Chemical Effects of Electric Current

This chapter builds in three stages, each answering one question that follows naturally from the last.

• Section 11.1 asks whether liquids conduct electricity at all, and builds two testers — one using a bulb, one using an LED and a compass needle — to find out; the answer sorts liquids into good conductors and poor conductors.
• Section 11.2 asks what happens to a conducting liquid once current actually flows through it, and finds that it undergoes chemical effects — bubbles, deposits, colour change — including the historic experiment that decomposed water into hydrogen and oxygen gas.
• Section 11.3 turns that chemical effect into a useful technology, electroplating, and surveys where it protects and decorates the objects you meet every day, from bicycle handlebars to food cans to bridges.

11.1 Testing liquids: good conductors and poor conductors

You built a tester in Class VII to check whether solids conduct electricity: a battery, a bulb, and two free wire ends that complete the circuit when touched together. The same tester works for liquids — dip the two free ends into the liquid, about a centimetre apart without touching, and watch the bulb. Always check the tester itself first by touching the free ends together directly; a dark bulb there means a loose connection, a fused bulb, or dead cells, not a problem with the untested liquid. Testing lemon juice, vinegar, tap water, vegetable oil, milk and honey this way shows two groups. Lemon juice and vinegar light the bulb — good conductors. Vegetable oil and honey leave it dark — poor conductors. Notice the wording: good conductor and poor conductor, not conductor and insulator, because under the right conditions most materials conduct at least a little — air is usually a poor conductor, yet lightning proves it can carry an enormous current. "Conducts" is a matter of degree, not a strict yes-or-no label. There is a catch with the bulb tester: a liquid can be conducting and still leave the bulb dark if the current is too weak to heat the filament. To catch weak currents, use an LED (needs far less current than a bulb) or, more sensitively, a coil of wire wound around a compass needle — even a small current creates a magnetic field strong enough to deflect it, the same magnetic effect of current met earlier, reused as a detector. This compass-needle tester reveals that liquids failing the bulb test are often weak conductors, not true non-conductors.

The brighter the bulb glows, the larger the current flowing through the tester, and since the same battery and bulb are used each time, brightness becomes a rough stand-in for how well the liquid conducts:

where: conductivity here just means how easily the liquid lets charge move between the two electrodes; a genuine numerical conductivity value is not needed at this level, only the comparison "more current means better conductor," and only when the compared currents are large enough to light the bulb at all. Distilled water is a good test case for this whole idea. Dipping the tester in distilled water leaves the bulb dark and the compass needle almost still — distilled water is a poor conductor. Dissolve a pinch of common salt into that same water, however, and the tester springs to life. This tells us that pure water itself barely conducts, but a solution made from it can conduct very well. Repeating the experiment with dilute acid (such as lemon juice or dilute hydrochloric acid) or a base (such as caustic soda) dissolved in distilled water gives the same result — good conduction — while dissolving sugar does not. So the rule that emerges is precise: most liquids that conduct electricity are solutions of acids, bases and salts, not just "anything dissolved in water." This immediately explains why natural water sources conduct. Tap water, well water, rain water and sea water are never chemically pure like distilled water; they carry small, naturally dissolved amounts of mineral salts, which make them good conductors. That is exactly why the human body's sweat (itself a weak salt solution) and any wet surface near a live wire are genuinely dangerous — not because water conducts, but because of what is dissolved in it.

11.2 Chemical effects of an electric current

Copper wire carrying current in a circuit comes out completely unchanged — the same wire, the same mass, before and after. A conducting liquid behaves very differently. Take two carbon rods (or iron nails) cleaned with sandpaper, wrap copper wire around their metal caps, connect them to a battery, and dip the rods (the electrodes) into salted or lemon-juiced water. Within a few minutes, gas bubbles appear near the electrodes. This is a genuine chemical change taking place inside the solution, driven by the electric current — very different from the physical, heating and magnetic effects of current studied earlier. The most famous version of this experiment is over two centuries old. In 1800, British chemist William Nicholson immersed two electrodes in water, passed a current through it, and found bubbles of two different gases forming — oxygen at the electrode connected to the positive terminal of the battery, and hydrogen at the electrode connected to the negative terminal. Passing current through water decomposes it into its two elements:

