Chemistry XI Chapter 4 Descriptive Questions

Question 4.29: What is difference between amorphous solids and crystalline solids.

Answer: The Solids which contain atoms, ions or molecules arranged in a definite three-dimensional pattern are called crystalline solids. The solids which contain atoms, ions and molecules arranged not in a definite three-dimensional pattern are called crystalline solids.

Question 4.30: What are cleavage planes?

Answer: The planes along which the crystalline solids are broken, are called cleavage planes. These planes make angle with each other. The values of these angles are different in different solids.

Question 4.31: Define the anisotropy. Why do solid crystals show anisotropic properties?

Answer: The physical properties of crystalline solids which change with change in direction of crystalline solids are called an isotropic properties and this phenomenon is known as anisotropy. For example, refractive index, coefficient of thermal expansion, electrical and thermal conductivities of some crystals. Solid crystals show anisotropic properties due to fact that the arrangement pattern of particles in crystalline solids is different in different direction. For example, electrical conductivity of graphite is greater in one direction than in other. Reason is that electrons are mobile for electrical conduction parallel to layers only.

Question 4.32: Why graphite is anisotropic in electrical conductivity?

Answer: The graphite is anisotropic in electrical conductivity because its electrical conductivity changes with change in its direction, i.e greater in one direction than in other. Reason is that electrons are mobile for electrical conduction parallel to layers only.

Question 4.33: Why solids are rigid and hard.

Answer: Solids particles are rigid and hard because in solids, particles are tightly packed with each other. It is not easy to displace these particles from each other by normal stress.

Question 4.34: Crystals have their own habits. Justify it.

Answer: The shape of crystals in which it usually grows is called habit of crystals. Crystals are obtained usually by slow cooling of saturated solution of the substance. A crystal can grow in various shapes. But if conditions under which crystals are grown, are changed then shapes of crystals also change. For example, cubic crystals of NaCl becomes needle like due to presence of 10% urea in NaCl solution.

Question 4.35: Symmetry is one of properties of crystalline solid. Justify it.

Answer: The repetition of faces, angles, or edges when a crystal is rotated by 360○ along its axis is called symmetry. There are various types of symmetry elements which can present in crystalline solids such as center of symmetry, plane of symmetry and axis of symmetry.

Question 4.36: Define the polymorphism and isomorphism. Give one example in each case.

Answer: The phenomenon in which a compound exists in more than crystalline forms is called polymorphism. For example, CaCO3 exists in trigonal and orthorhombic forms. The phenomenon in which two different substances exist in more than crystalline form is called isomorphism. NaF and MgO both show cubic crystals.

Question 4.37: Why does a polymorphic compound show same chemical properties but different physical properties?

Answer: A compound which exists in more than one crystalline form is called polymorphic compound. As these crystalline forms have same chemical formula, so their chemical properties are same, but their structural arrangement of particle is different, so their physical properties are different.

Question 4.38: What is allotropy? Give allotropic forms of sulphur and carbon.

Answer: The phenomenon in which a element exists in more than crystalline forms is called allotropy. For example, tin (Sn) exist in forms of white tin and grey tin. Carbon exists in diamond and graphite form.

Question 4.39: What is transition temperature? Give transition temperature and allotropic forms of tin.

Answer: The temperature at which two crystalline forms of the same substance can co-exist in equilibrium with each other is called transition temperature of that substance. Above and below this temperature only form exists.

Figure: Allotropic forms of tin and potassium nitrate at respective transition temperatures

Question 4.40: What are crystal lattice and crystallographic elements?

Answer: Crystal lattice: It is arrangement of points representing atoms, ions, or molecules of crystals at different sits in three-dimensional space. The points are also called lattice sites or lattice points.
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