Liquids and Solids Solved Exercise
Solved exercise for Liquids and Solids for Fsc pre engineering and pre medical
and for entry test preparation for engineering Universities and Medical Colleges.
and for entry test preparation for engineering Universities and Medical Colleges.
TEXT BOOK EXERCISE
Q.1. Multiple choice questions.
(a) molecules of water in liquid state
(b) atoms of helium in gaseous state at high temperature
(c) molecules of solid iodine
(d) molecules of hydrogen chloride gas
Acetone and chloroform are soluble in each other due to
(a) intermolecular hydrogen bonding
(b) ion-dipole interaction
(c) instantaneous dipole
(d) all of the above
NH3 shows a maximum boiling point among the hydrides of Vth group elements due to
(a) very small size of nitrogen
(b) I one pair of electrons present on nitrogen.
(c) enhanced electronegative character of nitrogen
(d) pyramidal structure of NH3
When water freezes at 0oC, its density decreases due to
(a) cubic structure of ice
(b) empty spaces present in the structure of ice
(c) change of bond lengths
(d) change of bond angles
In order to mention the boiling point of water at 110oC, the external pressure should be
(a) between 760 torr and 1200 torr
(b) between 200 torr and 760 torr
(c) 765 torr
(d) any value of pressure
Ans: (i) c (ii) a (iii) c (iv) b (v) a
Q.2. Fill in the blanks.
i The polarizability of noble gases ________ down the group and results in the increase in their
ii ________boiling points is developed in acetone and chloroform when they are mixed together.
iii Exceptionally weak ________ of HF is due to strong hydrogen bonding present in it.
iv The concept of dynamic equilibrium is the ultimate ________ of all reversible systems.
v HV of C6H14 should be ________ than that of C2H6.
vi During the formation of ice from liquid water there is a ________ % increase in volume.
vii The rate of increase of vapour pressure of water ________ at high temperatures
viii A layer of ice on the surface of water ________ the water underneath for further heat loss.
ix Evaporation is a ________ process.
x Liquid crystals are used in the display of ________ devices.
Ans: (i) increases (ii) H-bonding (iii) acidic strength (iv) result / goal (v) greater (vi) 9 (vii) increases (viii) insulates or prevents (ix) cooling (x) electrical.
Q.4. (a) What type of intermolecular forces will dominate in the following liquids.
(i) Ammonia, NH3 (ii) Octane, C8H18 (iii) Argon, Ar (iv) Propanone, CH3COCH3 (v) Methanol, CH3OH
(b) Propanone (CH3COCH3), propanol (C3CH2CH2OH) and butane have very similar relative molecular masses. List them in the expected order of increasing boiling point. Explain your answer.
Answers:
In ammonia (NH3) hydrogen bonding will dominate. It is due to high E.N of Nitrogen
Octane (C8H18) has weak Vander Waal’s Forces but no hydrogen bonding.
Argon has wear London dispersion forces.
Propanone (acetone, CH3COCH3) has dipole-dipole interactions.
Methanol (CH3OH) has hydrogen bonding. It is due to high E.N of oxygen
(a) The increasing order of boiling points is Butane < Propanone < Propanol
The propanol (CH3-CH2-CH2-OH) has hydrogen bonding. So it has maximum boiling point.Propanone has dipole-dipole interaction while butane has no such forces.
In the hydrogen bonded structure of HF, which is the stronger bond: the shorter covalent bond
or the longer hydrogen bond between different molecules.
In a very cold winter the fish in garden ponds owe their lives to hydrogen bonding?
Water and ethanol can mix easily and in all proportions.
The origin of the intermolecular forces in water.
Answer:
The hydrogen bonded structure of HF is shown in figure. The fluorine has maximum electro negativity.The shorter covalent bond. H-F is shorter than longer hydrogen bond H---F.When winter comes, then temperature falls down. The water at 4oC has maximum density. So it goes to the bottom of pond. The surface water freezes into ice. During ice formation the water molecules get regular arrangements due to H-bonds. In these arrangements there are many empty spaces and ice occupies 9% more volume than liquid water. Thus ice has less density and floats on water. Moreover ice is an insulator of heat. So it prevents underneath water from freezing. Therefore fish and other aquatic animals live (survive) under thick blanket of ice.Both water and Ethanol (C2H5OH) show H-bonding with each other. It is the reason that they can mix easily in all proportions.The oxygen atom has small size and high electro negativity. There is high electro negativity difference between oxygen and hydrogen. This high E.N difference between O-atom and H-atom is the origin of intermolecular forces in water.
Q.6. (a) Briefly consider some of the effects on our lives if water has only a very weak hydrogen bonding present among its molecules.
(b) All gases have a characteristic critical temperature. Above the critical temperature it is impossible to liquefy a gas. The critical temperatures of carbon dioxide and methane are 31.14oC and -81.9oC, respectively. Which gas has the stronger intermolecular forces? Briefly explain your choice?
Answer:
(a) If water had only very weak hydrogen bonding, then its freezing point would be less than 0oC of ice would not have been less than liquid water. These facts would create problems for human beings and aquatic animals.
