## Saturday 23 July 2011

### CBSE Class IX (9th) Science | Chapter 1. MATTER IN OUR SURROUNDINGS | Lesson Exercises

Question 1. Convert the following temperatures to the celsius scale.
(a) 293 K (b) 470 K.

Answer : As we know 0° C =273.16 K. For convenience, we take 0° C = 273 K after rounding off the decimal.
To change a temperature on the Kelvin scale to the Celsius scale we have to subtract 273 from the given temperature

(a) 293 K =  293-273  = 20° C
(b) 470 K = 470-273   = 197° C

Question2. Convert the following temperatures to the Kelvin scale.
(a) 25°C (b) 373°C.

Answer : Kelvin is the SI unit of temperature, 0° C =273.16 K. For convenience, we take 0° C = 273 K
after rounding off the decimal. To  convert a temperature on the Celsius scale to the Kelvin scale we have to add 273 to the given temperature.

(a) 25°C = 25 + 273 = 298 K

(b) 373°C = 373 + 273 = 646 K

Question3. Give reason 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 balls disappear with time without leaving any solid. This is due to the property of sublimation. Some solids do not exist in their liquid form or changes from solid to liquid; instead they directly evaporate from their solid form to vapour forms. Naphthalene is a sublime substance with very high vapors pressure,  that show the property of sublimation. So naphthalene balls disappear after some time due to the property of sublimation.

(b) We can get the smell of perfume sitting several metres away. This is due to high rate of  diffusion of perfume particles. Liquid generally vaporize at normal temperature. Perfume particles on vaporizing, intermix on their own with each other and air molecules around at a very fast rate. They do so by getting into the spaces between the particles. This intermixing of particles of two different types of matter on their own is called diffusion.Therefor the smell of perfume reach us sitting metres away.

Question4. Arrange the following substances in increasing order of forces of attraction between the particles. water, sugar, oxygen.

Answer : Substances in increasing order of forces of attraction between the particles.
• Oxygen
• Water
• Sugar

Question5. What is the physical state of water at.
(a) 25°C (b) 0°C (c) 100°C ?

Answer : Physical states of water at given temperatures :
(a) 25°C - Liquid as familiar water
(b) 0°C  - Solid  as ice
(c) 100°C- Gaseous as water vaporour
Question6. 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) water at room temperature is a liquid because its melting temperature is with in range of room temperature. As we know force of molecular attraction of a substance becomes high at low temperature and weak  at high temperature. Due to this  water at 0°C becomes solid due to increased molecular particles attraction.At room temperature arround 20 °C, due to increase in tempreture,  kinetic energy of the water particles increases. Due to the increase in kinetic energy, the particles start vibrating with greater speed. The energy supplied by heat overcomes the forces of attraction between the water particles. The
particles leave their fixed positions and start moving more freely. A stage is reached when the solid ice melts and is converted to water a liquid.

(b) an iron almirah is a solid at room temperature.This is because the melting point of iron is  1536 °C or 2797 °F or 1809 K where as room temperature is around 20°C. At this temperature, kinetic energy of the iron molecules is not enough to  overcomes the forces of attraction between the water particles. There for iron almirah is solid at room temperature.

Question7. Why is ice at 273 K more effective in cooling than water at the same temperature?
Answer : The heat energy  absorbed by ice to change to liquid state, without showing any rise in temperature is known as the latent heat. We already know, the  heat can change the physical state of a matter by overcoming the forces of attraction between the particles. For water freezing point and melting point is 0°C. So water has extra amount of hidden energy as compare to ice.

Question8. What produces more severe burns, boiling water or steam?
Answer : Steam produces more severe burns as compare to boiling water as it has extra high heat energy.

Question9. Name A,B,C,D,E and F in the following diagram showing change in its state

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