11th Class Chemistry Chapter four Short Questions with Answers Solved

Question: 1

Why the melting and boiling points of alkanes increase with increase in molar masses?

Answer: 1

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Alkanes are saturated hydrocarbons. Greater the length of carbon chain, greater the interaction of one molecule with the other. Higher alkanes are zig-zag in structure and they are tailed macromolecules as well. These features are responsible for the forces of interactions and cause the increase of M.P. and B.P. of alkanes.

Question: 2

Liquid boils at that temperature when its vapour pressure becomes equal to the external Pressure. Why?

Answer: 2

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Increase in temperature increases the vapour pressure of liquids. When the vapour pressure of the liquid becomes equal to the external pressure, then the vapour pressure inside the bubbles of the liquid is such that it can face the external pressure, so the bubbles burst in the outward direction on the surface. This bursting of the bubbles outwardly is called boiling.

Question: 3

Is it true that polar compounds are soluble in polar solvents?

Answer: 3

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Polar compounds have separation of positive and negative charges. If the 'solvent has also separation of positive and negative charges then the interaction takes place between the opposite poles. These interactions are responsible to mix them with each other.

Question: 4

The rate of increase of vapour pressure with respect to temperature of water is higher at higher temperature thin at lower temperature. Justify it.

Answer: 4

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The rate of evaporation depends on temperature. When water is close to room temperature, then its intermolecular forces are dominant. So the evaporation tendency of the water molecules is less. At higher temperature the intermolecular forces of water are less, so the evaporating tendency is sufficiently high.

Question: 5

The boiling point of water is different at Murree Hills and at mount Everest. Justify it.

Answer: 5

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Boiling point of a liquid changes as the external pressure changes. At murree Hills, atmospheric pressure is less than standard pressure (760 torr). So water boils at 98°C instead of 100^°C. At mount Everest atmospheric pressure is further reduced .So water boils at 69° C.

Question: 6

Why dipole-dipole forces are much stronger than dipole-induced dipole forces?

Answer: 6

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In dipole-dipole forces, the atoms have sufficient partial positive and partial negative charges to attract each other. Where as in cuse of Londonforces no +ve or - ye change present apparently.

Question: 7

Define boiling point. Is it related with the external pressure?

Answer: 7

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Boiling point is that temperature of the liquid at which the vapour pressure of the liquid is equal to the external pressure. If the external pressure is higher, then the boiling point of the liquid is increased. If the external presser is decreased, then the boding point decreases. The boiling of water is low at mountains. Due to the change in external pressure.

Question: 8

Ice floats on water. Justify it.

Answer: 8

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Ice is solid water. Water expands when it is solidified. This expansion is due J empty spaces which are left behind due to the hydrogen bonding. The density of ice is close to 0.91 g cm^-3 as compared to that of liquid water which is 1.00 gcm^-3 at 4^°c.

Question: 9

Why the melting boiling points of halogens increase down the group?

Answer: 9

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The atomic sizes increase from fluorine to iodine. The number of shells increase, polarizability increase and the overlapping of the orbitals increase. This makes the melting and boiling points high down the group.

Question: 10

Why intermolecular forces are weaker than intramolecular forces?

Answer: 10

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Intermolecular forces are present between two different molecules of the same kind or different kinds. Valence electrons are not responsible for these forces, that is why they are weak forces. Intramolecular forces are present within the same molecule due to sharing of electrons and overlapping of the orbitals, so that why they are strong.

Question: 11

Give the reason for 19w boiling point of HF (19.5^° C) as compared to H₂O (100^°C), although the hydrogen bonding in HF.is stronger than that of water.

Answer: 11

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In water each molecule can make 2 hydrogen bonds with two neighbouring water molecules. In this way the links of H₂O molecule are greater. In case of HF, these is only one partial positive hydrogen which can make only one H-bond with another fluorine atom.

Question: 12

How in a very cold winter fish in garden ponds owe their lives to hydrogen bonding?

Answer: 12

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When water is frozen at 0^°C, then it expands. The hydrogen bonding in the solid state of H₂O adjust the molecules of water in such a way that empty spaces are left behind. In this way the density of water in the solid state becomes less. So ice floats on water.

Question: 13

Define polymorphism and isomorphism? Give on example of each.

Answer: 13

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Isomorphism is the phenomenon in which two different substances exist in the same crystalline form, e.g; NaNO3, KNO3, KNO₃ are rhombonedral enystalline form. When a compound exists in more than one crystalline shape. Then the phenomenon is called polymorphism. AgNO₃ rhombohedral crystals and orthorhombic crystalline form.

Question: 14

Sodium is a good conductor of electricity but sodium chloride is not. Give reason?

Answer: 14

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Sodium is an alkali metal and free electrons are available in the crystal lattice. These free electrons are responsible for the passage of electrical current. Free electrons are not available in the NaCI crystal.

Question: 15

Why the metals are malleable and ductile?

Answer: 15

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In the metallic crystals the lattice points are occupied by positively charged ions and free electrons are responsible to hold them together. When stress is applied on the metals then the layers slide past over one another. The layers are bounded by the free electrons and they play the role of glue. Due to this reason metals are malleable and ductile.

Question: 16

Amorphous solid like glass is also called super cooled liquid. Why?

Answer: 16

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Amorphous solids like glass have random structures and their particles are disarranged just like liquids. So the amorphous solids are no doubt hard and rigid but look like liquids. That is why glass is called a super cooled liquid.

Question: 17

Molecularsolids are soft and easily compressible. Why?

Answer: 17

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The molecules in such crystals are present at the lattice points. There are van der waal's forces among the molecules of such solids. There forces are weak. So, there solids are soft and easily compressible.

Question: 18

Why the heat of sublimation of a substance is greater than that of heat of vaporization?

Answer: 18

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During sublimation two stages are crossed in a single step i.e., conversion of solid to liquid and liquid to vapours. In vaporization, liquid changes into vapours. Therefore, heat required for sublimation is greater than for vaporization.

Question: 19

How liquid crystals can act as temperature sensors?

Answer: 19

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Liquid crystals can reflect light. When any of the wavelength of light is reflected, the liquid crystal look coloured, when the temperature is changed the distances between layers of molecules are changed. So, the reflected light also changes colours. Due to this property of cholesteric liquid crystals, they are used as temperature sensors.

Question: 20

How unit cell is define by unit cell dimensions?

Answer: 20

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The distances between two adjacent particles along x, y and z axis are measured and denoted by 'a', 'b', and 'c'. These distances are called unit cell lengths. The angles in between these three axes are denoted by "33" and. These six parameters are also called Crystallographic elements.

Question: 21

How is that some of crystalline solid substances show anisotropy?

Answer: 21

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The variation of a certain physical property in different direction is known as anisotropy. Some of the crystalline - substances are anisotropic for certain properties. Like electrical conductivity, thermal conductivity, passage of light and cleavage.

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11th Class Chapter four Chemistry Short Questions

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