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Chemical Effects of Electric Current - Electrical Conductivity of Liquids

Grade 8CBSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Electrical conductivity in liquids depends on the presence of ions. Liquids that conduct electricity are called electrolytes.

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Most liquids that conduct electricity are solutions of acids, bases, or salts. Common examples include lemon juice (H3C6H5O7H_{3}C_{6}H_{5}O_{7}), vinegar (CH3COOHCH_{3}COOH), and tap water containing dissolved salts.

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Unlike metallic conductors where electrons flow, in liquids, the electric current is carried by the movement of positive ions (cations) and negative ions (anions).

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Distilled water is a poor conductor (insulator) because it lacks dissolved salts or ions, but it can be made conductive by adding a small amount of salt like NaClNaCl.

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Chemical effects of electric current include: (1) Formation of gas bubbles at the electrodes, (2) Deposition of metal on electrodes, and (3) Change in the color of the solution.

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The electrode connected to the positive terminal of the battery is the Anode, and the one connected to the negative terminal is the Cathode.

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Electroplating is a common application where a layer of a superior metal (like CrCr or AgAg) is deposited on a base metal (like FeFe or CuCu) using an electric current.

📐Formulae

2H2O(l)→Electric Current2H2(g)↑+O2(g)↑2H_{2}O(l) \xrightarrow{\text{Electric Current}} 2H_{2}(g) \uparrow + O_{2}(g) \uparrow

NaCl(s)→dissolved in H2ONa+(aq)+Cl−(aq)NaCl(s) \xrightarrow{\text{dissolved in } H_{2}O} Na^{+}(aq) + Cl^{-}(aq)

CuSO4(aq)⇌Cu2+(aq)+SO42−(aq)CuSO_{4}(aq) \rightleftharpoons Cu^{2+}(aq) + SO_{4}^{2-}(aq)

💡Examples

Problem 1:

Explain why a compass needle deflects when the free ends of a tester are dipped into lemon juice.

Solution:

The deflection occurs because lemon juice is a conductor of electricity.

Explanation:

Lemon juice contains citric acid, which dissociates into ions when dissolved in water. These ions allow the electric current to flow through the liquid. The flowing current creates a magnetic field around the wire, which causes the magnetic compass needle to deflect.

Problem 2:

During the electrolysis of water, which gas is collected at the electrode connected to the positive terminal of the battery?

Solution:

Oxygen gas (O2O_{2}) is collected at the positive electrode (Anode).

Explanation:

Water (H2OH_{2}O) decomposes into Hydrogen (H+H^{+}) and Oxygen ions. Since Oxygen ions carry a negative charge in this context (or are derived from the oxidation process), they are attracted to the positive electrode (Anode), while Hydrogen (H2H_{2}) is released at the negative electrode (Cathode).

Problem 3:

In the process of electroplating a copper spoon with silver, what should be used as the Anode and the Cathode?

Solution:

Anode: Pure Silver rod; Cathode: Copper spoon.

Explanation:

In electroplating, the object to be coated is always made the Cathode (negative electrode). The metal to be deposited is made the Anode (positive electrode). The electrolyte must contain ions of the metal to be deposited, such as a silver nitrate (AgNO3AgNO_{3}) solution.