Kerala Syllabus Class 8 Basic Science: Chapter 06 Metals and Non-Metals - Questions and Answers
Std 8: Chemistry: Chapter 06: Metals and Non-Metals - Questions and Answers
♦ Metals
Metals are substances that generally exist in solid state, show high conductivity of heat and electricity, metallic lustre and hardness.
♦ Complete the Table below.
• Generally, metals can be hammered and flattened into thin sheets. This characteristic property of metals is known as malleability.
• Gold is the most malleable metal.
• Silver, Copper, Aluminium, etc., are highly malleable.
♦ Write examples for utilising the malleability of metals.
• Metal sheets are used to make roofing of buildings.
• Aluminium foil is used to wrap food
• Making gold and silver ornaments
♦ Ductility
• Metals can be drawn into thin wires. This property of metals is known as Ductility.
• Gold and Platinum are the most ductile metals.
• Tungsten is used in filament bulbs to make filaments as it is highly ductile.
• Metals like gold and copper can be drawn into very fine wires.
♦ There are a lot of situations where the ductility of metals is utilised. List a few of them.
• Electric wires
• Manufacturing of filament in bulbs
• Production of heating coils
♦ Electrical Conductivity
The ability of a material to conduct electricity through it is called electrical conductivity. All metals are electrical conductors.
♦ Complete the circuit as shown in the figure.
| Object | Bulb glows/does not glow | Conductor of electricity/ not a conductor of electricity |
|---|---|---|
| A folded piece of paper | Does not glow | Not a conductor of electricity |
| A piece of plastic scale/ ruler | Does not glow | Not a conductor of electricity |
| A piece of razor blade | Bulb glows | Conductor of electricity |
| A fine iron wire | Bulb glows | Conductor of electricity |
| Aluminium wire | Bulb glows | Conductor of electricity |
| Glass rod | Does not glow | Not a conductor of electricity |
| Copper wire | Bulb glows | Conductor of electricity |
| Rubber band | Does not glow | Not a conductor of electricity |
• All metals conduct electricity. But all the metals do not conduct electricity to the same extent.
• Silver is the best electrical conductor among the metals.
• The electrical conductivity of the metals such as silver, copper, gold, and aluminium decreases in the following order.
Silver > Copper > Gold > Aluminium
♦ Which metal is used in household electrical wiring?
Copper
♦ Which metal is used to make electric lines for the public distribution of electricity?
Aluminium
♦ Why is silver not used for this purpose?
For making electrical transmission lines, aluminium is used. Copper, silver, etc., are costly metals. So they are not practical for this purpose.
♦ Thermal Conductivity
• The ability of a substance to conduct heat is called thermal conductivity.
• Thermal conductivity is one of the fundamental characteristics of metals.
♦ Metal and Thermal conductors.
• Thermal conductivity of metals is used in cooking vessels.
• Silver is the best conductor of heat among metals.
• Metals such as aluminium and copper have relatively high thermal conductivities.
♦ Aluminium is extensively used for making cooking utensils. Find the reasons.
Aluminium and copper are good heat conductors and comparatively cheaper than silver. So they are extensively used for making cooking utensils.
♦ Sonority
• The ability of metals to produce characteristic sound when striked with a hard material is known as sonority.
• Jingling of anklets or cymbals and sound of bell is due to the sonority of metals.
♦ Metallic Lustre
Newly cut or scrubbed surfaces of metals shine. This shiny appearance of the surface of metals is known as metallic lustre.
♦ Identify the situations in which metallic lustre is made use of?
• For making ornaments
• Making of mirrors
• Making of shiny metal foils for wrapping and packing.
♦ Hardness
• Metals are generally hard. But there are soft metals also.
• Lithium, sodium and potassium are soft metals.
• Iron, copper, aluminium, gold, etc., are generally hard.
♦ High melting point.
• Metals generally have high melting point. That means most of the metals melt only at high temperatures.
