealed container with a gas inside. If you decrease the volume of the container while keeping the temperature constant, what happens to the pressure inside? Explain this phenomenon based on the particle nature of matter.

If you decrease the volume of the container while keeping the temperature constant, the pressure inside the container will increase. This is because the gas particles, which are in constant random motion, will now have less space to move around. As a result, they will collide more frequently with the walls of the container, leading to an increase in the force exerted per unit area, which is pressure.

BiologyClass 9CBSE

Why do we feel more severe burns from steam at 100°C than from boiling water at the same temperature?

Steam at 100°C contains more heat energy than boiling water at 100°C due to the latent heat of vaporization. When steam condenses on the skin, it releases this additional latent heat (approximately 22.5 × 105 J/kg) in addition to the heat released by cooling from 100°C. This extra heat causes more severe burns.

EnglishClass 9CBSE

What is latent heat of fusion? How does it differ from latent heat of vaporization?

Latent heat of fusion is the amount of heat energy required to change 1 kg of a solid into a liquid at its melting point without any change in temperature. Latent heat of vaporization is the amount of heat energy required to change 1 kg of a liquid into a gas at its boiling point without any change in temperature. The key difference is the phase transition involved: solid to liquid for fusion, and liquid to gas for vaporization.

BiologyClass 9CBSE

Consider the three states of water: ice, liquid water, and steam. Arrange them in increasing order of (a) kinetic energy of particles and (b) intermolecular forces of attraction.

(a) Increasing order of kinetic energy of particles: Ice < Liquid water < Steam. Justification: Kinetic energy is directly related to temperature and the freedom of particle movement. Particles in steam (gas) have the highest kinetic energy and move most freely, followed by liquid water, and then ice (solid) where particles only vibrate. (b) Increasing order of intermolecular forces of attraction: Steam < Liquid water < Ice. Justification: Stronger intermolecular forces hold particles closer together. In ice (solid), forces are strongest, followed by liquid water, and then steam (gas) where forces are weakest and negligible.

BiologyClass 9CBSE

Differentiate between boiling and evaporation based on at least three key characteristics.

1. Boiling is a bulk phenomenon, occurring throughout the liquid when it reaches its boiling point, whereas evaporation is a surface phenomenon, occurring at all temperatures. 2. Boiling involves rapid bubble formation, while evaporation is a slow and continuous process. 3. Boiling occurs at a fixed temperature (boiling point), while evaporation can occur at any temperature below the boiling point.Explanation: Boiling is a phase transition that requires a specific temperature and involves the entire mass of the liquid, whereas evaporation is a continuous process driven by the kinetic energy of surface particles and can occur at various temperatures.

BiologyClass 9CBSE

Explain why evaporation causes cooling. Give two everyday examples where you observe this phenomenon.

Evaporation causes cooling because the particles with higher kinetic energy escape from the surface of the liquid. This leaves behind particles with lower average kinetic energy, which translates to a lower temperature for the remaining liquid. Examples include: 1. Sweating: When sweat evaporates from our skin, it takes away heat, cooling our body. 2. Water kept in an earthen pot (matka) becomes cool: The porous surface allows some water to seep out and evaporate, taking latent heat from the remaining water.

BiologyClass 9CBSE

Why is diffusion fastest in gases compared to liquids and solids? Provide an example to illustrate your answer.

Diffusion is fastest in gases because their particles are far apart and move rapidly and randomly with high kinetic energy and negligible intermolecular forces. This allows for quick intermixing. For example, when an incense stick is lit in one corner of a room, its fragrance quickly spreads throughout the entire room due to the rapid diffusion of gas particles.

BiologyClass 9CBSE

Describe the arrangement and movement of particles in the three common states of matter: solid, liquid, and gas. How do these differences account for their characteristic properties?

In solids, particles are tightly packed in fixed positions, vibrating about their mean positions, giving solids a definite shape and volume. In liquids, particles are less tightly packed and can slide past each other, allowing liquids to take the shape of their container but maintain a definite volume. In gases, particles are far apart, move randomly and rapidly, and have negligible intermolecular forces, leading to no definite shape or volume and high compressibility.

BiologyClass 9CBSE

Explain how the particle nature of matter helps us understand why a sugar cube dissolves faster in hot water than in cold water.

When sugar is added to water, the water particles move into the spaces between the sugar particles, and the sugar particles move into the spaces between the water particles. In hot water, the particles have more kinetic energy, meaning they move faster. This increased movement leads to a more rapid intermixing of sugar and water particles, resulting in faster dissolution.The kinetic energy of particles increases with temperature. Faster-moving particles collide more frequently and with greater force, leading to a quicker breakdown of the sugar crystal structure and a more efficient distribution of sugar particles throughout the water.

