a. Reversible reaction  b. Rate of reaction

a. Reversible reaction :

i. Reactions which do not go to completion and occur in both the directions simultaneously are called reversible reactions.

ii. Reversible reactions proceed in both directions. The direction from reactants to products is the forward reaction, whereas the opposite reaction from products to reactants is called the reverse or backward reaction.

iii. A reversible reaction is denoted by drawing in between the reactants and product a double arrow, one pointing in the forward direction and other in the reverse direction (⇌ or ⇄).

iv. At high temperature in an open container, the CO₂ gas formed will escape away. 

Therefore,

It is not possible to obtain back 

e.g.,

a. H_2(g) + I_2(g) ⇌ 2HI_(g) 

b. CH₃COOH_(aq) + H₂O_(I) ⇌ CH₃COO^-_(aq) + H₃O⁺_(aq)

b. Rate of reaction :

Rate of a chemical reaction :

i. The rate of a chemical reaction can be determined by measuring the extent to which the concentration of a reactant decreases in the given time interval, or extent to which the concentration of a product increases in the given time interval.

ii. Mathematically, the rate of reaction is expressed as :

Rate = \(-\frac{d[Reactant]}{dT}\) 

= \(\frac{d[Product]}{dT}\)

Where, d[reactant] and d[product] are the small decrease or increase in concentration during the small time interval dT.