(b) Because critical temperature of CO2 is higher than that of CH4, SO, SO2 has stronger intermolecular forces and CH4 has very weak intermolecular forces.
Q.7. Three liquids have the properties mentioned against their names.
NO water propanone pentane
i Molecular formul H2O C3H6O C5H12
ii Relative molecular mass (amu) 18 58 72
iii Enthalpy change of vaporization (KJ mol-1) 14.1 31.9 27.7
iv Boiling Point (oC) 100 56 36
(a) What type of intermolecular force predominates in each liquid ?
(i) In water (ii) In propanone (iii) In Pentane
(b) What do you deduce about the relative strength of these forces in the liquids? Justify your conclusions.
(c) If the liquids are shaken together in pairs,
(i) Which pair would be unlikely to mix ?
(ii) Explain this immiscibility in terms of the forces between the molecules.
(iii) Choose one of the pairs that mix and say whether the enthalpy change on mixing would be positive or negative.
Answer:
(a) Water has hydrogen bonding, propanone has dipole-dipole interactions and pentane (C5H12) has London dispersion fores.
(b) From boiling points and enthalpy of vaporizations of three liquids we deduce that
H-bonding in water is very strong
Dipole-dipole interactions in propanone are weak
(c)
- The pair (water + pentane) would be unlikely to mix.
- Water is polar molecule and pentane is non-polar molecule or water has H-bonding and pentane has very weak
- Water and Propanone mix into each other because both liquids are polar. The process of mixing is an exothermic process and some heat will evolve. Therefore enthalpy change will be negative.
Q.8. Describe the various forces responsible for keeping the particles together in the following elements and compounds and their effect on physical properties making use of the data below:
Substances
|
Formula
|
Molar Mass (a.m.u)
|
M.P (oC)
|
Neon
|
Ne
|
20
|
-248
|
Argon
|
Ar
|
40
|
-189
|
Water
|
H2O
|
18
|
0
|
Sodium fluoride
|
NaF
|
42
|
993
|
Diamond
|
C
|
12
|
3350
|
Answer:
Neon:
Neon is a noble gas. It has very weak London dispersion Forces. Thus it has the least melting point (-248oC).
Argon:
Argon is also a noble gas. It has very weak London dispersion Forces. But it has high polarizability
than that of Neon. So its melting point is higher (-189oC) than neon (-248oC).
Water:
Water has hydrogen bonding. Due to strong intermolecular forces melting point of ice (water) is high.
Sodium Fluoride:
Sodium fluoride has ionic bond. The positive (Na+) and negative (F-) ions are strongly held by electrostatic forces. It is the reason that melting point of NaF is very high.
Diamond:
In diamond there is SP3 hybridization. Each carbon atom is bonded with four other carbon atoms. A huge network structure is formed. From this network structure a single unit can not be removed. It is the reason diamond has very high melting point (3350oC).
Q.9. The boiling and molar masses of hydrides of some first row elements are tabulated below:
Substance
|
Boiling Point (K)
|
Molar Mass (g mol-1)
|
CH4
|
109
|
16
|
NH3
|
240
|
17
|
H2O
|
373
|
18
|
Suggest reasons for the difference in their boiling points in terms of the type of molecules involved and the nature of the forces present between them.
Answer:
CH4:
CH4 is non polar molecule. It has only weak London dispersion Forces. It the reason that CH4has very low boiling point (109 K or -164oC).
NH3:
NH3 is a polar molecule. It has hydrogen bonding and dipole-dipole interactions. It is the reason that boiling point of NH3 is high (240 K or -33oC).
H2O:
H2O is a polar molecule. It has hydrogen bonding and dipole-dipole interaction. Because oxygen is more electronegative than Nitrogen. So wager has greater hydrogen bonding than ammonia. It is the reason H2O has higher boiling point (373 K or 100oC).
Q.10. Explain the term saturated vapour pressure. Arrange in order of increasing vapour pressure: 1dm3 water, 1dm3 ethanol, 50cm3 water, 50cm3 ethanol and 50cm3 of ether.
Answer:
See page No.
50cm3 ether > 50cm3 ethanol = 1dm3 ethanol > 1dm3 H2O = 50cm3 H2O
Q.11. While a volatile liquid standing in a breaker evaporates, the temperature of the liquid remains the same as that of its surrounding. It the same liquid is allowed to vapourize into atmosphere in an insulated vessel, its temperature falls below that of its surrounding. Explain the difference in behaviour.
Answer:
When a liquid evaporates in a beaker, it causes cooling. Now heat from surrounding enters into the liquid. It is the reason temperature remains same. But when a liquid is present in an insulated vessel, then evaporation of liquid causes cooling. But heat can not enter into liquid from surrounding due to insulated vessel. It is the reason that the temperature of liquid falls below that of surrounding.
Q.12. How does hydrogen bonding explain the following indicated properties of the substances?
