A physical combination of two or more substances where each keeps its own chemical identity and properties

A flask holds distilled water. Every particle in it is the same, and each one contains two hydrogen atoms joined to one oxygen atom. How is the contents of the flask best classified?

A diving tank is filled with air: nitrogen, oxygen and argon spread evenly through the tank. The shop can raise the oxygen share for a special dive. How is the gas in the tank best classified?

A student stirs sand and iron filings together in a dish. The gray specks stay visible in the sand, and a magnet held above the dish pulls the filings out. How is the sample best classified?

A balloon is filled with helium. Every atom inside has the same atomic number, 2, and nothing in the balloon can be broken down into a simpler substance. How is the gas best classified?

Salt is stirred into a beaker of water until it disappears. Gently heating the beaker until the water evaporates leaves the salt behind as white crystals.

Salt dissolved in water

Sand and water

Oil and water

Air

Fruit salad

Vinegar

Chocolate chip ice cream

Coffee with sugar completely dissolved

Pizza