• Gallium and caesium are metals having very low melting points. They start melting when placed on our palm. In other words, they exist in the liquid state on warmer days.
♦ Are there metals which exist in a liquid state?
Mercury is a metal which exists in a liquid state.
♦ High boiling points.
Metals generally have high boiling points, and we can't boil them easily.
♦ High density.
• Metals generally have high density. Most of the metals sink in water, i.e., they have higher densities than water.
• Iron, copper, silver, aluminium, gold, etc., are the familiar metals that sink in water.
• Metals like lithium, gallium, caesium, sodium and potassium float in water as they have lower densities than water.
| Metals are substances that generally exist in solid state, show high conductivity of heat and electricity, metallic lustre and hardness. |
|---|
♦ Find the appropriate relationship and choose the correct answer from the brackets, and fill in the blanks.
Electric wires: Electrical conductivity of metals.
Agricultural implements: ………………
(Hardness, sonority, density, metallic lustre)
Answer: Hardness
♦ Find the appropriate relationship and choose the correct answer from the brackets, and fill in the blanks.
Chilanga: Sonority of metals.
Utensils used for cooking: ……......
(Hardness, Sonority, Thermal Conductivity, metallic lustre)
Answer: Thermal Conductivity
♦ Which of the following metals has the highest ductility?
(Silver, Platinum, Iron, Aluminium)
Answer: Platinum
♦ Analyze the statements given below and choose the correct answer.
(i) Metals are malleable.
(ii) All metals are hard.
(iii) Metals are conductors of electricity.
(iv) Ductility of all metals is the same.
Statements (i) and (iii) are true; (ii) and (iv) are false.
Statements (i) and (ii) are true; (iii) and (iv) are false.
Only statement (i) is true.
Statements (i), (ii), (iii) and (iv) are true.
Answer: Statements (i) and (iii) are true; (ii) and (iv) are false.
♦ Analyze the statements given below and choose the correct answer.
Statement (i): Aluminium is used to make cooking utensils.
Statement (ii): Aluminium has high thermal conductivity.
Statements (i) and (ii) are wrong.
Statement (i) is correct (ii) is wrong.
Statement (i) is correct, and statement (ii) is a correct explanation of statement (i).
Statements (i) and (ii) are correct, but statement (ii) is not a correct explanation of statement (i).
Answer: Statement (i) is correct, and statement (ii) is a correct explanation of statement (i).
♦ Which of the following is a soft metal?
(Iron, Sodium, Aluminium, Gold)
Answer: Sodium
♦ Which of the following is a metal stored in kerosene?
(Mercury, Sodium, Calcium, Barium)
Answer: Sodium
♦ The properties of some substances are given below. Analyze them and choose the correct answer.
(i) They are usually electrical conductors.
(ii) They usually have a high boiling point.
(iii) They are usually low in density.
(iv) They are usually hard.
(i), (ii), and (iii) are physical properties of metals.
(i) and (ii), (iii) and (iv) are physical properties of non-metals.
(i) and (ii) are physical properties of metals, but (iii) and (iv) are of non-metals.
(ii) and (iv) are physical properties of metals, but (i) and (iii) are of non-metals.
Answer: (i) and (ii) are physical properties of metals, but (iii) and (iv) are of non-metals.
♦ Match the following and choose the correct answer from the following
| Metal | Use | Charactristic |
|---|---|---|
| A. Aluminium | P. To make Jewellery | S. Hardness |
| B. Gold | Q. To make vehicles | T. Electrical conductivity |
| C. Iron | R. Electric wires | U. Metallic lustre |
b) A-Q-T, B-P-S, C-R-U
c) A-P-S, B-Q-U, C-Q-S
d) A-R-T, B-P-U, C-Q-S
Answer: d) A-R-T, B-P-U, C-Q-S
♦ Analyze the statements given below and choose the correct answer
Statement (A): Sodium is stored in kerosene.
Reason (R): Sodium reacts very quickly with air and water
Statements (A) and (R) are correct but (R) is not the correct explanation of (A).
