Direct Current – IGCSE Chemistry Definition

Direct current, abbreviated d.c., is electric current that flows in one direction only. In a d.c. circuit, one terminal of the power supply is always positive and the other is always negative. This is in contrast to alternating current (a.c.), where the direction of flow reverses many times per second.

Why d.c. is needed for electrolysis

Electrolysis requires direct current because the electrodes must maintain a constant polarity. The anode must remain positive to attract negative ions (anions), and the cathode must remain negative to attract positive ions (cations). If alternating current were used, the polarity would switch back and forth, the ions would oscillate without reaching either electrode, and no net electrolysis would occur.

Common d.c. sources used in electrolysis experiments include batteries and d.c. power packs. The positive terminal is connected to the anode and the negative terminal is connected to the cathode.

In the laboratory

When setting up an electrolysis experiment, candidates must ensure they use a d.c. power supply, not a.c. The circuit diagram should show a battery symbol (long and short parallel lines) or a d.c. power supply. The positive and negative terminals must be correctly labelled and connected to the appropriate electrodes.

Exam context

Paper 2 and Paper 4 questions about electrolysis sometimes ask what type of current is required and why. The answer must state d.c. (direct current) and explain that it maintains constant polarity at the electrodes, allowing ions to move consistently to one electrode for discharge.

Worked exam question

State the type of electrical supply needed for electrolysis and explain why. (2 marks)

Direct current (d.c.) is needed [1]. D.c. keeps the anode positive and the cathode negative at all times, so cations always move to the cathode and anions always move to the anode, allowing them to be discharged [1].

Common mistakes

Candidates sometimes write “electricity” without specifying d.c. or a.c. The question requires the specific type. Another error is failing to explain why a.c. would not work. Simply saying “d.c. is needed” without the reason earns only one mark when two are available.