What will occur when solid calcium sulfate is added
towater?

Explanation:

Calcium sulfate is slightly soluble in water. The small amount of ions given by calcium sulfate on dissolving in water does not interact with water acting as a spectator ions. Due to presence of covalent bonds in calcium sulfate it doesn't dissolves in water which polar solvent.

Hardening of water occurs when solid calcium sulfate is added  to water.This hardening of water is due to presence of calcium ions.

let that person have a turn to speak : )

as given the equation is

2al + 6 hcl > 2 alcl3 + 3 h2(g)

as per equation two moles of aluminum reacts with six moles of hcl to give two moles of alcl3 and three moles of hydrogen gas

given: three moles of al are present and 7 moles of hcl

for three moles of aluminum nine moles of hcl will be required

however as 7 moles of hcl are present so the limiting reagent is hcl

hcl is going to decide the number of moles of product formed

when six moles of hcl are present = 3 moles of h2 are formed

when one mole of hcl is present == 3/6 mole sof h2 will be formed

when 7 moles of hcl are present = 3 x 7/ 6 moles of h2 will be formed = 3.5 moles of h2