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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

ScienceClass 9CBSE

‘Osmosis is a special kind of diffusion’. Comment.

Soln: Diffusion in liquids and gases is the movement of particles from low concentration to high concentration. Osmosis is the movement of particles from the low concentration to high concentration through the semi permeable membrane. This movement happens due to diffusion. Hence Osmosis is a special kind of diffusion.

ScienceClass 9CBSE

The non S I and S I units of some physical quantities are given in column A and column B respectively. Match the units belonging to the same physical quantity: (A) (B) (a) Degree Celsius (i) kilogram (b) Centimetre (ii) Pascal (c) Gram per centimetre cube (iii) metre (d) Bar (IV) kelvin Milligram (v) kilogram per metre cube

Soln: — (iv) Degree Celsius and kelvin are the units of — (iii) Centimetre and metre are the units of —(v) Gram per centimetre cube and kilogram per metre cube are the units of — (li) Bar and pascal are the units of - (i) Milligram and kilogram are the units of

ScienceClass 9CBSE

Match the physical quantities given in column A to their S I units given in column B: (A) (B) (a) Pressure (i) cubic metre (b) Temperature (ii) kilogram (c) Density (iii) pascal (d) Mass (iv) kelvin Volume (v) kilogram per cubic metre

Soln: — (iii) The SI unit of pressure is pastor (denoted by P). — (iv) The SI unit of temperature is -kelvin" (denoted by K). — (v) The SI unit of density is kilogram per cubic metre (kg/m3). — (if) The SI unit of mass is kilogram (kg). — (i) The SI unit of volume is

ScienceClass 9CBSE

Evaporation of a liquid at room temperature leads to a——— effect. (b) At room temperature the forces of attraction between the particles of solid substances are———than those which exist in the gaseous state. (c) The arrangement of particles is less ordered in the ——— state. However, there is no order in the ——— state. (d) ——— is the change of solid state directly to vapour state without going through the ———state. (e) The phenomenon of change of a liquid into the gaseous state at any temperature below its boiling point is called———.

Soln: Cooling Stronger Liquid, gaseous Sublimation, liquid Evaporation

ScienceClass 9CBSE

A sample of water under study was found to boil at 102°C at normal temperature and pressure. Is the water pure? Will this water freeze at 0°C? Comment.

Boiling point of pure water is 100 °C and melting point is 0°C at 1 atmospheric pressure. Here the water boils at 102 °C hence it is not a pure water and freezes at temperature below 0°C.

ScienceClass 9CBSE

In which of the following conditions, the distance between the molecules of hydrogen gas would increase? (i) Increasing pressure on hydrogen contained in a closed container (ii) Some hydrogen gas leaking out of the container (iii) Increasing the volume of the container of hydrogen gas (iv) Adding more hydrogen gas to the container without increasing the volume of the container (a) (i) and (iii) (b) (i) and (iv) (c) (ii) and (iii) (d) (ii) and (iv)

Answer is c) ii and iii Explanation To increase the intermolecular interaction either volume of Hydrogen gas should be reduced or container volume should be increase. By increasing the pressure or by adding Hydrogen without increasing container volume inter-molecular interaction would decrease. Water under study was found to boil at 102°C at normal temperature and pressure

ScienceClass 9CBSE

Which condition out of the following will increase the evaporation of water? (a) Increase in temperature of water (b) Decrease in temperature of water (c) Less exposed surface area of water Adding common salt to water

Answer is a) Increase in temperature of water Explanation: Increase in temperature of water increase the kinetic energy of the water which will make more particles to attain enough energy to convert into vapour state. This is how increase in temperature favours evaporation. On the other hand exposed surface is also responsible for the evaporation. Higher the exposed surface are higher will be the evaporation. When common salt is added to water then surface is occupied by the solvent as well as non-volatile solute particles. This make the escaping tendency of solvent particles decrease and thus the evaporation of water decreases. Hence options (b), (c) and (d) will decrease the evaporation of water.

ScienceClass 9CBSE

The boiling points of diethyl ether, acetone and n-butyl alcohol are 35°C, 56°C and 118°C respectively. Which one of the following correctly represents their boiling points in kelvin scale? (a) 306 K, 329 K, 391 K (b) 308 K, 329 K, 392 K (c) 308 K, 329 K, 391 K (d) 329 K, 392 K, 308 K

Answer is c) 308 K, 329 K, 391 K Explanation: Temperature in Celsius can be converted into temperature in kelvin by adding 273 to the Celsius value. Boiling points of diethyl ether, acetone and n-butyl alcohol are 308 K, 329 K, and 391 K respectively.

ScienceClass 9CBSE

Choose the correct statement of the following (a) Conversion of solid into vapours without passing through the liquid state is called sublimation. (b) Conversion of vapours into solid without passing through the liquid state is called vaporisation. (c) Conversion of vapours into solid without passing through the liquid state is called freezing. Conversion of solid into liquid is called sublimation.

Soln: Answer is a) Conversion of solid into vapours without passing through the liquid state is called sublimation. Explanation: Sublimations is a process in which a solid is converted into vapours without passing through liquid Vaporisation is a phase transition from liquid to Conversion of liquid to solid at substance’s freezing temperature is called as

ScienceClass 9CBSE

A few substances are arranged in the increasing order of ‘forces of attraction’ between their particles. Which one of the following represents a correct arrangement? (a) Water, air, wind (b) Air, sugar, oil (c) Oxygen, water, sugar (d) Salt, juice, air

Answer is c) Oxygen, water, sugar Explanation: Intermolecular force of attraction is less in gases than liquid and solid. Solids have higher intermolecular attraction compared to liquid. In the answer Oxygen is a gas which has lesser intermolecular attraction than water-a liquid and sugar- a crystalline solid.

ScienceClass 9CBSE

During summer, water kept in an earthen pot becomes cool because of the phenomenon of (a) diffusion (b) transpiration (c) osmosis evaporation

Soln: Answer is d) evaporation Explanation: Evaporation of water through pores of earthen pot reduces temperature of immediate surroundings. This makes the water cool after some time. Movement of water through plants and evaporation through its aerial parts is called tyranspiration. Diffusion is the movement of a substance from an area of higher concentration to area of lower concentration If molecules of a solvent tend to pass through a semipermeable membrane from a less concentrated solution into a more concentrated one it is called osmosis.

ScienceClass 9CBSE

The property to flow is unique to fluids. Which one of the following statements is correct? (a) Only gases behave like fluids (b) Gases and solids behave like fluids (c) Gases and liquids behave like fluids Only liquids are fluids

Soln: Answer is c) Gases and liquids behave like fluids Explanation: In gases and liquids intermolecular force of attraction between the particles is less and they facilitate flow of these states of matter.

ScienceClass 9CBSE

Seema visited a Natural Gas Compressing Unit and found that the gas can be liquefied under specific conditions of temperature and pressure. While sharing her experience with friends she got confused. Help her to identify the correct set of conditions (a) Low temperature, low pressure (b) High temperature, low pressure (c) Low temperature, high pressure High temperature, high pressure

Soln: Answer is c) Low temperature, high pressure Explanation: To compress gas into liquid, low temperature and high pressure is required. Between the particles of gas there is lot of space. On applying pressure particles get closer and they start attracting each other to form a liquid. Lot of heat is generated when gas is compresses hence it is necessary to cool it. Cooling lowers the temperature of compressed gas and helps in liquefying it.

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