(i) Structure of DNA
(ii) Hydrogen bonding in proteins
(iii) Formation of ice and its lesser density than liquid water
(iv) Solubilities of compounds
Ans: ////////// ??????????
Q.13. What are liquid crystals? Give their uses in daily life.
Ans: The turbid liquids with some degree of order are called liquid crystals. The liquid like crystals which exist between melting temperature and clearing temperature are called liquid crystals.First of all liquid crystals were discovered in 1888 by Frederick Reinitzer an Austrain botanist. He was studying an organic compound cholesteryl benzoate. This compound turns milky liquid at 145oC and becomes a clear liquid at 179oC. By cooling, the reverse process occurs and liquid crystals are formed. The liquid crystals are composed of long rod like molecules. They have some degree of order. On the basis of ordering, the liquid crystals are of three types.
(i) Nematic (ii) Smectic (iii) Cholestric
Properties:
(i) Liquid crystals are always isotropic.
(ii) They have optical properties.
(iii) They have fluidity, surface tension, viscosity etc.
(iv) Their properties are in between liquids and crystals.
Uses of Liquid Crystals:
(i) Liquid crystals are used as solvents in chromatography.
(ii) Liquid crystal screens are used in oscillographic and TV displays.
(iii) Liquid crystals are used in thermometers.
(iv) Liquid crystals are used in electrical circuits to find out potential failure.
(v) Liquid crystals are used in the display of digital watches, calculators and laptop computers.
(vi) Liquid crystals are used as temperature sensors. When white light is reflected from liquid crystal then its colour changes. When temperature changes, the colour of reflected light also changes. Thus temperature is accurately measured.
(vii) Liquid crystals are used to locate infection, tumors and breast cancer. They are also used to detect blockage of veins and arteries. These parts are warmer than other tissues. When liquid crystal is painted on them, they produce blue coloured spots. Thus indication of any problem is easy.
Q.14. Explain the following with reasons.
(i) Evaporation causes cooling.
(ii) Evaporation takes place at all temperatures.
(iii) Boiling needs a constant supply of heat.
(iv) Earthenware vessels keep water cool.
(v) One feels sense of cooling under the fan after bath.
(vi) Dynamic equilibrium is established during evaporation of a liquid in a closed vessel at constant temperature.
(vii) The boiling point of water is different at Murree hills and at Mount Everest .
(viii) Vacuum distillation can be used to avoid decomposition of sensitive liquid.
(ix) Heat of sublimation of a substance is greater than that of heat of vaporization.
(x) Heat of sublimation of iodine is very high as compared to other halogens.
Answer:
(i) The reason is that during evaporation first of all high energy molecules leave the liquid and low energy molecules are left behind. So temperature of the liquid falls and cooling is produced. To continue the evaporation heat moves from surrounding to the liquid. Thus temperature of surrounding also falls. For example when we put spirit on our hands then spirit evaporates and hand feels cooling.
(ii) Evaporation is a surface process. It takes place at all temperatures. When high energy molecules come at the surface of liquid, they escape out of surface. However by increasing temperature, the average K.E of molecules increases. Hence rate of evaporation increases by increasing the temperature.
(iii) When we supply heat to a liquid, then K.E of molecules and rate of evaporation increase. At boiling point the K.E of molecules becomes maximum. Thus heat supplied at boiling point is used to break intermolecular forces. So there is no increase in K.E of molecules. Hence temperature remains constant at boiling point. It is the reason that boiling needs a constant supply of heat.
(iv) Earthenware vessels have pores in them. Water evaporates from these pores and causes cooling. During evaporation, the escaping molecules get energy from neighbours to overcome intermolecular forces. Thus temperature of remaining water decreases. In the old earthenware’s pores are blocked with dust. So water can not evaporate. Thus they do not keep water cool.
(v) One feels sense of cooling under the fan after bath. It is due to evaporation process. After a bath some water molecules are present on the body. These molecules get heat from the body and evaporate. Thus body feels cooling.
(vi) See page No. 139
(vii) Atmospheric pressure is different at Murree hills and at Mount Everest . Due to different atmospheric pressure boiling point of water is different at two places. At Murree hills atmospheric pressure is 700 torr and boiling point of water is 98oC. At Mount Everest the atmospheric pressure is further reduced and boiling point of water is 69oC.
(viii) The distillation which is carried out at low pressure is called Vacuum distillation. It has many advantages.
(i) It decreases the time for distillation.
(ii) It decreases fuel cost for distillation.
(iii) It prevents decomposition of compounds
For example B.P of glycerine is 290oC. Pressure of 760 torr but at this temperature glycerine decomposes. Hence distillation of glycerine is impossible at 290oC. Therefore its vacuum distillation is done at 120oC at reduced pressure of 50 torr.
(ix) In sublimation a substance directly changes into vapours. It is two steps process (Solidliquidvapours) on other hand vaporization is single step process. It is the reason heat of sublimation of a substance is greater than that of heat of vaporization.
(x) Iodine has biggest atomic size than other halogens (F2, Cl2, Br2). So I2 has high polarizability. Due to high polarizability Iodine has greater 
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