Gaseous molecules are free to move in the container. The molecules collide with each other and with the wall of the container. On the wall the molecules exert force. The force per unit area is called the pressure of the gas.
Q. 27. What are the applications of interconversion of states of matter ?
The inter conversion of states of matter is used to :
(i) generate electricity in thermal power plant.
(ii) separate nitrogen and oxygen from air by liquefaction.
(iii) prepare machine parts.
(iv) prepare room fresheners.
(v) prepare ice-creams.
Q.28. What happens to a gas if its inter molecular space is reduced ?
If the inter molecular distance between molecules of a gas is reduced, it changes to liquid. Further reduction it is changed to solid.
Q. 29. Which of the following substances is most compressible?CO2, H2O, NaCl.
Q. 30. Which property of a gas results in steady pressure of the gas ?
Answer: The constant bombardment of the gas molecules with the walls of the container
Q. 31. In which of the following substances, weakest inter molecular force is expected: H2O, CH3OH,
Answer: Al, He. He
Q. 32. One gas mixes with another gas easily. What is this property called ?
Q. 33. Describe briefly (i) Melting point and (ii) Boiling point.
(i) Melting point :- The melting point of a solid is that temperature at which it changes into the liquid. From the beginning to the end of melting, the temperature does not change.
(ii) Boiling point :- The boiling point of a liquid is that temperature at which a liquid starts boiling at the atmospheric pressure.
Q. 34. How would you find out whether a sample of sodium chloride is pure or impure ?
Answer: Pure substance melts at its melting point. If sodium chloride melts at 97ºC,then the sample is pure otherwise impure.
Q. 35. How will you find out whether a sample of water is pure or impure ?
Answer: The boiling point of water at one atmospheric pressure is 100ºC. If the given sample of water boils at 100ºC then it is pure otherwise impure.
Q. 36. Why do solids have a fixed shape and gases have neither a fixed shape nor a fixed volume ?
Answer: In solids the molecules are close due to intermolecular force. The molecules are arranged in a fixed pattern. The movement of molecules are not possible. Hence its shape are fixed .
In gases the intermolecular force are negligible. Molecules are free to move in any direction. The distance between the molecules is very large. Hence gases do not have fixed shape and fixes volume.
Q. 37. What is Vaporization ?
Answer: The change of liquid into its gaseous form (vapour) when temperature of liquid is increased is called vaporization.
Q. 38. What is Sublimation ?
Answer: A change of state directly from solid to gas without changing into liquid state or vice-versa is called sublimation.
Q. 39. What is Condensation ?
Answer: A change of gaseous state to liquid state or solid state is called condensation.
Q. 40. What is Deposition ?
Answer: A change of vapour to solid is called deposition. It is the reverse process of sublimation
Q. 41. What is Liquefaction ?
Answer: A substance which is gas in normal condition, when changed to liquid by cooling it under pressure is called liquefaction.
Q. 42. What is Solidification ?
Answer: A substance which is a liquid in normal condition, when changed to solid to by cooling it under pressure is called solidification.
Q. 43. What is difference between vapour and gas.
Answer: Vapour is used to denote the gaseous state of fluids which exists as liquids under normal conditions, while gas is used to denote the gaseous state at normal temperature. We always speak water vapour and carbon dioxide gas.
Q. 44. Why do the three states of matter differ ?
Answer: The three states of matter differ due to :
(i) Difference in packing and arrangement of molecules in the three states.
(ii) Intermolecular force of attraction are different in the three states.
Q. 45. Why does the temperature remain constant until whole of the solid changes into liquid, though the heat energy is constantly supplied ?
Answer: During melting, temperature of the liquid phase remains the same as that of the liquid phase. The heat energy supplied is utilized to destroy the crystal pattern and is stored in the liquid phase as potential energy.
Q. 46. Why does the temperature remain constant during boiling though heat is constantly supplied ?
Answer: The heat energy supplied is utilized to destroy the intermolecular force amongst the molecules of the liquid and is stored as potential energy.
Q. 47. Why does a gas fill a vessel completely ?
Answer: The molecules of a gas are moving continuously with a high speed in all direction and intermolecular force amongst the molecules are negligible. Hence it fills the vessel completely.
Q. 48. How does the state of matter changes from solid to liquid and then to gas on heating ?
Answer: In solids particles are very close. When heat is given to solid, the distance between particles increases and it takes the shape of the container. On further heating the distance between the particles increases in such an extent that the molecules are free to move. This is a gaseous state.
Q. 49. Explain the term boiling on the basis of kinetic theory of gases.
When a liquid is heated up to its boiling point, the heat is absorbed by the molecules and stored in the form of potential energy . When potential energy of the molecule is is increased, the intermolecular distance is increased. It means intermolecular force of attraction reduces to zero. The molecules start escaping in air causing the liquid to boil.
Q. 50. Explain the term melting on the basis of kinetic theory.
Answer: The molecules of solids vibrate about its mean position. When it is heated, its kinetic energy is increased and it starts vibrating vigorously. At the melting point the intermolecular force of attraction is reduced and particles can not hold each other with strong force to hold them in their fixed position. The crystalline structure is destroyed and it starts melting.
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