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1. FULLMARKS EDUCARE (Discover Yourself) SCIENCE Matter in Our Surroundings CLASS - IX In-Text Questions (Page 3) Q.3 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. Ans. (a) The intermolecular forces in gases are very weak. As a result, gas molecules have large kinetic energy and are free to move throughout the container. Therefore, gases fill entirely fill the container. (b) Gas molecules have large kinetic energy and are in random continuous motion. Upon striking the walls of the container, these molecules exert some force on the walls. And we know that pressure is force per unit area. That is why gases exert pressure on the walls of their container. (c) A table is rigid, incompressible and has definite shape and volume. Thus, it is solid. (d) Gases have almost negligible intermolecular forces and large intermolecular spaces. As a result, there is very small resistance when we move our hand through air. But the intermolecular forces solid are very large and the intermolecular spaces, very small. As a result, a lot of force is required to separate the molecules of a solid. It is for the same reasons that we need karate expert to break a block of wood. Q.1. Which of the following are matter? Chair, air, love, smell, hate, almonds, thought, cold, cold-drink, smell of perfume. Ans. Chair, air, almonds, cold-drink and smell of perfume are matter. Q.2. Give reasons for the following: The smell of the hot sizzling food reaches you several metres away, but to get smell from cold food, you have to go close. Ans. The rate of diffusion of a gas increases as its temperature increases. Therefore, when the food is hot, diffusion of its smell is faster and we can smell it metres away. But, in case of cold food, diffusion is slower. Hence, we have to go close to the cold food to inhale its smell. Q. 3. A diver is able to cut to through water in a swimming pool. Which property of matter does this observation show? Ans. A diver is able to cut through water in a swimming pool. It shows that the attractive force between water molecules is not very strong. Q. 4. What are the characteristics of particles of matter? Ans. The main characteristics of particles of matter are as following: (a) Particles of matter have vast empty spaces between them. (b) These particles possess kinetic energy. Therefore these are continuously in motion. (c) The particles of matter are held together by the attractive force between them. The strength of these forces varies from matter to matter. Q. 4. Liquids generally have lower density as compared to solids. But you must have observed that ice floats on water. Find out why? Ans. When water freezes (i.e. becomes ice), its volume increases. As a result, its density decreases. Ice, being lighter than water, therefore floats on it. In-Text Questions (Page 6) Q.1. The mass per unit volume of a substance is called density (Density = Mass/Volume). Arrange the following in the order of increasing density: air, exhaust from chimneys, honey, water, chalk, cotton and iron. Ans. Air, exhaust from chimneys, cotton, water, honey and iron. Q.2. Tabulate the differences in the characteristics of states of matter by commenting upon the following: Rigidity, compressibility, fluidity, filling a container, shape, kinetic energy and density. Ans. In-Text Questions (Page 9) Q. 1. Convert the following temperatures to Celsius scale : (a) 300 K (b) 573 K. Ans. We know that Temperature in °C = Temperature in Kelvin – 273. Therefore, (a) 300 K = (300 – 273) = 27°C (b) 573 K = (573 – 273) = 300°C S o l i d s L i q u i d s G a s e s Rigidity Very high rigidity. Very low rigidity Not rigid at all Q. 2. What is the physical state of water at: (a) 250°C? Ans. (a) At 250 °C, water is in gaseous (steam). (b) At 100 °C, water is in liquid state. But, if we continue to heat it at this temperature, it starts vaporizing while temperature remains at 100°C. Compressibility Incompressieble Very low compressibility Highly compressible (b) 100°C? Do not flow at normal temp Flow from higher to lower level. Fluidity Flow in all directions Container filled equal to volume of liquid poured Fill the entire container Filling a container Container not needed Take the shape of container Shape Definite shape Gases are shapeless More KE than solid molecules. Kinetic energy Molecules have least KE Maximum KE Q.3 For any substance, why does the temperature remain constant during change of state? Gases have least Density Solid have maximum density Liquid have lesser density than solids Density NCERT SOLUTIONShttp://www.fullmarkseducare.com/Ph:. 9911968787

2. Ans. When a solid is heated it becomes liquid at a temperature called its melting point.Temperature represents the average kinetic energy of the molecules of a substance. At the melting point or boiling point of a substance, all the heat energy supplied goes into increasing the inter- molecular spaces. This in turn decreases the intermolecular forces and hence the state changes from a solid to liquid or liquid to gas. However the the average kinetic energy of the molecules does not increase. Therefore, temperature remains constant during the change of state. Hence, 25 °C = (25 + 273) = 298 K. 373°C = (373 + 273) = 646 K. Q 2. Give reason for the following observations: (a) Naphthalene balls disappear without leaving any solid. (b) We can get the smell of perfume sitting several metres away. Ans. (a) Sublimation is the direct conversion of a solid into gaseous state without the intervening liquid state. Naphthalene balls take heat from their surroundings, which cause their sublimation. Thus, solid naphthalene just disappears without any trace. (b) Perfumes are highly volatile i.e. their molecules rapidly diffuse into air. Thus, we can smell the perfume