Solved examples for class 8 science : Chapter 4

    Materials: Metals and Non-Metals

    I. Multiple Choice Questions (MCQs).
    Q.1. The metal which reacts violently with cold water is
    a) sodium
    b) magnesium
    c) aluminium
    d) copper

    Answer: (a) sodium
    Q.2. Which among the following can be displaced by zinc?
    a) Calcium
    b) Magnesium
    c) Aluminium
    d) copper
    Answer. (d) copper

    Q.3. The characteristic shine of metals is called
    a) malleability
    b) ductility
    c) lustre
    d) none of these

    Ans. (c) lustre

    Q.4. Which of the following is used for making matchstick?
    a) Aluminium
    b) Phosphorus
    c) Nitrogen
    d) Iodine

    Answer. (b) Phosphorus

    Q.5. The metal which plays an important role in the conduction of messages to the nervous system, is
    a) sodium
    b) copper
    c) calcium
    d) zinc
    Answer. (a) sodium
    II. Fill in the blanks by selecting appropriate words from the given list : List : atoms, metallic, solid, compound, Hydrogen

    Q.1. When two or more elements combine chemically, they form …………………….
    Q.2. An element contains only one kind of ……………………..
    Q.3. Metals are generally………………………….. at room temperature.
    Q.4. ………………………..gas bums with a pop sound.
    Q.5. Food items like pickles, curd, tamarind, citrus fruits, etc., are advised not to store in……………………… containers.

    1- compound,
    2- atoms,
    3- solid,
    4- Hydrogen,
    5- metallic

    Match the items of Column A with those of Column B.

    Answer. 1- c, 2- d , 3- b, 4-e , 5- a

    III. Write ‘T’ for the true and ‘F’ for the false statements.

    Q.1. Metalloids possess characteristics of both metals and non-metals.
    Q.2. Gold and silver are the most malleable and ductile metals.
    Q.3. The handles of the utensils are made of plastic, since it is conductor of heat.
    Q.4. In the presence of moisture most of the metals get corroded.
    Q.5. Graphite is a bad conductor of electricity.
    Answer. 1-T, 2-T, 3-F, 4-T, 5-F,

    V. Very Short Answer Questions.

    Q.1. Name the metal which is liquid at room temperature.
    Answer: Mercury

    Q.2. Name the reaction in which a more reactive metal displaces a less reactive metal from its salt solution.
    Answer: Displacement reactions
    Q.3. Name the metal, which is a constituent of haemoglobin.
    Answer: Iron

    Q.4. Write the metals which are constituents of enzymes.
    Answer: Copper and zinc
    Q.5. Write the metals whose compounds are constituents of cells of the plants.
    Answer: Calcium and magnesium
    VI. Short Answer Questions.

    Q.1. Why do some metals lose their lustre when exposed to the atmosphere?

    Answer: The lustre of some metals are Lost when exposed to the atmosphere. This is because the metals react with air and moisture present in the atmosphere to form a thin coating of an oxide layer on its surface.

    Q.2. Why are potassium and sodium stored in kerosene?

    Answer: Potassium and sodium are so reactive that they catch fire immediately when brought in contact with air. They form their respective oxides. That is why, these metals are stored under kerosene.

    Q.3. Why is reaction between lead and concentrated hydrochloric acid stopped after some time?

    Answer: Lead combines with concentrated hydrochloric acid to form Lead chloride and hydrogen. But the layer of lead chloride formed on the metal is hard to penetrate. This slows down and finally stops the reaction.

    Q.4. What is a metal? Discuss some of its physical properties.
    Answer: The elements which possess the characteristic property of lustre, hardness, sonority, malleability, ductility and conductivity for heat and electricity are called metals

    • Physical State: Metals are generally solids at room temperature.
    • Lustre: Metals have a shiny appearance.
    • Most of the metals are hard and strong, except sodium and potassium which are soft.
    • Metals are sonorous i.e., they produce a ringing sound when struck.
    • Malleability: Metals can be beaten into thin sheets.
    • Ductility: Metals can also be drawn into thin wires.
    • Conductivity: Metals are good conductors of heat and electricity.

    Q.5. What is a non-metal? Discuss some of its physical properties.
    Answer: The elements which are dull, soft, non-sonorous, non-malleable, non-ductile and non-conductor of heat and electricity are known as non-metals .

    • Non-metals occur in all the three states-solids, Liquids and gases at room temperature.
    • Non-metals have a dull appearance. Graphite, diamond and iodine are the only non-metals which have lustre.
    • Non-metals which are solid, are not as hard as the metals. In fact they are soft and brittle. They break into pieces on applying pressure.
    • Non-metals like carbon (as in wood), oxygen, nitrogen are non-conductors of heat.
    • Non-metals are either bad conductors or non-conductors of electricity. Graphite (carbon) which is a good conductor of electricity is an exception.