Statements (A) and (R) are correct. (R) is the correct explanation of (A).
Statement (A) is correct, but (R) is wrong.
Statements (A) and (R) are wrong.
Answer: Statements (A) and (R) are correct. (R) is the correct explanation of (A).
♦ Which of the following metals has the lowest melting point?
(Sodium, Gallium, Calcium, Magnesium)
Answer: Gallium
♦ Find the appropriate relationship and fill in the blanks.
Haemoglobin: Iron
Bone: ………………
Answer: Calcium
♦ We use metals for various purposes in our daily life. Which property of metals is utilized in the uses given below?
a. Tungsten is used to make filaments.
b. Aluminium is used to make roofing sheets.
Answer:
a. Tungsten - Ductility
b. Aluminium - Malleability
♦ Why are cooking utensils made of metal but their handles are made of plastic?
Cooking utensils are made of metal because metals conduct heat well for cooking, while their handles are made of plastic or wood since these materials are poor conductors of heat and keep the handles safe to touch.
♦ What are the characteristics of non-metals in comparison with metals?
• Generally, non-metals are less hard.
• They do not show sonority.
• They generally do not conduct heat.
• Generally, non-conductors of electricity
• Possess lower densities
• Non lusture
• Generally not malleable and ductile compared to metals.
♦ Reactions of metals with atmospheric air
Most of the metals react with air and its components. Due to this, their lustre fades as keeping in the air.
♦ Burn a piece of magnesium in air. What is left behind?
Magnesium reacts with oxygen in the air to form the compound magnesium oxide.
Magnesium + Oxygen → Magnesium oxide
2Mg+ O₂ → 2MgO
♦ Dissolve the magnesium oxide formed, in water taken in a beaker. What is the colour change when a red litmus paper is dipped in it?
• Red litmus turns blue, showing that the solution is basic. The compound magnesium hydroxide produced by the dissolution of magnesium oxide in water is basic.
• Metals react with atmospheric air, usually producing metal oxides. Metal oxides are generally basic in nature.
♦ Do you know how soda water is prepared?
Soda water is prepared by dissolving carbon dioxide in water at high pressure.
CO₂ + H₂O → H₂CO₃
♦ Dip a blue litmus paper in soda water. Notice the change in colour.
• Soda water turns blue litmus to red. This is because CO₂ dissolved in water forms an acidic solution.
• Soda water is acidic in nature. Carbonic acid is present in soda water.
• Oxides of non-metals are generally acidic in nature.
♦ Corrosion of Metals
Metals react with different components of air and form various compounds. This is known as corrosion of metals.
♦ Let us do an experiment
Materials required: Clean and dry test tubes-4, cork-4, rust-free and shiny iron nails -4, anhydrous calcium chloride, table salt, ordinary water, distilled water, boiled distilled water and oil.
Procedure: Take four test tubes. Put a rust-free and shiny iron nail in each of them. Add distilled water in the first test tube, salt solution in the second test tube and distilled water soon after boiling in the third test tube. Iron nails should be fully immersed in these three test tubes. Add oil in the third test tube in such a way that the surface of the water is completely covered by oil. Take anhydrous calcium chloride or quicklime in the fourth test tube. All four test tubes should be closed immediately with a cork after putting the materials in them. Observe the changes after a few days.
Test Tube - 1: Iron nail rusted. (Test tube contains air and water)
Test Tube - 2: Iron nail rusted more. (Test tube contains air, water and salt)
Test Tube - 3: Iron nail not rusted. (Test tube has water but not air. Dissolved air in distilled water is expelled on boiling)
Test Tube - 4: Iron nail not rusted. (There is air in the test tube. But moisture is not present. Moisture is absorbed completely by unhydrous calcium chloride or quiklime.)
Inference:
• Iron rust due to the contact of air and water content in the atmosphere.
• Presence of salts in moisture/water increases the process of rusting.
• Rusting can be prevented by avoiding the contact of iron materials with air (oxygen) and water.
