Living Science 2020 2021 Solutions for Class 8 Science Chapter 15 - Chemical Effects Of Electric Current

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Only tap water will conduct electricity. Because tap water contains dissolved salts in it. When electricity passed through tap water, the salts dissociate into ions, which are responsible for the electrical conductivity of tap water. Whereas distilled water does not contain any dissolved salts. When electricity is passed through it, there are no charge carriers present in it, hence it does not conduct electricity.

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No, conduction of electricity in liquids does not occur through electrons but due to the presence of ions. These ions are responsible for the flow of electricity in a liquid when a potential difference is applied. 

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When an electric current is passed through a liquid, it breaks chemically into dissolved ions, these ions conduct electricity within a liquid. The process of breaking up of a liquid chemically into ions upon passing electricity is known as electrolysis and the liquid is known as the electrolyte. 

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To electroplate copper on a metal object, we will use a solution of copper sulfate as an electrolyte, a copper rod or plate as an anode and the metal object to be electroplated as the cathode. 

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Electroplating is done most of the times to protect metals from corrosion and to make them wear resistance for a longer period of time. 

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d) either positive or negative charge depending on the material

In a solid conductor (copper wire, aluminium wire, etc.), current is the flow of negative charges (electrons). The direction of current is opposite to the direction of electrons. In a liquid conductor (a salt solution), current is the flow of positive ions (positive charges). Flow of current is the same as the direction of positive ions, but opposite to the direction of negative ions.

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(c) petrol

Petrol is a hydrocarbon; therefore, it does not have free electrons or ions to conduct electricity.

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(b) liquid that conducts electricity and breaks up chemically in the process

A substance that conducts electricity in its liquid state or when dissolved in water, and breaks up chemically during the process is called an electrolyte.

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(b) copper is formed at cathode.

On passing electricity through copper sulphate solution, the salt ionizes to its constituent ions in the solution. These ions travel towards the oppositely charged cathode, i.e. copper, while the sulphate ions travel towards the anode thereby conducting electricity.

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(c) tap water

In tap water, the current is not conducted by ions.

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(c) zinc

Iron is coated with zinc to protect it from corrosion and formation of rust. The zinc coating will also give it a shiny appearance.

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A positively-charged body has a deficit of electrons.

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False.
Since electrons flow from a negatively charged body (excess of electrons) to a positively charged one (deficit of electrons), electronic current will also flow from the negative to the positive charge.

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True.

Electromotive force is the driving force in an electric circuit, which pushes the electrons carrying the charge around the circuit. This force is created by an electric cell or a battery.

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Electrons are responsible for the flow of electric current. They flow from a negatively charge body (with excess of electrons) to a positively charged one (with deficit of electrons).

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True.

Metals are conductors of electricity, as they allow electric current to flow through them.

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LED light is better for use in a tester for testing conductivity of liquids because it lights up even with a small amount of current.

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Most liquids that conduct electricity are solutions of acids, bases or salts.

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When electricity is passed through tap water, the gases hydrogen and oxygen are evolved.

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True.
Ions may be positively charged or negatively charged. Positively charged ions are called cations, while the negatively charged ions are called anions.

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An electric current brings about chemical changes in most conducting liquids.

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During electroplating, copper is deposited at the cathode.

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False.
Objects made from less expensive metals are electroplated with expensive metals, so that they look more attractive and beautiful.

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A continuous and directed flow of negative charges (i.e. electrons) through a wire is called electric current.

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Conventional current flows from a positively charged body to a negatively charged one, while the electronic current flows from a negatively charged body to a positively charged one. This is the difference between electronic current and conventional current. 

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A substance which conducts electricity in its liquid state or when it is dissolved in water to break-up chemically is called an electrolyte. Most soluble salts, acids and bases are electrolytes. Some examples are NaCl, KCl, NaOH, etc.
 

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Electroplating means coating a thin layer of a metal over another metal.
Zinc is electroplated on iron to prevent the latter from rusting. Hence, iron electroplated with zinc has various applications in the construction and automobile industries.

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(a) The electrons will flow from the negative to the positive terminal.
(b) Conventional current will from from the positive to the negative terminal.

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Chromium is a shiny metal that never corrodes and also resists scratches. Therefore, it is electroplated on iron to make it scratch-resistant and free from corrosion.

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When an electrolyte is dissolved in water, it breaks into cations (positively charged ions) and anions (negatively charged ions).
These ions then move freely in the aqueous solution. When an electric current is passed through this solution, the cations move towards the negatively charged electrode, i.e. the cathode, and the anions move towards the positively charged electrode, i.e. the anode.

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Tap water is impure and contains salts of metals dissolved in it. Therefore, when electricity is passed through tap water, it will start conducting the electricity.

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When copper sulphate is dissolved in water, it breaks into positively charged ions (cations) and negatively charged ions (anions).
Cations move towards the negatively charged electrode (cathode), and anions move towards the positively charged electrode (anode), thereby depositing copper at the cathode. This experiment shows that an electric current can bring about a chemical change and the phenomenon is called electrolysis.

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Electroplating is the process of coating a thin layer of a metal such as gold, silver, chromium, etc., over another metal.
This is done to protect the metal or to lend it an attractive look.
For electroplating copper in the laboratory, the following steps are to be taken:

• Fill three-forth of a beaker with copper sulphate solution.
• Hammer a copper wire to flatten it; then connect it to the positive terminal of a battery.
• Connect a spoon to the negative terminal of the battery.
• Dip the copper wire as well as the spoon in the copper sulphate solution, and let the current pass.

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Four uses of electroplating are:

1. Iron used to make bridges and automobiles is electroplated with zinc in order to prevent rusting.
2. Gold or silver is electroplated on jewellery made from a cheaper metal in order to make the jewellery look attractive.
3. Iron used to make food-storage utensils is electroplated with tin to prevent the food from getting spoiled by reacting with iron.
4. Chromium is a shiny metal that never corrodes and also resists scratches. It is electroplated on a cheaper metal like iron to make it corrosion and scratch-resistant. Chromium-plated iron is used for manufacturing car parts, wheel rims, taps, etc.

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An LED light consumes less electrical energy than an electric bulb.

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No.
Neither mercury produce ions in a solution nor in its liquid state; therefore, it cannot be used as an electrolyte. However, it has free electrons that can well-conduct electricity.

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One of the essential requirements for conducting electricity is the presence of free electrons or ions. In its solid state, the ions of copper sulphate (ionic compound) are immobile; therefore, it cannot conduct electricity. But when it is dissolved in water, it dissociates into copper ions and sulphate ions. These dissociated ions of copper sulphate conduct electricity.

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Yes, we agree that copper conducts electricity through electrons, while a salt solution conducts electricity through ions.
Copper is a metal and has free electrons to conduct electricity, whereas a salt solution is an electrolytic solution. The salt ionizes into its constituent ions in the solution. These ions travel towards the oppositely charged electrodes and conduct electricity.

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Water is a good conductor of electricity. So, if a person touches an electrical appliance with wet hands, he/she may get electrocuted.

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If a student interchanges between the copper wire and the spoon during electroplating, the copper ions deposited on the spoon will move back to their place of origin.

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Water is a good conductor of electricity. Therefore, if we pour water in order to put out a fire in an electric appliance, we may get electrocuted. So, we should not use water to put out a fire in an electric appliance.

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Different metals have different properties and on the basis of these properties they are used. Chromium is only used for electroplating metals in order to increase their life span. Besides this, chromium is expensive. So, instead of making the whole item with chromium, a layer of chromium is only added to lend a lustrous look to the item.