    Q.6. How do metals and non-metals react with oxygen? Mention some of the reactions for them.
    Answer: Reaction of metals with oxygen.

    Metals react with oxygen (of air] under different conditions to produce metal oxides. Some of these reactions are vigorous while some are very slow.
    Potassium and sodium are so reactive that they catch fire immediately when brought in contact with air. They form their respective oxides.

    Magnesium, aluminium, iron and copper react slowly with air (and moisture). A thin layer of oxide is formed on their surface. When burnt in air, magnesium burns with a dazzling white light and forms an ash like powder.

    $ 2Mg + O_{2} \rightarrow 2MgO$

    Non-metals react with oxygen to form acidic or neutral oxides.

    $ S + O_{2} \rightarrow SO2 $

    $ C + O_{2} \rightarrow CO2 $

    VII. Long Answer Questions.

    Q.1. How do metals react with water? Describe reaction between sodium and water through an activity. Also mention the reactions of potassium, calcium and magnesium with water separately.

    Answer. Metals lying above hydrogen in the activity series displace hydrogen from water to form oxides and hydroxides of the metals and hydrogen gas.

    Aim: To observe the reaction between sodium and water.
    Requirements : A small piece of sodium metal, a trough of water, a pair of tongs, knife, filter paper, red and blue litmus paper.
    Procedure: Cut a small piece of sodium kept in kerosene with a knife. Remove it with a pair of tongs and dry it between the folds of a filter paper. Drop the metal into the trough of water. Observation: A reaction takes place instantly. Sodium melts to a silvery globule which darts about the surface of water with a hissing sound. Bubbles of a gas are produced around it. The gas pushes the metal against the side of the trough where it catches fire and burns with a golden yellow flame. We can also hear pop sounds off and on, until the metal piece disappears. The water in the trough if tested with red litmus which changes to a blue colour. The water in the trough feels hot to touch.

    Explanation and Conclusion: The heat produced in the reaction is so large extent that sodium melts and starts burning with a golden yellow flame. The hydrogen produced also catches fire and burns with a pop sound. The reaction between sodium and water produces sodium hydroxide, an alkali, which turns red litmus blue.

    Sodium [2Na] + Water $[H_{2}O] \rightarrow $ Sodium hydroxide [2NaOH] + Hydrogen [$H_{2}$]

    Potassium also floats on water and bursts into a flame immediately. The flame is lilac in colour.
    Hydrogen bubbles are produced around it and a clear solution of potassium hydroxide is formed.
    Potassium [2K] + Water [$2H_{2}O]\rightarrow $ Potassium hydroxide [2KOH] + Hydrogen [$H_{2}$]

    The reaction of calcium and water is less violent and the hydrogen liberated does not catch fire.
    Calcium [Ca] + Water [$2H_{2}O]\rightarrow$ Calcium hydroxide $[Ca(OH)_{2}] + Hydrogen [H_{2}]$

    Magnesium does not react with cold water. It reacts with boiling water or steam to form magnesium oxide and hydrogen. The metal floats on the surface of water due to the bubbles of hydrogen gas formed which keeps sticking to its surface. A burning piece of magnesium continues to burn when brought in contact with steam.
    Magnesium [Mg] + Boiling water [$H_{2}O] \rightarrow Magnesium oxide [MgO] + Hydrogen [$H_{2}$]

    Q.2. How do metals and non-metals react with acids? Describe the rate of reaction of a dilute acid with different metals through an activity.

    Answer. Metals react with dilute hydrochloric acid or dilute sulphuric acid to form the corresponding salt and hydrogen gas. The metals which lie above hydrogen in the activity series displace hydrogen from the acids but those lying below hydrogen do not.
    The metals like sodium, potassium and calcium react violently with dilute acids. The reactivity of the metals decreases as we move down the activity series.

    Non-metals do not react with dilute acids. However hot concentrated nitric acid oxidises the non-metal like sulphur, carbon, etc., to their oxides along with the liberation of nitrogen dioxide gas.

    Aim: To find the rate of reaction of a dilute acid with different metals.

    Requirements: Six test tubes; a test tube stand; very thin pieces of metals like magnesium, aluminium, zinc, iron, lead and copper, sand paper; dilute hydrochloric acid and a matchbox. Procedure: Place the test tubes in the stand and fill one-fourth of each with dilute hydrochloric acid. If the metal samples are tarnished, clean them by rubbing with sand paper. Drop the piece of magnesium in the test tubes containing dilute hydrochloric acid. Bring a matchstick near the mouth of the test tube as soon as the metal piece is dropped in it. Heat the test tube and note the change in the rate of the reaction. Repeat the experiment with other metal pieces.