BiologyClass 9CBSE

Which of the following processes requires heat energy from the surroundings?(a) Freezing(b Condensation(c) Evaporation(d) Deposition

(C): Freezing, condensation, and deposition are processes where a substance releases heat as it changes state (exothermic). Evaporation (liquid to gas), melting (solid to liquid), and sublimation (solid to gas) are processes that absorb heat (endothermic).

ScienceClass 9CBSE

1.      Why does the temperature of a substance remain constant during its melting point or boiling point?

The temperature of a substance remains constant during melting and boiling points till the completion of melting and boiling because of latent heat of fusion used by the substances. Latent heat of fusion helps to overcome the force of attraction between particles of solid to change into liquid when they melt. Hence temperature remains constant. I the same way during formation of vapours latent heat helps solid substance to convert into gaseous state. Hence temperature of a substance remain constant at boiling point.

ScienceClass 9CBSE

Comment on the following statements: (a) Evaporation produces cooling. (b) Rate of evaporation of an aqueous solution decreases with increase in humidity. Sponge though compressible is a solid.

Soln: When liquid evaporates it takes latent heat from the object it This will make the object gets cool. Hence evaporation causes cooling. When there is increase in the humidity water vapours in the atmosphere will be Hence atmosphere will not take water vapours easily which decreases the process of evaporation. Sponge is a solid because it has definite shape and volume which does not change until you press It has minute pores in which air is filled. When you press air passes out through pores this makes it a compressible solid.

ScienceClass 9CBSE

You want to wear your favourite shirt to a party, but the problem is that it is still wet after a wash. What steps would you take to dry it faster?

Drying of clothes is due to process of evaporation hence following steps should be taken to dry the shirt faster. Dry the shirt under fan with high speed. With increase in speed of fan water vapour move away with wind. This decreases water vapour in the surroundings which will increase the evaporation and the shirt gets dried faster. Spread the shirt on a hanger which will increase surface Increase in surface area makes the shirt dry quickly. Dry the shirt in Because of high temperature more water vapour gets kinetic energy to get converted to vapour. This will fasten the process of evaporation and the shirt gets dried easily. Iron the Heat increases the speed of vaporization and evaporation. This will make the shirt gets dried faster.

ScienceClass 9CBSE

It is a hot summer day, Priyanshi and Ali are wearing cotton and nylon clothes respectively. Who do you think would be more comfortable and why?

Hot summer leads to lot of sweating. Cotton absorbs lot of moisture whereas Nylon will not absorb the moisture as efficient as cotton. Hence cotton absorbs the sweat faster that Nylon does. Evaporation of moisture from the cotton cloth will give a cool feeling, especially when wind flows. Hence It can be concluded that Priyanshi will be more comfortable in summer days.

ScienceClass 9CBSE

Alka was making tea in a kettle. Suddenly she felt intense heat from the puff of steam gushing out of the spout of the kettle. She wondered whether the temperature of the steam was higher than that of the water boiling in the kettle. Comment

Steam and the water temperature boiling in the kettle is the same. But the particles of steam have more energy in the form of latent heat of vaporisation than the particles of water. This energy is released when the steam condenses to water. Therefore, steam is hotter than the boiling water.

ScienceClass 9CBSE

Water as ice has a cooling effect, whereas water as steam may cause severe burns. Explain these observations.

Water as ice has less energy and water molecules absorb energy from our body to give cooling effect whereas steam has high energy and transfers high energy into our body which transforms as heat to cause burns.

ScienceClass 9CBSE

Classify the following into osmosis/diffusion Swelling up of a raisin on keeping in Spreading of virus on Earthworm dying on coming in contact with common Shrinking of grapes kept in thick sugar Preserving pickles in Spreading of smell of cake being baked throughout the Aquatic animals using oxygen dissolved in water during respiration

Osmosis Diffusion Osmosis Osmosis Osmosis Diffusion Diffusion Explanation: Swelling up of a raisin on keeping in water is osmosis because the concentration of solutes is more outside. Hence, water moves water from higher to lower concentrations through a semipermeable membrane, causing the raisin to swell. Spreading the virus through sneezing is diffusion because the virus enters the air in tiny droplets. They remain suspended for a long time and move here and there in the air from higher to lower Earthworms die on coming in contact with common salt is osmosis because of variations in the concentration of water and solute. When salt is sprinkled on them, their skin gets dried, due to which their body shrinks, and they die. The shrinking of grapes in thick sugar syrup is Osmosis because water will move down from higher to lower concentrations via a semipermeable membrane. The Preserving of pickles in salt is Osmosis because salt helps to remove water from the cells and helps to preserve pickles. The spreading of the smell of cake being baked throughout the house is Diffusion because the particles of cake move from a region of higher concentration to lower concentration through diffusion hence, producing the smell. Aquatic animals using oxygen dissolved in water during respiration is osmosis because fish absorb oxygen from water (high concentration) through gills and skin (semipermeable membrane) to the lower

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