States of Matter Class 11 Important Questions Chemistry Chapter – 5
Class 11 Important Questions Chemistry
Que 1. What is Absolute Temperature?
Ans 1. It is also called thermodynamic temperature (or) kelvin temperature. It is a temperature on the absolute (or) Kelvin scale in which zero at -273.16 0C.
∴ T= (t0C + 273.16 K)
Que 2. What is Absolute Zero?
Ans 2. It is the lowest temperature theoretically possible at which the volume of a perfect gas is zero.
Que 3. Why Helium is used in balloons in place of hydrogen?
Ans 3. Helium is used in balloons in place of hydrogen because H2 is highly inflammable but He is incombustible.
Que 4. What is the effect of an increase of temperature on surface tension and viscosity in a liquid?
Ans 4. Both decrease with an increase in temperature.
Que 5. Why vegetables are cooked with difficulty at hill stations?
Ans 5. At the hill station, the atmospheric pressure is less and so the boiling point decreases.
Que 6. What is the difference between normal boiling point and standard boiling point?
Ans 6. When the external pressure is equal to one-atmosphere pressure, the boiling point is called the normal boiling point. When the external pressure is 1 bar, it is called stranded boiling points.
Que 7. What is the difficulty faced by the mountaineers with respect to the air present around them? How is this difficulty solved?
Ans 7. At altitude, the atmospheric pressure is low. Hence, the air is less dense. As a result, less oxygen is available for breathing. The person feels uneasiness, headache etc. This is called altitude sickness. This difficulty is solved by carrying oxygen cylinders with them.
Que 8. Boyle’s law states that at a constant temperature if pressure is increased on a gas, volume decreases and vice versa. But when we fill the air in a balloon, volume as well as pressure increase. Why?
Ans 8. The law is applicable only for a definite mass of the gas. As we fill air into the balloon, we are introducing more and more air into the balloon. Thus, we are increasing the mass of air inside. Hence, the law is not applicable.
Que 9. What are STP Conditions?
Ans 9. STP means Standard Temperature and Pressure conditions.
→ Standard temperature is 00C = 273 K
→ Standard pressure is 1 atmosphere = 76 cm = 760 mm. of Hg.
Que 10. What is Boyle Temperature?
Ans 10. The temperature at which a real gas exhibits ideal behaviour for a considerable range of pressure is called Boyle’s temperature.
Class 11 Important Questions Chemistry
Que 11. What would have happened to the gas if the molecular collisions were not elastic?
Ans 11. On every collision there is a loss of energy, so molecules would have slowed down & settled down in the vessel and pressure reduce to 0.
Que 12. Why do liquids diffuse slowly as compared to gases?
Ans 12. In liquids, the molecules are less free than in a gas. i.e intermolecular forces in liquid are greater than in gas.
Que 13. A tyre tube with a pinhole is first filled with oxygen to a pressure of 30 lb/sq inch and allowed to leak out. Then it is filled with N2 gas to the same pressure and allowed to leak out again. In which case, the time taken will be more and why?
Ans 13. The time taken will be more in case the tyre tube is filled with O2. This is because O2 has a higher molar mass than N2 and the rate of effusion is inversely proportional to the square root of molar mass.
Que 14. What is the difference between vapour and gas?
Ans 14. A substance that is in a gaseous state at room temperature is called a gas. However, if a substance is not in the gaseous state at room temperature but changes into the gaseous state on heating, then the gaseous state obtained is called vapour or when gas is below its critical temperature, it is called vapour.
Que 15. Why do liquids have a definite volume but no definite shape?
Ans 15. This is because the intermolecular forces are strong enough to hold the molecules together but not so strong as to fix them into definite positions (as in solids). Instead, they possess fluidity and hence no definite shape.
Que 16. What would have happened to the gas if the molecular collisions were not elastic?
Ans 16. On every collision, there would have been a loss of energy. As a result, the molecules would have slowed down and ultimately settle down in the vessel. Moreover, the pressure would have gradually reduced to zero.
Que 17. Explain the following
(a) Why are aerated water bottles kept underwater during summer?
(b) Which property of liquid is responsible for the spherical shape of drop?
(c) Why is moist air lighter than dry air?
(d) Define aqueous tension?
(e) What are units of a and b which are van der Waal’s constant?
Ans 17. (a) To reduce the temperature so as to reduce pressure, otherwise, the bottle may burst.
(b) Surface Tension.
(c) Moist air has water vapours which lower vapour density, so it is lighter.
(d) It is the pressure of water vapours at a given T.
(e) Unit of a in atm. L2 mol-2, b is L mol-1.
Que 18. Why at one time gases like helium, hydrogen, oxygen and nitrogen were called permanent gases but not now?
Ans 18. These gases were called permanent gases because they could not be liquefied at room temperature by applying any amount of pressure on them. Now, they are not called so because they can be liquefied by first cooling them to a temperature below a particular temperature called critical temperature of the gas and then applying suitable pressure on the gas.
Que 19. Give reasons for the following.
(i) The size of a weather balloon becomes larger and larger as it ascends into higher altitudes.
