Could someone explain to me where the additional energy goes when applied to an object in the state of melting (such as water) so that the temperature of the object does not increase, and could someone please explain to me why Helium has a specific latent heat.
When you heat a substance (not at its melting point or boiling point), the energy supplied goes to increase the kinetic energy of the atoms/molecules in the substance.
When the same heat is applied to the substance at its melting point (or boiling point), the heat goes to increase the potential energy of the atoms/molecules in the substance. You could refer to the potential energy as the energy used to break the inter-particle bond. In ice, there are relatively strong forces of intermolecular attraction, which makes the ice hard.
Now, temperature is directly related to the kinetic energy of the atoms/molecules of a substance, and since the kinetic energy of the atoms/molecules does not increase during a change of state, the temperature remains constant.
At least, this is what happens at a macroscopic level and in the long term. But if you heat ice at one particular point (at particle level and for a short period of time), that point will melt faster and the temperature will tend to rise there, but this heat energy will then be distributed to the other parts of the ice still cold for melting.
It is only when all the ice has molten than the heat supplied will contribute once again to the increase in kinetic energy, that is, temperature.
And for the last part, every substance that undergoes a change of state has latent heat.