When you are balancing chemical reactions ('equations'), what must you never touch? the normal-looking numbers to the left of the molecules or compounds. the coefficients, the number in subscript

The correct answer option is: the number in subscript.

Explanation:

When you are balancing any chemical equation, you must never touch or change the number in subscript of the molecule of a chemical or compound.

The number in subscript of any molecule or compound shows the ratio of atoms that makes up that certain molecule or compound.

You can only change or alter the number written left to the molecule of compound to balance the equation.

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