Q1. A drop of ink put into water gradually spreads and changes the colour of water. Name the property involved in the process.
Ans. Diffusion.
Q2. Give example to justify that the particle of matter attracts each other.
Ans. The particles of matter attract each other it can be justified as if you try to break iron nail, it will be difficult which shows particles of iron are held by strong attractive forces.
Q3. A rubber band can change its shape on stretching but still we call it solid. why?
Ans. Rubber band is solid. It does not change its shape on stretching but the shape is regained when stretching forces is removed.
Q4. Why is oxygen called a gas? Give two reasons.
Ans.
- Oxygen neither has a fixed volume nor fixed shape.it completely fills the vessel in which it is kept.
- Oxygen exert pressure due to collision of the molecules on the walls of the containing vessel.
Q5. Why balloons sometimes burst in sunlight?
Ans. Balloons burst sometimes in sunlight because sunlight causes increase in kinetic energy of particles of air filled in balloons due to which pressure inside balloons increases and the balloons burst when they are not able to withstand the pressure.
Q6. What is dry ice? How is it prepared?
Ans. Dry ice is solid carbon dioxide. For preparing dry ice, carbon dioxide gas is subjected to high pressure at low temperature. This leads to liquefacation of carbon dioxide. Liquid carbon dioxide is injected into blocks where it gets stored as dry ice.
Q7. Why does surgeon spray some ether on skin before performing minor surgery?
Ans. Surgeon spray some ether on skin before performing minor surgery because ether has a low boiling point (308 K) due to which it evaporates quite fast. As a result, the temperature of the skin becomes so low that it almost becomes numb. Due to this numbness, the patient does not feel much pain when minor cut is made in the skin to perform the surgery.
Q8. Define
(i) latent heat of fusion
(ii) melting point
Ans.
(i) latent heat of fusion – The amount of heat required to convert one kilogram of a solid into liquid at the atmospheric pressure at its melting point is known as latent heat of fusion of the substance.
(ii) Melting point - The temperature at which a solid change into liquid at the atmospheric pressure is called melting point of the solid.
Q9. On heating a mixture of sodium chloride and ammonium chloride which one will change into vapours and why?
Ans. Ammonium chloride undergoes sublimation while sodium chloride does not.
Q10. Write full form of notations, LPG and CNG.
Ans.
LPG - Liquefied Petroleum Gas.
CNG - Compressed Natural Gas.
Q11. What happens when solid is heated?
Ans. On heating solid first changes into liquid then to gaseous state. For example, ice on heating, first get converted into liquid water and then to steam. Solid may directly convert into gaseous state without passing through liquid state. For example, naphthalene on heating directly gets converted into the gaseous state. This process is called SUBLIMATION.
Q12. Water as ice has a cooling effect whereas water as a steam may cause burns. Explain
Ans. Ice and steam both are different physical states of water. They differ in their nature. When ice contacts with skin (at higher temperature) absorbs energy as latent heat of fusion from the skin and melts. since energy has been taken from the body, a cooling sensation is experienced. But when steam contacts with skin (lower temperature), it releases energy which causes severe burns.
Q13. Define kelvin scale.
Ans. This is also called absolute scale. Kelvin is S.I unit of temperature. Temperatures on kelvin scale are always positive.
Q14. What is Atmospheric Pressure?
Ans. The pressure exerted by atmospheric air is called atmospheric pressure. The value of atmospheric pressure is different at different places. At sea level the atmospheric pressure is 1atm. As we go higher the atmospheric pressure decreases.
Q15. Evaporation is surface phenomenon. Explain
Ans. During evaporation, high energy molecules of liquid escape into vapour phase. Escape of particles of the liquid takes place at the surface at the surface of the liquid only. Therefore, evaporation occurs only at the surface of the liquid.
Q16. If two ice cubes are pressed hard between the palms, what will happen when pressure is released after some time? Give reason.
Ans. The ice cubes will join. When pressure is applied some of the ice between the two cubes will melt due to decrease in the freezing point. Energy required for melting is taken from surroundings. When pressure is released, energy equal to latent heat of fusion is given out. This causes water to freeze and ice cubes join back.
Q17. Why we sprinkle water on the roof or open courtyard in summer?
Ans. We sprinkle water on the roof of the house or open courtyard as when water evaporates, it absorbs large amount of heat as latent heat of vaporization and thus help the hot surface to become cool.
Q18. List two precautions you must take while finding the melting point of ice.
Ans.
(i) The bulb of the thermometer must not be dipping in ice. It must touch ice.
(ii) The constant stirring must be done.
Q19. It is a hot summer day, Priya and Ali are wearing cotton and nylon clothes. Who do you think would be more comfortable and why?
Ans. Priya would be more comfortable than Ali as she is wearing cotton clothes. Cotton clothes are porous in nature hence will absorb moisture from the body. As it evaporates, it will give cooling comfort to the body. Ali is wearing nylon clothes which have hardly any pores, he will feel uncomfortable.
Q20. How can gas be converted into liquids and solids?
Ans. When a gas is cooled, its particle loose kinetic energy and their motion slows down and forces of attraction between the particles become stronger and they move closer finally a stage is reached when the particles become close and the vapour change into liquid. The change of state from gas to liquid is called condensation.
When a substance in liquid state is further cooled, its particle looses more kinetic energy. The force of attraction between the particles become strong and the motion of the particles gradually gets restricted to only vibratory motion. At this point the liquid changes into solid, the change of state from liquid to solid is called freezing.
Thus, we can interconvert the states of matter by changing temperature.
