Which of these is an indication that a chemical reaction has occurred?

a. freezing of a substance
b. melting of a substance
c. boiling of a substance
d. formation of a gas

The correct option is D.

A chemical change is a type of change in which a new substance is formed and which is always irreversible. Chemical changes involves the breaking and forming of bonds and the original composition of the substances involved are usually altered. The indicators that show that a chemical change has occur include: change in temperature, change in colour, formation of precipitate and formation of gas bubbles.

2b2t

explanation:

2b2t

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