Chemical Effects of Electric Current — NCERT Solutions, AI Tutor & Practice

Understand: Electric current is a flow of charge. For a liquid to conduct, it must contain freely moving charged particles (ions).

Analyse:

• Liquids that contain free ions (charged particles) conduct electricity. These are called good conductors. Examples: salt solution, lemon juice, dilute acids, and tap water (contains dissolved salts).
• Liquids that have no free ions do not conduct. These are poor conductors or insulators. Examples: distilled water, sugar solution, vegetable oil.

Conclude: A liquid conducts electricity only when it contains free ions to carry the charge. Distilled water is pure and has no ions, so it does not conduct, but when a little salt is added, it becomes conducting.

Set-up: Take a tester with two electrodes (e.g. pencil-lead or metal ends) connected through an LED and a cell (battery). Dip both electrodes into the liquid without letting them touch each other.

Observation:

• If the LED glows, the liquid is a good conductor.
• If the LED does not glow, the liquid is a poor conductor.

Why an LED is preferred:

• Some liquids conduct only a very small current, which is not enough to heat a bulb filament and make it glow.
• An LED glows even when a very weak current passes through it.
• Note: An LED has two wires (leads). The longer wire must connect to the positive terminal of the battery for it to work.

Conclusion: The LED tester can detect even feeble currents that a bulb would miss.

Definition: When an electric current passes through a conducting liquid, it may cause chemical changes in the liquid. These changes are called the chemical effects of electric current.

Signs that a chemical effect has taken place:

1. Bubbles of gas may form at the electrodes.
2. Deposits of metal may appear on one of the electrodes.
3. Change in colour of the solution.
4. Change in the colour of the electrodes.

Conclusion: Any of these observations shows that a chemical reaction is occurring due to the passage of current.

Why acid is added: Pure water is a poor conductor, so a few drops of acid (or salt) are added to make it a good conductor. This process is safe as it only helps current flow.

What happens:

• When current passes, water breaks up (decomposes).
• Bubbles of gas appear at both electrodes.
• At the electrode connected to the negative terminal, hydrogen gas is released.
• At the electrode connected to the positive terminal, oxygen gas is released.

Note: The volume of hydrogen collected is about twice the volume of oxygen, since water is $H_2O$.

Conclusion: This is a clear chemical effect — the current splits water into hydrogen and oxygen.

Definition: Electroplating is the process of depositing a thin layer of a desired metal over another material (usually a metal) using electric current.

Uses / Why it is useful:

1. Chromium plating — done on car parts, taps, and bicycle handles because chromium is shiny, hard, and does not corrode; using solid chromium would be costly.
2. Zinc / tin coating — iron cans used for food storage are coated with tin to prevent iron from reacting with food.
3. Silver and gold plating — cheaper metals are plated with silver or gold to make attractive, less-expensive jewellery.
4. Protection from rust and to give a better appearance.

Conclusion: Electroplating combines the strength/low cost of the base metal with the appearance or protective quality of the coating metal.

Materials: A copper plate, the object to be plated, a copper sulphate ($CuSO_4$) solution, a battery, and wires.

Set-up:

• Take copper sulphate solution in a container.
• Connect a pure copper plate to the positive terminal — this is the anode.
• Connect the object to be plated to the negative terminal — this is the cathode.

Process:

1. When current flows, copper sulphate dissociates into copper and sulphate.
2. Copper from the solution deposits on the object (cathode).
3. The copper anode dissolves into the solution, replacing the copper that was removed.
4. Thus the concentration of copper in the solution stays roughly constant.

Result: A shiny, even layer of copper forms on the object.

Understand: Conduction of electricity in a liquid needs free ions.

Analyse:

• Distilled water is pure water with no dissolved salts, hence it has almost no free ions.
• Therefore, distilled water is a poor conductor — the student's claim is incorrect.

How to make it conduct:

• Dissolve a little common salt (or acid) in it.
• This adds free ions to the water, making it a good conductor.

Conclude: The conducting ability of ordinary water comes from the salts dissolved in it, not from the water itself.

Observation 1 — Bubbles at the electrodes:

• This is a sign of a chemical effect of electric current — a chemical reaction is releasing gases at the electrodes.

Observation 2 — Deflection of the compass needle:

• A current flowing through the wire produces a magnetic effect, which deflects the nearby compass needle.
• This also confirms that current is actually flowing through the circuit and the liquid is conducting.

Conclusion: Together these show that the liquid is a good conductor, current is flowing (magnetic effect), and that current is producing a chemical effect in the solution.

Understand: Food cans are made of iron, but iron is reactive.

Analyse:

• Iron reacts with food and can spoil it, and it also rusts easily.
• Tin is less reactive than iron and does not easily react with food substances.
• A thin layer of tin is electroplated onto the iron.

Conclude: Tin is used because it is less reactive and safe with food, protecting both the iron and the food, which a cheaper reactive metal could not do.

| Liquid | Conductor type | Reason | |---|---|---| | (a) Lemon juice | Good conductor | Contains acid → free ions | | (b) Vegetable oil | Poor conductor | No free ions | | (c) Tap water | Good conductor | Contains dissolved salts (ions) | | (d) Distilled water | Poor conductor | Pure water, no ions | | (e) Vinegar | Good conductor | Contains acetic acid → free ions |

Conclusion: Liquids containing acids, bases, or dissolved salts conduct electricity, while pure or oily liquids do not.

Understand: In electroplating, metal is deposited on the object (cathode) from the solution.

Analyse:

• As plating continues, the metal ions in the solution get used up because they deposit on the object.
• If the anode is made of the same plating metal, it slowly dissolves into the solution, replacing the ions that were removed.
• This keeps the concentration of metal in the solution constant, so plating continues smoothly.

Conclude: Using the same metal for the anode ensures a continuous, even, and lasting supply of metal ions for uniform plating.

Understand: Water is $H_2O$, so it decomposes into hydrogen and oxygen in the ratio 2 : 1 by volume.

Given: Volume of hydrogen $= 40$ mL.

Calculation: $\frac{\text{Volume of hydrogen}}{\text{Volume of oxygen}} = \frac{2}{1}$ $\text{Volume of oxygen} = \frac{40}{2} = 20 \text{ mL}$

Electrodes:

• Hydrogen (40 mL) forms at the negative electrode (cathode).
• Oxygen (20 mL) forms at the positive electrode (anode).

Conclusion: 20 mL of oxygen is collected, and the 2:1 ratio confirms the chemical formula of water.