Reversible Reactions: A reversible reaction is a reaction that can be reversed, i.e., the products can be converted back to the reactants. A reversible reaction is represented by a double arrow with their heads in the direction opposite to each other. Thus, a reversible reaction can be kept

Explanation: The compound 'AB' undergoes decomposition to form the products 'A' and 'B'. It is the forward reaction. As soon as the products are formed, they combine together to form 'AB': It is the backward reaction
PCl₅(g) ⇌ PCl₃(g) + CI₂(g)
Irreversible Reactions: The reaction that cannot be reversed is called irreversible reaction. Consider the combustion of coal into carbon dioxide and water.
C_(s) + O_2(g) ⟶ CO_2(g)+ Heat
Coal oxygen Carbon dioxide
In this reaction, solid coal burns with oxygen and gets converted into carbon dioxide gas and water. As the product is a gas, as soon as it is formed it escapes out of the reaction container. It is hard to decompose a gas into a solid. Thus, the backward reaction is not possible in this case. So, it is an irreversible reaction

Medical applications of radioisotopes can be divided into two parts
i) Diagnosis ii) Therapy Radio. 
Isotopes are used as tracers to diagnose the nature of blood circulatory disorders, defects of bone metabolism, to locate tumors, etc. Some of the radio isotopes which are used as tracers are: hydrogen, carbon, nitrogen, sulfur, etc.
(i) Radio sodium (Na²⁴) is used for the effective functioning of heart.
(ii) Radio - Iodine (I¹³¹) is used to cure the goiter.
(iii) Radio iron is (Fe⁵⁹) is used to diagnose anemia and also provide treatment for the same.
(iv) Radio phosphorous (P³²) is used in the treatment of skin diseases.
(v) Radio cobalt (Co⁶⁰) and radio gold (Au⁷⁹) are used in the treatment of skin cancer.
(vi) Radiations are used to sterilize the surgical devices as they can kill the germs and microbes.

Linear expansion:
(i) When a body is heated or cooled, the length of the body changes due to change in its temperature. This expansion is said to be a linear or longitudinal expansion.
(ii) The ration of increase in length of a body per degree celsius rise in temperature to its unit length is called the coefficient of linear expansion. Its SI unit is per Kelvin. The value of coefficient of linear expansion is different for different materials.

(iii) The equation relating the change in length to the change in temperature of a body is given below:
\(\frac { \triangle L }{ { L }_{ 0 } } \) =α_L ΔT
ΔL  - Change in length
L₀ - Original length
ΔT - Change in temperature
α_L - Coefficient of linear expansion.

(i) The object is always placed on the left side of the lens.
(ii) All the distances are measured from the optical centre of the lens.
(iii) The distances measured in the same direction as that of incident light are taken as positive.
(iv) The distances measured against the direction of incident light are taken as negative. (v) The distances measured upward and perpendicular to the principal axis is taken as positive.
(v) The distances measured downward and perpendicular to the principal axis is taken· as negative.

(i) If the difference in electronegativity between the two elements is 1.7, the bond has 50% ionic character and 50% covalent character.
(ii) If the difference is less than 1.7, the bond is considered to be covalent.
(iii) If the difference is greater than 1.7, the bond is considered to be ionic.

(i) When birds fly they push the air downwards by their wings (Action).
(ii) The air pushes the bird upwards (Reaction).
(iii) When a person swims he pushes the water using hands backwards (Action), the water pushes the swimmer in forward direction (Reaction).
(iv) Rockets expel gas at high velocity (Action). The downward moving gas pushes the rocket in upward direction (Reaction).
(v) When we fire a bullet, the gun recoils back, Bullet is moving forward (action). The gun equalise this forward action by moving backward (reaction).

Applications of Avogadro's hypothesis
(i) It explains Gay-Lussac's law.
(ii) It helps in the determination of atomicity of gases.
(iii) Molecular formula of gases can be derived using Avogadro's law.
(iv) It determines the relation between molecular mass and vapour density.
(v) It helps to determine Gram molar volume of all gases (i.e. 22.4 lit at S.T.P)