    Observation: Bubbles of gas are seen to escape through the liquid. The reaction is vigorous in case of magnesium and it decreases in the order: aluminium > zinc > iron. The test tube feels hot to touch. In case of lead and copper no bubbles are seen to escape and the temperature of the test tube remains the same.
    When the test tube is heated the rate of escaping of bubbles becomes faster. But with lead and copper there is still no change. When a lighted matchstick is brought near the mouth of the test tube! except in case of lead and copper) we observe that the gas burns with a pop sound.
    Explanation and Conclusion: The reaction of the metals with acids decreases in the order : Magnesium > aluminium > zinc > iron.

    This explains why the rate of formation of the bubbles is the fastest in case of magnesium and least in the case of iron. The gas formed in the reaction is hydrogen which burns with a pop sound when a lighted matchstick is brought near the mouth of the test tube. The test tube feels hot to touch since the reaction is exothermic i.e., it releases a lot of heat. Lead and copper do not react with dilute hydrochloric acid even on heating.

    Q.3. (a) What do you mean by displacement reaction? Describe them with some examples.
    (b) How do metals react with bases? Describe with an example.
    (c) Discuss uses of some common metals and non-metals.
    Ans a) Displacement reactions are those reactions, in which a more reactive metal displaces a less reactive metal from its salt solution. For example, magnesium, aluminium and iron can displace copper from copper sulphate solution. This is because these metals are more reactive than copper.
    $ Mg + CuSO_{4} \rightarrow MgSO_{4} + Cu $
    If a copper strip is placed in magnesium sulphate or aluminium sulphate or iron sulphate solution no reaction takes place. This is because copper being less reactive than magnesium or aluminium or iron cannot displace these metals from their salt solution.

    $ Cu + MgSO_{4} \rightarrow No Reaction $

    b) Only the metals like aluminium, zinc and lead react with the alkalis sodium hydroxide and potassium hydroxide to form salt and hydrogen gas.

    $2Al + 2NaOH + 2H_{2}O \rightarrow 2NaAlO_{2} + 3H_{2}$

    The gas can be tested with a lighted matchstick which burns with a pop sound.
    Freshly prepared sodium hydroxide solution is used for the reaction. It is prepared by dissolving a few pellets of sodium hydroxide in water. The pellets of sodium hydroxide should not be touched with bare hands since they are highly corrosive.

    c) Main uses of some metals
    Iron : For making bridges, automobile parts, machines, utensils, steel and for construction purposes. Copper : For making boilers, electric wires, cables, utensils, alloys, coins, statues, car radiators and calorimeters.
    Aluminium : For making aeroplanes, ships, cars, buses, trains, electric wires, cables, utensils, alloys, foil for wrapping, metallic paints and in thermite process.
    Lead : For making lead pipes, automobile batteries, alloys such as solder, bullets, chemical compounds and paints and protective screen for X-ray machines. Mercury : For making amalgams and in thermometers and barometers.
    Zinc : For galvanisation of iron, in making alloys, such as brass, bronze, german silver and in dry cells.
    Chromium and nickel : For electroplating and for making steel.
    Gold : For making jewellery, for electroplating.
    Silver: For making jewellery, in photography, for electroplating and for silver coating of mirror. Platinum : For making jewellery, electrodes, etc.

    Main uses of some non-metals
    Carbon in the form of diamonds : For making jewellery, cutting and grinding equipment. In the form of graphite : For making high temperature crucibles, electrodes, black lead pencils and dry lubricants.
    Sulphur : For the manufacture of sulphuric acid, gun powder, in the vulcanisation of rubber as an antiseptic in making skin ointments and in a number of medicines.
    Oxygen: For respiration by living organisms, for artificial respiration, supports burning and in oxyacytelene flame for cutting and welding metals.
    Nitrogen : For making ammonia, nitric acid and in many fertilizers.
    Phosphorus : For making matchstick, phosphate fertilizers, crackers, rat poison and smoke screens.
    Hydrogen : As oxy-hydrogen flame used for cutting and welding metals in hydrogenation of vegetable oils and fuels for rockets.
    Chlorine : For sterlizing water, for bleaching and manufacture of chlorine compounds. Silicon : For making semiconductors, glass, cement, binder in ceramic industry, alloys in solar cells, used for making greases, polishes, insulating material for electrical appliances and for making silicon carbide used as a grinding tool.
    Iodine : For making tincture iodine used as an antiseptic (purple solution) and in iodised common salt.

    Reviewed By:
    Krishna Kant Majee
    M.Sc., B.Ed.

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