where: the upward arrows mark the two gases escaping as bubbles at their respective electrodes; this pattern of which gas appears at which electrode is exactly the clue used to spot a hidden battery's positive terminal, since the two gases (and, in salt or lemon-juice solutions, other visible changes) always sort themselves the same way relative to the terminals. A famous accidental version of the same idea happened to Boojho. Testing whether a potato conducts electricity, he inserted the two copper wires of a tester into a cut potato and forgot about it for half an hour. When he returned, a greenish-blue spot had appeared on the potato around the wire connected to the positive terminal, but not around the other wire. This is a chemical reaction between the potato's natural juices and the metal of the electrode — and because the spot only ever forms on the positive-terminal side, this trick can be used to identify the positive terminal of a battery hidden inside a sealed box. Generalising from these observations: passing an electric current through a conducting solution causes chemical reactions. Depending on which solution and which electrodes are used, the visible result may be gas bubbles at the electrodes, a metal deposit building up on an electrode, or a colour change in the solution. These are collectively called the chemical effects of electric current — the third major effect of current, alongside the heating effect and the magnetic effect studied in earlier grades.

11.3 Electroplating

The most useful application of the chemical effect of current is electroplating — depositing a thin layer of a desired metal onto another object by passing current through a solution containing that metal. A shiny bicycle handlebar, a "gold" bangle that is really gold-coated silver, and a food can that never rusts are all everyday products of electroplating. The classic demonstration uses copper sulphate solution and two copper plates as electrodes, connected to a battery. When current flows, the copper sulphate in solution splits into copper and sulphate. The free copper is drawn to the electrode connected to the negative terminal of the battery and deposits there as a shiny coating. Meanwhile, at the other electrode — the copper plate connected to the positive terminal — an equal amount of copper dissolves into the solution, replacing exactly what was lost. Copper is, in effect, being transferred from one electrode to the other, and the process continues steadily as long as the current flows.

As current flows, the copper leaving the solution and depositing at the cathode must be replaced from somewhere, or the solution would run out of copper and the process would stop; that replacement comes from the anode dissolving at essentially the same rate, so the two masses of copper stay in step throughout the plating:

where: $m$ stands for the mass of copper transferred in a given interval of time, measured in the same units (such as milligrams) at both electrodes; the approximation sign allows for small side losses, such as sludge or minor side reactions, which real electroplating baths always show to a small degree. Electroplating is used across industry because a metal's surface property can be borrowed without making the whole object out of that often-expensive metal. Chromium is shiny and resists corrosion and scratches but is costly, so car parts, bath taps and bicycle handlebars are made of a cheaper metal with just a thin chromium coating. Tin cans are iron plated with tin, which is less reactive than iron and keeps food from spoiling. Iron bridges and pipes are coated with zinc (galvanising) to resist rust. Jewellers plate silver or gold onto cheaper metals for an affordable precious look. In every case, disposing of the used electroplating solution safely is an environmental concern, since it is a polluting waste with specific disposal guidelines.

Board level - Comparing two liquids by bulb brightness

A tester is used to check the conduction of electricity through two liquids, A and B. The bulb glows brightly for liquid A, but only very dimly for liquid B. What can you conclude, and when should you be cautious about this method?

Step 1. Brighter bulb means larger current, since a larger current heats the filament more.

Step 2. The same battery and bulb were used for both liquids, so the only variable was the liquid itself. A brighter glow for A means more current flowed through A, so A lets charge move through it more easily than B does.

Step 3. Conclusion, in the chapter's own language: liquid A is the better conductor. This rules out "B is better," "both are equal" and "conducting properties cannot be compared this way."

Step 4. Caution: this method only works when both currents are large enough to light the bulb at all. If B's current were too weak to glow the bulb, you could not tell a poor conductor from a non-conductor without switching to an LED or compass-needle tester.

Liquid A is the better conductor, but the brightness method is trustworthy only when both liquids are strong enough to light the bulb in the first place.

Board level - Diagnosing why the bulb fails to glow

The free ends of a tester are dipped into a liquid, but the bulb does not glow at all. List the possible reasons and explain how you would tell them apart.

Step 1. Rule out the tester first: touch the two free ends together directly, with no liquid. If the bulb still stays dark, the fault is a loose connection, a fused bulb, or dead cells — nothing to do with the liquid.

Step 2. If the tester passes, the liquid is either genuinely a poor conductor (like distilled water), so almost no current flows, or it conducts a little but the current is too weak to heat the filament.

