If you dilute a concentrated solution, the concentration of the obtained solution is smaller than that of the initial solution. But if you add more solute to a concentrated solution, the concentration of the obtained solution is greater than that of the initial.
Group 13 metals like aluminum lose three electrons to form an ion with a 3+ charge
The reaction will be like

Now these H will wonder around means they will diffuse here and there (the random transnational movement of particles is known as diffusion) and once the two hydrogen atoms come across each other they will form bond and will form hydrogen molecule
so answer is : diffusion