Question 5
In the reaction CaCO3(s) + 2HCl(aq) → CaCl2(aq) + H2O(l) + CO2(g), which is easiest to observe?
The right answer is the one marked above.
The rate at which CaCO3 is used up, or CO2 is given off. The first can be weighed and watched as it disappears; the second leaves as bubbles.
CaCl2 and H2O both stay in the solution and give no outward sign at all. HCl being used up is just as invisible.