Which sample of matter is a mixture?
(1) br2(ℓ) (3) kbr(s)
(2) k(s) (4) kbr(aq)

               KBr (aq) is a mixture.

Explanation:

                       Mixture is defined as the combination of two non reacting species. Let's discuss each option one by one.

a) Br₂(l):

              This Bromine is not a mixture because the bracket specifying the state of Br₂ is liquid. Hence, as bromine is a liquid at room temperature, so this compound is purely Br₂.

b) KBr (s):

                KBr is also present in pure form because in bracket solid is written.

c) K (s):

              Potassium is also present in pure form as the bracket is specifying solid, as KBr is solid at room temperature.

d) KBr (aq):

                   This is a mixture of KBr and water. Because KBr being an electrolyte when dissolved in water it is hydrated as follow,

                             KBr (s) water> K⁺ (aq) + Br⁻ (aq)