Cambridge O Level Chemistry · Syllabus 5070 · Atoms, Elements and Compounds
Mobile ions
What is Mobile ions?
Charged particles that are free to move through a substance and so can carry an electric current. An ionic compound contains ions in every state, but they are locked in fixed positions in the solid lattice; melting or dissolving breaks the lattice apart and allows the ions to move, which is why an ionic compound conducts only when molten or in aqueous solution. The charge carriers in these cases are ions, never electrons.
This definition is part of the Atoms, Elements and Compounds chapter in Cambridge O Level Chemistry.
Questions students ask about Mobile ions
How do you explain the properties of a substance from its structure in an exam answer?
Walk the chain: identify the particles, follow the outer electrons, name the structure, then read the property off the structure. For melting point, say which particles are held together, by which attraction, and how much energy is needed to overcome it. For conductivity, name the mobile charged particle — delocalised electrons in a metal or graphite, mobile ions in a molten or aqueous ionic compound — or state that there is none. Marks are given for each link in the chain, not for the final word alone.