sitting several metres away. Q. 4. Suggest a method to liquefy atmospheric gases. Ans. First, the gases should be compressed to a very high pressure and then allowed to expand suddenly through a small nozzle. The sudden expansion will cool down the gases. When repeated many time, this process will liquefy the atmospheric gases. Q3. Arrange the following substances in the increasing order of forces of attraction between the particles: water, sugar and oxygen. Ans. Oxygen, water, sugar. In-Text Questions (Page 10) Q.1. Why does a desert cooler cool better on a hot, dry day? Ans. The rate of evaporation depends upon the temperature of air and the humidity. In hot and dry air, evaporation is very fast. Hence, desert cooler is very effective on such days. During rainy season, atmospheric moisture i.e. humidity is very high. Thus, the rate of evaporation is very low. This makes desert coolers ineffective during rainy season. Q4. What is the physical state of water at (a) 25°C (b) 0°C (c) 100°C Ans. (a)At 25°C, water is in liquid state. (b)At 0°C, water can be in solid or liquid state depending upon how much it has been cooled. (c) At 100°C, water may be in liquid or gaseous state, depending upon how much it has been heated. The change of state from a liquid to a gas or vapour occurs at a constant temperature called the boiling point. Remember, during change of state, there is no change in temperature. Q.2. How does the water kept in an earthen pot (matka) become cool? Ans. As the water oozes through the fine pores in the earthen pot, it evaporates, taking away some of the heat energy from the water in the pot. As a result, water in the earthen pot cools down. Q5. Give two reasons to justify. (a) Water at room temperature is a liquid. (b) An iron almirah is solid at room temperature. Ans. (a) Whether a substance will be in solid, liquid or gaseous state depends upon the strength of the intermolecular bonds. The strength of bonds in turn depends upon, among other things, the temperature of the substance. As the temperature of a substance is gradually increased, the intermolecular bonds are weakened. As a result, the state of the substance changes from solid to liquid to gaseous. At room temperature, the intermolecular bonds in water are considerably weak. At room temperature, intermolecular spaces in water and the KE of water molecules are large. Hence, water is liquid at room temperature. (b) In an iron almirah at room temperature, the (i) Intermolecular forces are very large, and (ii) Intermolecular spaces as well as kinetic energy are very small. Thus, the molecules are held together very tightly. Therefore, the almirah is solid at room temperature. Q.3. Why does our palm feel cold when we put some acetone or petrol or perfume on it? Ans. Acetone, petrol, and perfume are highly volatile substances i.e. these have a high evaporation rate even at room temperature. So, when put on our palm, these take heat from the palm and quickly evaporate making our palm cool. Q4. Why are we able to sip hot tea or milk faster from a saucer than a cup ? Ans. The rate of cooling of a liquid depends upon its surface area. A saucer has much larger surface area than a cup. When we pour hot tea or milk into a saucer, surface area of liquid increases, which results in its faster cooling. Thus, we are able sip hot teaor milk faster from a saucer than from a cup. Q5. What type of clothes should we wear in summers? Ans. During summer, we sweat a lot. Cotton is a good absorber of water. So, cotton cloths absorb sweat and expose it to atmosphere for easy evaporation which makes our body cool. Q6. Why is ice at 273 K more effective in cooling than water at the same temperature? Ans. We have to remove 336000 J of heat energy from 1 kg of water (at 273 K) to convert it into 1 kg of ice at 273 K. So, ice at 273 K would extract an equal amount of energy from its surroundings in order to melt into water at 273 K. Therefore, it is more effective in cooling. Exercises (Page 12) Q1. Convert the following temperatures to Kelvin scale: (a) 25°C (b) 373 °C. Ans. We know that Temp in Kelvin = Temp in °C + 273 NCERT SOLUTIONShttp://www.fullmarkseducare.com/Ph:. 9911968787

3. During change of state, there is no change in temperature. Q7. What produces more severe burns, boiling water or steam? Ans. To convert 1 g of water at 100 °C into steam at the same temperature, we need to give an extra 2260 J of energy. Therefore, steam contains much more energy than boiling water. So, steam causes more severe burns than boiling water does. Q8. Name A, B, C, D, E and F in the following diagram showing change in its state. Increase heat and decrease pressure E A B SOLID LIQUID GAS D C F D i ecrease ncrease heat and pressure Ans. A : Solid changes into liquid state by the process offusion. B : Liquid changes into gaseous state by the processof boiling or vaporisation. C : A gas changes into liquid state by the process ofliquefaction. D : A liquid changes into solid state by the process offreezing or solidification. E : Solid changes into gaseous state by the process ofsublimation. F : Gas change into solid state by the process ofsublimation. Let’s take some water at room temperature. As you start cooling it, its temperature keeps dropping, till eventually it reaches 0. As soon as you reach 0, if you stop, it will be in liquid state. Now if you keep removing heat, the temperature remains 0, while the liquid starts turning to solid by rejecting its latent heat fusion. As the last of the liquid part turns to ice, you have a solid at 0 degrees Celsius. Similarly, if you reverse the process and you heat ices and it reaches 0, it is solid at 0 degrees, and continue heating till you reach completely liquid at 0 degrees Celsius. NCERT SOLUTIONShttp://www.fullmarkseducare.com/Ph:. 9911968787