Typically calcium carbonate is poorly soluble (for O Level Chemistry, we take calcium carbonate to be insoluble in water).

The calcium carbonate works to neutralise the nitric acid present. When added in excess, any extra quantity of calcium carbonate beyond the neutralisation point will not dissolve in water to form an alkaline solution.

This is in contrast to sodium hydroxide and ammonia in which extra quantities added result in an alkaline solution.

will recommend the word 'sparingly' in place of 'poorly'

Water : Only dilutes the nitric acid, doesn't neutralise it. (i.e H+ ions concentration decreases but does not eliminate or remove any free H+ ions)

Sodium hydroxide :

HNO3 + NaOH → NaNO3 + H2O

The acid is neutralised is the H+ ions react with the OH- ions to form water molecules. (Note that at higher levels, it's actually hydronium/hydroxonium ions (H3O+) instead)

Excess NaOH will yield a strongly alkaline/basic solution due to near complete dissociation of OH- ions in the water.

But in the long run when a lot of water is added, the acidity level would be very low.

Though a pH increase by 1 would require a ten-fold increase in the quantity of water required for this purpose.

The question wants complete neutralisation ,so CaCO3 will be better than water.

Furthermore, adding water to acid (and especially a strong and concentrated one) is highly discouraged due to the very exothermic enthalpy of hydration.

The solution may vaporise readily or splatter. What we don't want are fumes/vapour filling up the entire area.

In the lab I used to work in, a PhD candidate accidentally dropped a 5l bottle of 70% sulfuric acid onto the floor.

The anti-slip flooring got corroded immediately and guess what? This person went to pour water all over the spill.

We all rushed to get out of the lab to avoid the fumes.

He never thought of a spill kit or using bases to neutralise it eg. Sodium carbonate/bicarbonate.

It must have been done in a state of panic, since it’s quite likely that even rational humans make irrational decisions in such situations.

Not really.

It was well known that in his country of origin, the awareness and emphasis on lab safety and culture/attitudes towards it are lax.

For a PhD candidate, this is unacceptable.

Not the first time something like this happened. On another occasion, one left a reflux set up alone, without realising that acid was dripping out through one of the outlets.

Happily went off to to have lunch and we had to clean up the mess.

Also, he and a few others from the same country been caught multiple times for lab safety violations.

Strongly reprimanded and stern official warnings were issued.

Anyway, he failed his candidature eventually.