If we take NH3 gas (a colorless gas) and mix it with HCl gas (another colorless gas) a reaction producing solid NH4Cl will take place. Where the NH3 and HCl gas come in contact with each other a white cloud forms. This white cloud is made up of very small solid crystals of solid NH4Cl (Ammonium chloride).
The chemical reaction can be written as:
NH3 (g) + HCl (gas) = NH4Cl (s)
As a word equation it would be written:
Ammonia gas plus Hydrogen chloride gas yields solid Ammonium chloride:
The particles of NH4Cl that form a so small that they look like smoke and take a long time to settle. After some time we could collect the Ammonium chloride crystals. If we were to heat the crystals up we could cause the NH4Cl to decompose back into NH3 (g) + HCl (g).
In answer to our question, two gases can react to form a solid. In this case NH3 (g) + HCl (g) produces the NH4Cl (s).
