Scientists must be careful when combining hydrochloric acid with NaClO, sodium hypochlorite. This is because this chemical reaction produces a yellow, poisonous gas called chlorine gas. Low levels of exposure to chlorine gas can cause fluid build-up in the lungs, and high exposure can cause frostbite to the skin and eyes.

Question

NaClO + HCl →
NaOH + Cl2
Clear Liquid
Clear Liquid

Clear Liquid

Yellow Gas

Which of these products or reactants provides evidence that a chemical reaction occurs?

A The total number of atoms remains unchangedThe total number of atoms remains unchanged
B A liquid is presentA liquid is present
C Chlorine gas is toxicChlorine gas is toxic
D A yellow gas is produced

Answer 1

The best answer that provides evidence that a chemical reaction occurs is:

D A yellow gas is produced.

The production of a new gas (in this case, chlorine gas) indicates that a chemical change has taken place, as new substances have formed as a result of the reaction.