Which sample of matter is a mixture?

(1) Br2 (ℓ) (3) KBr (s)

(2) K (s) (4) KBr (aq)

The correct answer is option 4. KBr (aq) is an example of a mixture. The symbol (aq) means that the substance is in aqueous form. Thus, potassium bromide is dissolved in water. Mixture is defined as a system which contains two or more substances mixed but not combined chemically. The two substances here are KBr and water.