Question

Which releases H+ cations when place in water?

A) CH_4
B) NaHC0_3
C) HCI
D) CO_2

Answer 1

D) Co2!

it reacts with water to form carbonic acid...

that has H+ ions!

Answer 2

C)
`\text{HCl}` hydrochloric acid and, in case that the question allows for more than one choices, D)
`\text{CO}_2` carbon dioxide as well.

Methane molecules
`\text{CH}_4` are nonpolar and barely dissolve in water.

Sodium hydrocarbonate
`\text{Na}\text{HCO}_3` undergoes hydrolysis to release hydroxide ions
`\text{OH}^(-)` that can end up consuming
`\text{H}^(+) \; (aq)`:

`\text{NaHCO}_3 \; (aq) \to \text{Na}^(+) \; (aq) + \text{HCO}_3^(-) \; (aq)\\\text{HCO}_3^(-) \; (aq) + \text{H}_2\text{O} \; (l) \leftrightharpoons \text{H}_2\text{CO}_3\; (aq) + \text{OH}^(-) \; (aq)`

Hydrochloric acid, a strong acid, ionizes to produce protons
`\text{H}^(+) \; (aq)` and chloride ions when dissolved in water:

`\text{HCl}\; (aq) \to \text{H}^(+) \; (aq) + \text{Cl}^(-) \; (aq)`

Carbon dioxide reacts with water to produce hydrocarbonic acid, a weak acid that slightly ionizes, also producing protons
`\text{H}^(+) \; (aq)` but to a significantly lesser extent than hydrochloric acid does.

`\text{CO}_2\; (g) + \text{H}_2\text{O} \; (l) \leftrightharpoons \text{H}_2\text{CO}_3\; (aq)\\\text{H}_2\text{CO}_3\; (aq) \leftrightharpoons \text{H}^(+)\; (aq) + \text{HCO}_3^(-) \; (aq)`