Aexists in the same physical state as that of the reaction it catalyzes.

A Homogeneous Catalyst exists in the same physical state as that of the reaction it catalyzes.

Explanation:
                     Catalysts are classifies as follow,
                                                      1. Homogeneous Catalyst
                                                      2. Heterogeneous Catalyst

Homogeneous Catalyst are those which are in same phase as that of reactants are.

Example:

         Acetic Acid + Ethanol + H₂SO₄  →  Ethyl Acetate + H₂O + H₂SO₄

In this reaction catalyst and reactants all are in same phase i.e. Liquids.

Heterogeneous Catalyst are those which are in different phase as that of reactants are.

Example:

                                 Ethene + H₂ + Ni  →  Ethane + Ni

In this reaction ethene and hydrogen are gases while Nickle catalyst is in solid state. 

The air is going to come out because of the low pressure.