Wait until a while ring of solid ammonium chloride forms where the hydrogen chloride and ammonia gases meet. The ammonia gas, having a lower molecular weight than the hydrogen chloride, will diffuse ...

When writing chemical equations for reversible reactions ... can break down to form ammonia and hydrogen chloride. It also shows that ammonia and hydrogen chloride (colourless gases) can react ...

The mass of hydrogen is the lowest of all the atoms. These low masses help explain why polyethylene has a low density. Another reason is that these long skinny molecules are loosely packed together.