how does the acid convert the rust into soluble salt?

Answer 1

acid reacts with the oxide (Rust) to form a salt.
Example

FeO + H2SO4 = FeSO4 + H2O

Fe2O3 +3 H2SO4 = Fe2(SO4)3 + 3H2O

Answer 2

Rust is an oxide and does not readily dissolve in water. Your question is to convert rust to be a soluble salt using acid.

Any acid that that reacts with the iron in rust to form soluble salts will dissolve it. For example you react it with HCl or HNO3 because the salt formed is ferrous/ferric chloride or ferric nitrate. Note that nitrates and chlorides or iron are soluble in water. And this happens also because the basic iron oxides react with the acid, a neutralization reaction, being favored because the products formed are stable soluble ions.

Another way to make soluble salt is to use an acid that will form complexes with the iron. An example is citric acid, oxalic acid etc. The iron is now bound to the complexing agent instead of the oxide. In essence, the acid has competitively displaced the oxide to form a soluble complex salt.

Answer 3

Iron Iii Oxide Solubility

Answer 4

Phosphoric acid may be used by direct application to rusted iron, steel tools or surfaces to convert iron(III) oxide (rust) to a water soluble phosphate compound.
Oxalic acid is widely used as an acid rinse in laundries, where it is effective in removing rust and ink stains because it converts most insoluble iron compounds into a soluble complex ion. For the same reason, it is the chief constituent of many commercial preparations

Answer 5

Rust is essentially iron(III)oxide, which is basic in nature.

By using an acid, there will be a neutralisation reaction between the basic oxide and the acid, producing salt and water. The iron salt produced will be soluble depending on the type of acid used.

For example, if using nitric acid, then the salt produced is iron (III) nitrate, which is soluble in water.