When the hydrogen chloride concentration doubles, the number of collisions between the reactants (1. decreases 2. increases 3. remains constant), which causes the rate of the forward reaction to (1. decrease 2.increase 3. remain constant)

The correct answers :  2. increases ; 2. increase.

The rate reaction is depends on the number of factors such as concentration, temperature and catalyst use. The concentration of reactants is one such factors. If the concentration of the reactants doubles, more reactant particles move together, thus collision between reactants increases and the reaction rate is also increased. Therefore, higher the concentration of reactants, the faster the rate of a reaction.  

the density(ratio between mass and volume as m/v) decreases from top to bottom in periodic table group as the size increases and nuclear charge decreases. but in periods density increases from left to right as atom size decreases and nuclear charge increases.