Chemistry XI Chapter 4

Question No. 4.18: How does rate of evaporation depend on the surface area and temperature? 
Answer: Evaporation occurs from liquid surface, so if surface area is increased then more molecules can escape from the surface at a time. Thus, liquid evaporates more quickly. For liquids having same surface area, evaporation rate increases with increase in temperature. This is because at high temperature, number of molecules with greater energy than average energy is increased, so rate of evaporation increases.

Question 4.19: How does vapor pressure depend on intermolecular forces and size of molecules? 
Answer: Vapor pressure is related to tendency of molecules to escape from surface of liquid. Greater the intermolecular force, lesser is the evaporating tendency of molecules. This due fact that, greater intermolecular forces do not allow molecules to evaporate easily. As a result, vapor pressure decreases. In addition, evaporation tendency of molecules also decreases with increase in size of molecules. 

Question 4.20: Why is heat of vaporization of water is greater than that of CH4? 

Answer: Due to extensive hydrogen bonding in water, high energy is required to separate the water molecules from each other. While in CH4, there are weak London dispersion forces. So less energy is required to convert liquid in vapors 

Question 4.21: Why the vapour pressure of water, ethyl alcohol and diethyl ether are different from each other at 0 ○C? 

Answer: Different vapor pressures of water, ethyl alcohol and diethyl ether can be explained based on their structural formulae and types of intermolecular forces in these compounds. Water has extensive hydrogen bonding; ethyl alcohol has hydrogen bonding while ether has no hydrogen bonding. Although order of London’s dispersions is opposite to that of hydrogen bonding in these compounds, but it is less significant than hydrogen bonding. For this reason, vapour pressures of water, ethyl alcohol and ether are different from each other and are in the following order: water vapour pressure < ethyl alcohol vapour pressure < ether vapour pressure. 

Question 4.22: What is meant by term boiling point? How is related with vapour pressure? 

Answer: The temperature of the liquid at which the vapour pressure of the liquid is equal to the external pressure is called boiling point. Boiling point of liquid is directly related to external pressure. Greater the external pressure, higher is boiling point of the liquid and vice versa. Due to this reason, boiling point of water is low at mountains as compared to sea level. 

Question 4.23: Vacuum distillation can be used to avoid decomposition of a sensitive liquid. Explain with example. 

Answer: Distillation is a process by which a organic compound can be separated from a mixture at its boiling point. The distillation when carried out at relatively low pressure is called vacuum distillation. By vacuum distillation decomposition of many sensitive liquids can be avoided. For example, boiling point of glycerin is 290 ○C at 1 atm pressure, but it decomposes before reaching this temperature. However, under vacuum, the b.p. of glycerin reduces to 250 ○C at 0.2 atm. It is distilled at this temperature without decomposition. 

Question 4.24: What are advantages of vacuum distillation? 

Answer: In vacuum distillation, the boiling point of the liquid is decrease by decreasing external pressure. vacuum distillation: i. reduces the time. ii. avoids the decomposition of sensitive liquids iii. saves the fuel. 

Question 4.25: Why does A liquid boil at temperature when its vapour pressure becomes equal to external vapour pressure? 

Answer: Vapour pressure of a liquid increases with increase in temperature. When vapour pressure of the liquid becomes equal to external pressure, then vapour pressure inside the bubble of the liquid is such that it can face the external pressure, so bubble rises in upward direction and burst upon the surface. 

Question 4.26: Why the things can be easily cooked in a pressure cooker? 

Answer: Pressure cooker is a closed container. Water vapors formed inside the pressure cooker are not allowed to escape. Thus, they start gathering above the surface of water and build up more pressure on the surface of water in the cooker. So boiling point of the water increases. To reach at boiling point, more and more heat is absorbed, and temperature of water is increased which helps in cooking the food very rapidly. 

Question 4.27: How can a liquid crystals act as temperature sensors? 

Answer: Liquid crystals can reflect light. When one of wavelengths of white light is reflected from liquid crystals it looks colored. If we change the temperature of liquid crystals, the color of reflected light is also changed. Reason is that the variation in temperature causes change in distance between the layers of the molecules of liquid crystal. Thus, wave length of reflected light also changes. 

Question 4.28: State the role of liquid crystals in biological applications? 

Answer: Liquid crystals are usually used to locate the veins, arteries, infections and tumors. As affected parts of body due to these diseases are warmer than surrounding tissues. So, these parts can be detected by liquid crystals which are temperature sensor. They are also used to check breast cancer by painting the liquid crystals on breast because liquid crystal, on hot area, looks blue.       Next