Step 3. Tell the two apart with a more sensitive tester: an LED needs far less current than a bulb, and a compass needle deflects for an even smaller current. A response from the LED or needle means the liquid was conducting weakly all along; no response from either means it is a genuine poor conductor.

The bulb can fail from a tester fault or from a liquid current too weak for a filament bulb; retesting with an LED or compass-needle tester tells the two apart.

Higher-order thinking (HOTS / olympiad-style) - Purifying copper by electrorefining

In electrorefining, a thin plate of pure copper and a thick rod of impure copper are the two electrodes, dipped in copper sulphate solution, with the goal of moving copper from the impure rod onto the pure plate. (a) Which electrode goes on the positive terminal, and why? (b) If 480 mg of copper dissolves off the impure rod in a given time, how much pure copper appears on the plate, and why might the real gain be slightly less?

Step 1. By the rule from the second derivation, the anode (positive terminal) loses metal and the cathode (negative terminal) gains metal. To move copper from the impure rod to the pure plate, the impure rod must dissolve — so it is the anode, on the positive terminal, and the pure plate is the cathode, on the negative terminal.

Step 2. This purifies the copper because only the anode dissolves, carrying its impurities into solution too, while the cathode gains fresh, separate copper; the impurities mostly settle out as sludge instead of depositing.

Step 3. By mass balance, the copper entering solution from the anode reappears at the cathode: $m_{\text{cathode}} \approx m_{\text{anode dissolved}} = 480\ \text{mg}$.

Step 4. The real gain is a touch under 480 mg because a small fraction of current goes to side reactions and some copper-rich sludge flakes away before dissolving — but the mass balance remains the dominant term. Timing note: state the anode-and-why reasoning in one sentence first, since that is where the "why" marks sit, before the arithmetic — reasoning and calculation are usually graded separately.

The impure rod is the anode on the positive terminal, the pure plate is the cathode on the negative terminal, and by mass balance close to 480 mg of pure copper deposits on the cathode.

• Calling liquids simply "conductors" or "insulators." NCERT deliberately uses good conductor and poor conductor because most materials, even air, conduct to some degree under the right conditions (lightning through air being the extreme example).
• Assuming a liquid does not conduct just because the bulb of the tester stays dark. A weak current can complete the circuit without lighting a filament bulb, yet still light an LED or deflect a compass needle.
• Treating distilled water and tap water as the same because they look identical. Only distilled water is genuinely a poor conductor; tap, rain and sea water all carry dissolved mineral salts and are good conductors.
• Mixing up which electrode gains metal during electroplating. The object being coated must be connected as the cathode (negative terminal); connecting it as the anode (positive terminal) would dissolve it instead of coating it.
• Believing the chemical effect of current "creates" new metal out of nothing. In electroplating, metal is transferred from the source electrode's plate to the object's electrode, not manufactured from the current itself.
• Forgetting the safety link between the two halves of the chapter. Since salty, mineral-laden water conducts well, wet hands, wet floors and flood water near live wiring are genuinely dangerous — because of what is dissolved in the water, not because pure water itself conducts.

• Explain why liquids are classified as good conductors or poor conductors of electricity rather than as conductors and insulators.
• Build and test a simple tester (battery plus bulb or LED) and check that it is working before testing any liquid with it.
• Explain why a bulb tester can miss a weak current that an LED or compass-needle tester would detect, and connect this to the magnetic effect of current.
• State which everyday liquids are good conductors and which are poor, and explain the difference in terms of dissolved acids, bases and salts.
• Explain why distilled water is a poor conductor while tap water, rain water and sea water are all good conductors.
• Describe, with an example, what a chemical effect of electric current is, including gas bubbles, metal deposits and colour changes.
• Recall Nicholson's 1800 experiment on the electrolysis of water and state which gas forms at which electrode.
• Define electroplating and correctly identify which electrode — the anode (positive terminal) or the cathode (negative terminal) — an object must be connected to in order to receive a metal coating.
• List at least three real-world uses of electroplating (chromium, tin, zinc, gold or silver) and explain what property each coating provides.
• Explain, in terms of dissolved salts, why wet hands, wet floors and flood water are dangerous near live electricity.

Ask the tutor: want to work through why seawater deflects the compass needle more strongly than tap water, or design your own step-by-step electroplating setup for a rusty iron key?