(ii) Tyres of automobiles is inflated to lesser pressure in summer than in winter.
Ans 19. (i) As we go to higher altitudes, the atmospheric pressure decreases. Thus, the pressure outside the balloon decreases. To regain equilibrium with the external pressure, the gas inside expands to decrease its pressure, Hence, the size of the balloon increases.
(ii) In summer, due to higher temperature, the average kinetic energy of the air molecules inside the tyre increase, i.e., molecules start moving faster. Hence, the pressure on the wall of the tube increases. If the pressure inside is not kept low at the time of inflation, at a higher temperature, the pressure may become so high that the tyre may burst.
Que 20. The liquid ammonia bottle is cooled before opening the seal. Explain.
Ans 20. The liquid ammonia bottle contains the gas under very high pressure. If the bottle is opened as such, then the sudden decrease in pressure will lead to a large increase in the volume of the gas. As a result, the gas will come out of the bottle all of a sudden with force. This will lead to the breakage of the bottle and also some serious accidents. However, if the bottle is cooled under tap water for some time, this will lead to a decrease in the volume of the gas to a large extent. If the seal is opened now, the gas will come out of the bottle at a slow rate, thus, reducing the chances of spilling gas.
Class 11 Important Questions Chemistry
Que 21. Ice has a lower density than water. Give a reason.
Ans 21: It is because of the presence of different types of hydrogen bonding in both of the compounds. Ice consists of intermolecular hydrogen bonding and a cage-like 3D structure. When ice melts, some hydrogen bonds break, allowing water molecules to fill the empty gaps. The molecules in liquid water are more densely packed than those in ice. As a result, ice is denser than water.
Que 22. What is meant by the term: Non-ideal or real gas?
Ans 22. The gas which does not obey the Cas law:
Boyle’s law, Charles’ law, Avogadro law at all temperatures and pressures is a called-non-ideal or real gas. Most of the real gases show ideal behaviour at low pressure and high temperature.
Que 23. What do the critical temperature, critical pressure, and critical volume for a gas stand for?
Ans 23. The critical temperature is the temperature above which a gas cannot be liquified however large may be the pressure applied on it. The pressure sufficient to liquiíy a gas at its critical temperature is called its critical pressure. The volume of the gas at its crìtical temperature is called its critical volume.
Que 24. What are the factors responsible for the strength of hydrogen bonds? Ans 24. The strength of the hydrogen bond depends on the distance between the lone pair of negatively charged electrons of the electronegative atom, and the partially positively charged hydrogen atom of the other molecule.
Que 25. Explain how the function PV/RT can be used to show gases behave non-ideally at high pressure. Ans 25. According to the ideal gas equation, PV = nRT so the ratio given above shows us the number of moles of an ideal gas. Now, on changing the pressure, there should be no effect on the number of moles, as it is fixed for every ideal gas. However, if it changes, it means that the gas is not behaving like an ideal gas.
Class 11 Important Questions Chemistry
Que 26. Give one application of Dalton’s law of partial pressures.
Ans 26. One application of Dalton’s law of partial pressures is in determining the/pressure of dry gas. Gases are generally collected over water and, so they contain water vapours. The pressure exerted by the water Vapours at-a particular temperature is called the Aqueous Tension. By subtracting the aqueous tension from the ‘ vapour pressure of the moist gas, the pressure of the dry gas can be calculated.
Pdry gas= Pmoist gas – Aqueous Tension (at t°C).
Que 27. What are the postulates of the kinetic theory of gases?
Ans 27. The important postulates of the theory are:
i. All gases are made up of a very large number of minutes particles called molecules.
ii. The molecules are separated from one another by large distances. The empty spaces among the molecules are so large that the actual volume of the molecules is negligible as compared to the total volume of the gas.
iii. The molecules are in a state of constant rapid motion in all directions. During their motion, they keep on colliding with one another and. also with the walls of the container and, thus, change their directions.
iv. Molecular collisions are perfectly elastic, no loss of energy occurs when the molecules collide with one another or with the walls of the container. However, there may be. redistribution of energy during the collisions.
v. There are no forces of interaction (attractive or repulsive) between molecules. They move completely independent of one another.
vi. The pressure exerted by the gas is due to the bombardment of its molecules on the walls of the container per unit area.
vii. The average kinetic energy of the gas molecules is directly proportional to the absolute temperature.
Que 28. Mention the two assumptions of the kinetic theory of gases that do not hold good.
Ans 28. The two assumptions of the kinetic theory that do not hold good are:
i. Molecules of a gas do not have any force of attraction between them.
ii. The volume of the molecules of a gas is negligibly small when compared to the space which is occupied by the gas.
Que 29. Why do liquids have a definite volume, but no definite shape?
Ans 29. It is due to the fact that in liquids intermolecular forces are strong enough to hold the molecules together, but these are strong enough to hold the molecules together, but these forces are not strong enough to fix them into definite or concrete positions as in solids. Hence they possess fluidity but no definite shape.
Que 30. Why falling liquid drops are spherical?
Ans 30. The surface area of a sphere is minimum. In order to have minimum surface area drops of the liquid become spherical.
Class 11 Important Questions Chemistry
