(2)

The mole ratio of hydrogen and oxygen gases liberated during the electrolysis of water is 2 : 1. Since two atoms of hydrogen and one atom of oxygen are present in the water molecule respectively:

$2H_{2}O\to 2H_2+O_2$

(3)

The displacement reaction is the one wherein the atom or a set of atoms is displaced by another atom in a molecule. For instance, when iron is added to a copper sulphate solution, it displaces the copper metal.

A + B - C $\to$ A - C + B

(2)

The law of conservation of mass states that “mass can neither be created nor destroyed.” So, both the sides of the chemical reactions have to be balanced to ensure that this law is followed and thus the number of the reactant and the product molecules remains the same on both sides of the equation.

(2)

Formation of crystals by process of crystallisation is the reaction in which the original identity of the substance remains as such, since on heating the crystals they can again be converted in the powder form and again on cooling they can recrystallise.

(2)

A physical change is the one in which the chemical properties of a substance do not change.
No chemical change is observed when we store oxygen gas under pressure, or during liquefaction of air, or when petrol is kept open in a china dish.
However, when we heat a copper wire in the presence of air, copper oxide is formed.

The reaction is as follows: $\underset{copper}{2Cu\left(s\right)}+\underset{oxygen}{O_2\left(g\right)}\stackrel{Heat}{\to }\underset{ccopper \left(ii\right) oxide}{2CuO\left(s\right)}$

Thus, heating of copper wire in presence of air at high temperature is an example of a chemical reaction.

(3)

Hydrogen and oxygen occur in the gaseous state in nature, whereas when they combine to form water, the physical state of the matter converts to the liquid state.

(3)

When carbon dioxide is passed through lime water, it turns milky due to the formation of white precipitate of calcium carbonate.

$Ca{\left(OH\right)}_2+C{O}_2\to CaC{O}_3+H_{2}O$

(3)

The given reaction is an example of displacement and redox reaction since oxygen replaces hydrogen from the ammonia molecule leading to the formation of NO. Similarly, $O_2$ is getting reduced in the reaction and ammonia is getting oxidised. Thus, a redox reaction takes place in this case. reaction.
Therefore, the below reaction is both the displacement and the redox reaction.
$4N{H}_3\left(g\right)+5O_2\left(g\right)⟶4NO\left(g\right)+6H_{2}O\left(g\right)$

(3)

The downward arrow symbol "$\downarrow$" represents the formation of the precipitate in a chemical reaction.

(4)

Copper + Zinc sulphate is not the feasible reaction since zinc is more reactive than copper and copper cannot displace it from its salt solution as per the reactivity series of metals