Chemical Effects of Electric Current

We know that many solids, like metals, allow electric current to pass through them — they are conductors. But what about liquids? Some liquids conduct electricity and some do not. A liquid that conducts electricity does so because it contains charged particles (called ions) that are free to move and carry the current. Liquids that conduct electricity are called electrolytes (or conducting solutions), while liquids that do not conduct are non-conductors.

To test whether a liquid conducts electricity, we use a simple circuit. The liquid is placed in a container, and two rods or plates (called electrodes) connected to a battery are dipped into it; a bulb or an LED is included in the circuit as an indicator. If the liquid conducts electricity, the circuit is completed, current flows, and the bulb glows (or the LED lights up). If the liquid does not conduct, no current flows and the bulb does not glow. An LED is often used instead of a bulb because it can detect even a very weak current that may not light an ordinary bulb.

Using this test, we find that most liquids that conduct electricity are solutions of acids, bases, and salts in water. For example, dilute acids (like dilute sulphuric acid), salt solution (common salt dissolved in water), and lemon juice conduct electricity, because they contain ions. Even ordinary tap water conducts electricity to some extent, because it contains dissolved salts and minerals that provide ions. These conducting solutions are the electrolytes.

On the other hand, some liquids do not conduct electricity because they have no free ions. Pure (distilled) water is a poor conductor, because it has almost no dissolved ions; it is the dissolved salts in ordinary water that make it conduct. Liquids like vegetable oil, kerosene, and alcohol also do not conduct electricity. So whether a liquid conducts depends on whether it contains free ions: solutions of acids, bases, and salts conduct (they are electrolytes), while pure water and oils do not. This passing of current through conducting liquids leads to interesting chemical effects, which we study next.

When an electric current is passed through a conducting liquid (electrolyte), it can cause a chemical change in the liquid — this is one of the chemical effects of electric current. The process in which an electric current passing through a conducting liquid causes it to break up (decompose) into simpler substances is called electrolysis. The word means "splitting by electricity". Electrolysis shows that an electric current can do chemical work, not just light a bulb or produce heat.

In electrolysis, two electrodes are dipped into the electrolyte and connected to a battery. The electrode connected to the positive terminal of the battery is the anode (positive electrode), and the one connected to the negative terminal is the cathode (negative electrode). When current flows, the substances in the liquid are decomposed, and the products of the decomposition are released or deposited at these electrodes. Different substances appear at the anode and the cathode.

A classic example is the electrolysis of water. When an electric current is passed through water (to which a little acid is added to make it conduct), the water decomposes into its two gases — hydrogen and oxygen. Hydrogen gas is released at the cathode (negative electrode) and oxygen gas at the anode (positive electrode). The volume of hydrogen collected is about twice that of oxygen, since water contains hydrogen and oxygen in a 2:1 ratio. This experiment beautifully demonstrates that electricity can split water into its elements.

Another important example is the electrolysis of copper sulphate solution. When current is passed through a solution of copper sulphate using copper electrodes, copper is deposited on the cathode (the negative electrode), while copper dissolves from the anode into the solution. So a reddish-brown layer of copper builds up on the cathode. This shows that during electrolysis, a metal can be transferred from the solution onto an electrode. The fact that electrolysis deposits metals on the cathode is the basis of a very useful process called electroplating, which we study next. So electrolysis — the decomposition of a conducting liquid by an electric current — is a key chemical effect of electricity with important applications.

One of the most useful applications of the chemical effect of electric current (electrolysis) is electroplating. Electroplating is the process of depositing a thin layer of one metal over another metal (or object) using an electric current. In other words, electroplating uses electrolysis to coat an object with a metal such as silver, gold, chromium, nickel, or copper. The thin metal coating improves the object's appearance, protects it, or gives it useful properties.

The principle of electroplating is the same as the electrolysis we studied: when a current is passed through a solution containing the metal to be coated, the metal is deposited on the object connected as the cathode (negative electrode). To electroplate an object, the object to be plated is made the cathode, a piece of the coating metal is made the anode, and both are dipped in a solution (electrolyte) containing the coating metal. When the current flows, metal from the solution is deposited on the object, while the anode dissolves to replace it in the solution. This continues until a thin, even layer of the metal covers the object.

