What will the presence of h+ ions in a solution cause?

a bitter taste when consumed

a reaction with magnesium to release hydrogen gas

a slippery feeling

a reaction with an acid

H+ refer to acid solution HCI (aq). the presence of H+ ions in a solution cause : b. a reaction with magnesium to release hydrogen gas.
the equation H+ react with magnesium to release hydrogen gas is Mg + 2H + --> Mg(2+) H2(g)

hope this help

The mass has escaped into a gas which you can not see so the law stays true.