Let's start with a few elements:

Hydrogen (H):

Hydrogen is a colorless, odorless, tasteless gas. Its density is the lowest of any chemical element, 0.08999 grams per liter. Hydrogen changes from a gas to a liquid at a temperature of -252.77°C (-422.99°F) and from a liquid to a solid at a temperature of -259.2°C (-434.6°F).

Oxygen (O):

Oxygen is a colorless, odorless, tasteless gas. It changes from a gas to a liquid at a temperature of -182.96°C (-297.33°F). Liquid oxygen can then be solidified or frozen at a temperature of -218.4°C (-361.2°F). The density of oxygen is 1.429 grams per liter.

Sodium (Na): 

Sodium is a solid at room temperature with a melting point of 97.82°C (208.1°F) and boiling point of 881.4°C (1,618°F). Its density is slightly less than that of water, 0.968 grams per cubic centimeter. Sodium is a good conductor of electricity.

Chlorine (Cl):

Chlorine is a dense gas with a density of 3.21 grams per liter.  Chlorine changes from a gas into a liquid at a temperature of -34.05°C (-29.29°F) and from a liquid to a solid at -101.00°C (-149.80°F). The gas is soluble (dissolvable) in water.

So taking two gases, hydrogen and oxygen, you can make a liquid molecule of the compound water, with an atom of oxygen left over:

O2 + H2 ---> H2O + O

Sometimes the reaction includes that extra oxygen, and you make another liquid called hydrogen peroxide:

O2 + H2 ---> H2O2

Hydrogen peroxide is used as an antiseptic.

Taking the element sodium, a solid, and combining it with Chlorine, a gas, makes common table salt, or sodium chloride:

Na + Cl ---> NaCl