• Iron grills in buildings near the seashore rust quickly due to the presence of salt in the moisture there.
♦ Besides iron, which other metals are used in our daily life that undergo corrosion in the presence of atmospheric air?
• Copper
• Aluminium
♦ Iron window bars in houses close to sea shore corrode faster. Why?
Iron window bars in houses near the seashore corrode more quickly due to the presence of salt in the moisture.
♦ Very reactive metals like sodium and potassium are stored in kerosene. What are the reasons for it?
These reactive metals react with atmospheric air fast and undergo corrosion. They are kept in kerosene to avoid contact with air and water.
♦ Why do iron knives used to cut lemons rust quickly?
Citric acid is present in lemon. The presence of acid increases rusting.
♦ Why are food items having sour taste, such as pickles and curd, not stored in vessels made of aluminium and iron?
Food items which are sour in taste contain organic acids. Metals like iron, aluminium, etc. react with them, and vessel get corroded.
♦ Gold never loses its shine when kept in an atmosphere. Why?
Gold is a metal with the lowest reactivity. So it never reacts with atmospheric air. So its shine lasts.
♦ What are the disadvantages of the corrosion of metals?
• Objects made of metals break due to the deterioration of their strength.
• Electric circuits stop working.
• Results in economic loss.
♦ Write any two ways to prevent corrosion of iron.
• Keeping iron articles (knives, farming equipments etc.) by coating with oil or grease.
• Painting
♦ Why do we use stainless steel utensils instead of iron utensils?
Iron utensils rust easily when exposed to moisture and air. Stainless steel is an alloy of iron. It prevents rusting and corrosion.
♦ Alloy
• Alloys are substances formed when metals are combined with other metals or non-metals. This is done to obtain materials with desirable properties and to resist corrosion.
• A very small amount of non-metals like carbon, phosphorus, etc. are also added to such alloys.
• For example:- stainless steel, brass, bronze
♦ Give examples of metals essential for the human body.
Sodium, potassium, calcium, iron, cobalt, and zinc are examples of such essential metals.
♦ There are various metals present in the human body. Which metal is present in teeth and bones? Which metal is present in the haemoglobin molecule?
• Calcium
• Iron
♦ Let us assess
1. Match the following.
A Name of Metal |
B Characteristic property | C Symbol |
|---|---|---|
| Silver | high ductility | Au |
| Platinum | the highest malleability | Ag |
| Gold | high electrical conductivity | Pt |
2. The names of certain metals are given in the box.
| Calcium, Mercury, Gallium, Copper, Silver |
|---|
a. Reacts with the components of air and forms tarnish on the surface.
b. It is the best conductor of heat.
c. The metal that melts when placed on the palm.
d. It exists in the liquid state in normal temperature.
e. This metal is present in bones and teeth.
Answer:
a. Copper
b. Silver
c. Gallium
d. Mercury
e. Calcium
3. Certain metals are given in the following table. Find any two uses of each and the characteristic property responsible for it.
Metal |
Use | Characteristic property |
|---|---|---|
| Copper | Electric wire, Cooking vessel | High electrical conductivity, High thermal conductivity |
| Gold | Ornaments, Gold plating | Metallic lustre, no corrosion |
| Aluminium | Cooking vessel, Roofing sheets | High thermal conductivity, high ductility, and low corrosion |
| Iron | Body of vehicles, Rods for reinforced concrete | Ductility, Hardness, Tensile strength |
| Silver | Ornaments, Electronic circuit boards | Metallic lustre, low corrosion, High electrical conductivity |
4. Oxides of elements A and B are dissolved in water in two separate beakers.
a) How can you identify the acidic solution and the basic solution?
b) Which of these elements is more likely to be a metal? Why?
Answer:
a. Can be identified using litmus - Acidic solution turns blue litmus into red. Basic solution turns red litmus into blue.
b. Which turns red litmus into blue because metal oxides are generally basic in nature.
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