The process of electrolysis: Passing an electric current through electrolytes causes the ions to move to the electrodes. Positively charged ions move to the negative electrode (the cathode), and negatively charged ions move to the positive electrode (the anode). Ions are discharged at the electrodes producing elements. This process is called electrolysis.

The idea

Electrolysis needs a direct current through an electrolyte, which is an ionic compound that is molten or dissolved so its ions can move. The power supply pushes electrons through the wires, making the cathode negative and the anode positive. Positive ions, called cations, are attracted to the negative cathode; negative ions, called anions, are attracted to the positive anode. At each electrode the ion gains or loses electrons and is discharged, forming a neutral element. For example, in molten lead bromide, Pb²⁺ ions reach the cathode and form lead metal, while Br⁻ ions reach the anode and form bromine. The products are elements, so electrolysis decomposes the electrolyte.

What earns the marks

• An electrolyte is an ionic compound that is molten or dissolved in water so that its ions are free to move and carry charge.
• A direct current from a power supply makes one electrode negative (the cathode) and the other positive (the anode).
• Positively charged ions (cations) move towards the negative electrode, the cathode.
• Negatively charged ions (anions) move towards the positive electrode, the anode.
• At the electrodes, ions gain or lose electrons and are discharged, producing neutral atoms or molecules of elements.
• The overall process of breaking down an ionic compound using electricity is called electrolysis.

Common mistakes and how to put them right

• Thinking that positive ions move to the positive electrode.

  Put it right by linking opposite charges attract, so positive ions go to the negative cathode.

• Believing that solid ionic compounds conduct during electrolysis.

  Put it right by stating that the ions must be molten or dissolved so they can move.

• Writing that ions are 'turned into electricity' or 'melted' at the electrodes.

  Put it right by saying ions are discharged, gaining or losing electrons to form elements.

Exam tips

• Name the cathode and anode and state their charges before describing ion movement, so your answer is precise.
• Use the words cation and anion correctly, and link each to its electrode.
• When asked to explain products, trace one positive ion to the cathode and one negative ion to the anode, then name the element formed.