Electroplating has many important applications. Jewellery and ornaments made of cheaper metals are electroplated with gold or silver to make them look attractive and valuable. Tin is electroplated onto iron to make tin cans for storing food, because tin is non-toxic and does not rust easily. Taps, bicycle handlebars, and car parts are electroplated with chromium, which is shiny, hard, and resists corrosion. Nickel and chromium are used to plate many objects for a bright, protective, rust-resistant finish.

The advantages of electroplating explain why it is so widely used. It gives objects an attractive, shiny appearance; it protects metals from rusting and corrosion by covering them with a layer of a metal that does not corrode easily; and it lets us give a cheap, strong metal the surface properties of an expensive one (for example, the look of gold or the hardness of chromium) at low cost. So electroplating, an everyday use of the chemical effect of electric current, beautifies and protects countless objects around us, from jewellery and food cans to car parts and taps.

In electrolysis and electroplating, we have seen that a metal is deposited on the cathode. A natural question is: how much metal is deposited? Can we control the thickness of the coating? The answer was discovered by the scientist Michael Faraday, and at this level we study it as a conceptual law: the amount of a substance (metal) deposited during electrolysis depends on the strength of the current and the time for which it flows.

This means two things. First, the greater the current passed through the electrolyte, the more metal is deposited in a given time — a stronger current carries more charge and deposits more metal. Second, the longer the time for which the current flows, the more metal is deposited — the deposit builds up steadily as time goes on. In short, more current and more time both give a thicker deposit of metal on the cathode.

We can understand this with an everyday comparison. Think of water flowing from a tap into a bucket: the faster the flow (more current) and the longer you leave the tap on (more time), the more water collects in the bucket. In just the same way, a larger electric current and a longer time cause more metal to be deposited during electrolysis. So the total amount of metal deposited depends on both how strong the current is and how long it flows.

This principle is very useful in practice, because it lets us control the thickness of the coating in electroplating. If a thin coating is wanted, a smaller current is passed for a short time; if a thicker coating is needed, a larger current is passed for a longer time. Manufacturers use this to deposit exactly the right amount of metal — for example, a thin layer of gold on jewellery, or a thicker, harder layer of chromium on car parts. So this conceptual law of electrolysis — that the amount deposited depends on the current and the time — gives us control over electroplating, completing our understanding of how the chemical effect of electric current is put to practical use.

We have studied how an electric current can decompose a liquid (electrolysis) and deposit metals (electroplating). It is helpful to step back and see that electric current produces several different effects, of which the chemical effect is one. Whenever an electric current flows, it can produce a heating effect, a magnetic effect, and — in conducting liquids — a chemical effect. Recognising these effects together shows how versatile and useful electricity is.

The chemical effect of electric current is the one this chapter has focused on: when current passes through a conducting liquid (electrolyte), it causes a chemical change, such as the decomposition of the liquid (electrolysis), the release of gases, the deposition of metals on electrodes, or a change in the colour of the solution. The appearance of bubbles of gas at the electrodes, deposits of metal, and changes in the colour of the electrolyte are all signs that a chemical effect is taking place. These effects occur only in conducting liquids, not in solid metal wires.

A current also produces a heating effect: when current flows through a wire that has some resistance, the wire becomes hot. This heating effect is used in electric heaters, irons, geysers, and the filaments of bulbs. A current also produces a magnetic effect: a wire carrying current behaves like a magnet and can deflect a compass needle. This magnetic effect is the basis of electromagnets and electric motors (which we study in a later chapter). So a single electric current can heat, magnetise, and cause chemical changes, depending on the circuit.

The chemical effect of electric current, the focus of this chapter, has given us electrolysis (splitting water and other compounds) and electroplating (coating objects with metal), both of great practical value. It also has further uses, such as the extraction and purification of metals on an industrial scale, where electricity is used to obtain pure metals from their compounds. By understanding that electricity has chemical, heating, and magnetic effects — and especially how the chemical effect leads to electrolysis and electroplating — we complete this chapter and prepare for the study of electric circuits and magnetism that follow.