The gas evolved at anode during electrolysis of brine is chlorine (G) When chlorine gas is passed through dry \(Ca(OH)_2\ (Y)\) produces bleaching powder (Z) used for disinfecting drinking water.
\(Ca(OH)_2+Cl_2\rightarrow CaOCl_2+H_2O\)
Since Y and Z are calcium salts, therefore X is also a calcium salt and is calcium carbonate.
\(CaCO_3+2HCl\rightarrow CaCl_2+CO_2+H_2O\)
\(Ca(OH)_2+CO_2 \rightarrow CaCO_3+H_2O\)

The water molecules associated with a crystalline substance is called 'water of crystallisation' .
Activity  \(\longrightarrow\)Take few crystals of blue copper sulphate in dry boiling tube. Heat it on flame, the blue crystals change to white dry powder and the inner surface of the test tube condenses with water.
This proves that crystals contain water molecules in it. If we add some water in the white powder it regains the lost water molecules and become blue again.

Working of Human eye
Human eye consists of various parts which helps us in seeing the objects, the function of various parts are:
(a) Cornea: It is the transparent membrane which refracts the light entering our eye.
(b) Iris: Iris controls the size of pupil.
(c) Pupil: It allows the light entering our eye to pass through it.
(d) Lens: Refracts the light of the objects at different places on retina.
(e) Ciliary muscles: Helps in changing the focal length of the lens.
(f) Retina: It is the screen of the eye on which image is formed. It consists of rods and cones .
(g) Optic nerve: It carries the electrical signals from retina to brain.

The three refractive defects of vision are
(i) Myopia:(a) Myopia or short-sightedness-Eye can see objects at short distance. Inability of the eye in viewing long distant objects. The image falls before retina.
Causes: (i) Elongation of eye ball.
(ii) Excessive curvature in cornea and focal length of lens decreases.
Correction: By using concave lens (diverging lens), which shifts the image to the retina by diverging the rays further.
(b) The near point of human eye is 25 cm which cannot be changed further hence the objects within 10 cm  of the eye cannot be viewed (ii) Hypermetropia (iii) Presbyopia
(ii)Hypermetropia: Also called long-sightedness. Person can see long distant objects but cannot see nearby objects distinctly. This is because the eye ball becomes smaller, focal length increases. It can be corrected by using converging lens called convex lens.

(iii)Presbyopia: The person suffering from this disease cannot see nearby as well as far off objects. It is caused due to weakening of ciliary muscles. This defect is caused due to ageing. The lens is not able to change the focal length. It can be corrected by using bi-focal lens.

(i) (a) Bi-focal lenses
(b) These lenses prepared in which upper part of lens is concave and lower part is convex.
(ii) As we know, power of lens, \(P=\frac{1}{f}\)
\(\Rightarrow \quad f=\frac{1}{P}\)
Hence, for first lens, focal length
\(f=\frac{1}{+3}=+0.33\)
or +33.33 cm (convex lens)
Similarly, for second lens focal length
\(f=\frac{1}{-3}=-0.33 \mathrm{~m} \text { or }-33.33 \mathrm{~cm} \text { (concave lens) }\)

Plaster of Paris is calcium sulphate hemihydrate \((CaSO_4.{1\over2}H_2O).\)

Gypsum, when heated at a temperature of about 380 K in a kiln. loses its three-fourth water of crystallisation and changes to plaster of Paris.

Following is the reaction involved:

\(\underset{Gypsum}{CaSO_4.2H_2O}\xrightarrow {Heat} \underset{Plaster\ of\ Paris}{CaSO_4.{1\over2}H_2O}+\underset{Water}{1{1\over2}H_2O}\)

(i) Sodium carbonate (washing soda) is used in glass, soap and paper industries.

(ii) It is used in the manufacture of sodium compounds such as borax.

(iii) Sodium carbonate can be used as a cleaning agent for domestic purposes.

(iv) It is used for removing permanent hardness of water.

Acids which ionize completely in aqueous solution-strong acids.
Acids which ionize partially in aqueous solution-weak acids.
Hydrochloric acid, nitric acid-strong acid.
Acetic acid, formic acid-weak acid.

(i) Tomato - Malic acid and citric acid
(ii) Vinegar - Acetic acid
(iii) Tamarind - Tartaric acid and citric acid.

(i) The acid is to be added slowly in water to prevent the mixture to be splashed. The reaction is highly exothermic, therefore, constant cooling should be done.
(ii) The process is called dilution.

Fovea: In the eye, a tiny pit located in the macula of the retina that provides the clearest vision of all. Only in the fovea are the layers of the retina spread aside to let light fall directly on the cones, the cells that give the sharpest image.

The changes in the size of the pupil to control the amount of light entering the eye is called pupil reflex. In bright light, pupil size is reduced as too much light falling on the retina could damage it. The reduced size of the pupil allows only limited amount of light to enter. In dim light, pupil she is increased to allow as much light as possible to enter the eye. The retina detects the brightness of light that is entering the eye. An impulse pass to the brain along sensory neurons and the message travels back to the muscles of the iris by motor neurons, triggering a response - the change in size of the pupil due to contraction of radial or circular muscles.

(a) The defect is hypermetropia, as the image of near point is formed beyond retina.
(b) Two causes
(i) Focal length of the lens increases
(ii) Eye ball becomes smaller
(c) Correction: It can be corrected by using a convex lens. It is a converging lens which shifts the image of the object on the retina. 

(i) The eye suffering from myopia, has longer eyeball than that of a normal eye due to which the retina is at a larger distance from the eye lens. This results in the formation of the image in front of the retina. 
(ii) The eye suffering from hypermetropia has short eyeball than that of normal eye due to which the retina is at smaller distance from the eye lens. This results in the formation of the image behind the retina. 

Tyndall effect. The earth's atmosphere is a heterogeneous mixture of minute particles of smoke, tiny water droplets, suspended particles of dust and molecules of air.
So, when a beam of light strikes such particles of smoke filled room through a small hole, the path of the beam of light become visible. This happens due to Tyndall effect. It's a phenomena based on scattering of light.
The colour of scattered light depend on the size of the particle of the medium through which the beam of light passes and wavelength of light.
and scattering \(\alpha\) diameter. [Where d = diameter of particle of medium]