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Solutions for Chemistry, Class 7, ICSE
Give the common name and chemical name of an ore of –
(a) zinc
(b) Aluminium
(c) iron
The common name and chemical name of ores of following elements are:
| S. No. | Metal | Common Name | Chemical Name |
|---|---|---|---|
| (a) | Zinc | Zinc blende | Zinc sulphide [ZnS] |
| (b) | Aluminium | Bauxite | Aluminium oxide [Al2O3.2H2O] |
| (c) | Iron | Haematite | Iron [III] oxide [Fe2O3] |
Distinguish between the physical properties of metals and non-metals with reference to —
(a) lustre
(b) malleability
(c) ductility
(d) tensile strength
(e) sonority
(f) conduction of heat and electricity
(g) melting and boiling point
(h) density
| S. No. | Properties | Metals | Non-metals |
|---|---|---|---|
| (a) | Lustre | Shows brightness | Shows no brightness |
| (b) | Malleability | Can be beaten into sheets | Cannot be beaten into sheets. |
| (c) | Ductility | Can be drawn into wires | Cannot be drawn into wires. |
| (d) | Tensile strength | Have high tensile strength | Have low tensile strength |
| (e) | Sonority | Produce sound when struck | Do not produce sound when struck |
| (f) | Conduction of heat and electricity | Can conduct heat and electricity | Poor or non-conductors of heat and electricity |
| (g) | Melting and boiling point | Have high melting and boiling point | Have low melting and boiling point |
| (h) | Density | Have high density | Have low density |
With reference to the physical properties of metals and non-metals, state the following exceptions.
(a) a metal which is liquid at room temperature
(b) a non-metal which sublimes and is lustrous
(c) a non-metal which has high tensile strength
(d) a non-metal which conducts electricity
(e) a metal which floats on water
(a) Mercury
(b) Iodine
(c) Carbon fibre
(d) Graphite
(e) Potassium
With reference to the following metals state their – use in daily life
Metals:
(a) copper
(b) iron
(c) aluminium
(d) magnesium
(e) zinc
(f) lead
(g) silver
(a) Copper is used in :
(b) Iron is used in :
(c) Aluminium is used in :
(d) Magnesium is used in :
(e) Zinc is used in :
(f) Lead is used in:
(e) Silver is used in :
Describe a simple experiment to prove that – a copper wire conducts electricity, but a piece of coal does not.
Experiment to prove that a copper wire conducts electricity.
Procedure — Join a wire with the battery, copper and bulb
Observation — The bulb glows when current passes through the circuit.
Conclusion — Copper conducts electricity, hence it is a good conductor of electricity.
Experiment to prove that coal does not conduct electricity.
Procedure — Join a wire with battery, coal and bulb
Observation — The bulb does not glow.
Conclusion — Coal does not conduct electricity hence, it is a bad conductor of electricity.
Differentiate between metals and non-metals with reference to –
(a) number of electrons in outer or valence shell
(b) formation of cation and anion
(c) reaction with dilute acids
| S. No. | Chemical Property | Metals | Non-metals |
|---|---|---|---|
| (a) | No.of electrons in outer shell | Metals have 1, 2 or 3 electrons in the outer or valence shell. | Non-metals have 4, 5, 6 or 7 electrons in the outer or valence shell. |
| (b) | Formation of cation and anion | Metals lose or donate valence electrons and form cations. | Non-metals accept valence electrons and form anions. |
| (c) | Reaction with acids | Metals react with dilute acids to give hydrogen gas. | Non-metals do not react with dilute acids to give hydrogen gas. |
Give a reason why – properties of metals are related to their activity or reactivity series of metals. Explain the same with reference to potassium and iron in the series.
Activity series of metals is a series of arrangement of metals in order of their reactivity. The most active metal is at the top of the series & the least active metal is at the bottom of the series.
Potassium is at the top of the series, hence it is the most reactive metal. It reacts vigorously even with cold water and explosively with acids to liberate hydrogen gas.
Iron lies in the middle of the series at number 7, hence it is moderately reactive. It does not react with cold water but reacts smoothly with steam and less vigorously with acids to liberate hydrogen gas.
Give the basic demarcation of the Modern Periodic Table into metals, metalloid and non-metals with special reference to halogens and noble gases.
In the Modern Periodic Table:
Give a reason why the non-metals – halogens and noble gases are placed in separate groups of the periodic table with special reference to their properties and uses.
Reasons for keeping halogens and noble gases in separate groups of the periodic table with special reference to their properties and uses are:
State what are metalloids. Name the elements recognized as metalloids. Compare the properties of metalloids with those of metals. State the uses of three different metalloids.
Metalloids are elements that show the properties of both metals and non-metals. They have physical and chemical properties, intermediate between those of metals and non-metals.
Elements like Boron [B], Silicon [Si], Germanium [Ge], Arsenic [As], Antimony [Sb], Tellurium [Te] are recognized as metalloids.
Comparison of properties:
| Property | Metal | Metalloids |
|---|---|---|
| State | Solids at room temperature | Solids at room temperature |
| Nature | Generally, hard in nature. | Brittle in nature. |
| Conductivity | Good conductors of heat and electricity. | Semi conductors of electricity and show average transmission of heat. |
| Lustre | Show brightness | Show brightness |
| Malleability & Ductility | Malleable and Ductile | Generally malleable and Ductile |
Use of metalloids:
Explain the term – rusting and give a word equation for the formation of rust. If polished iron nails are kept in three separate test tubes, state the contents in each test tube required, to prove the conditions for rusting.
Rusting involves slow oxidation of iron in presence of air and moisture resulting in formation of hydrated iron [III] oxide called rust.
Iron + Oxygen ⟶ Iron [III] Oxide Hydrated Iron [III] Oxide [rust]
| Test Tube A | Test Tube B | Test Tube C | |
|---|---|---|---|
| Contents | Anhydrous calcium chloride | Boiled tap water with oil layer on top | Ordinary tap water |
| Condition present | Oxygen | Moisture | Moisture & oxygen |
| Condition absent | Moisture | Oxygen | Nil |
| Result | Iron nails do not rust | Iron nails do not rust | Iron nails rust |
| Conclusion | Moisture is essential for rusting | Oxygen is essential for rusting | Moisture and oxygen both are essential for rusting |
State why telegraph poles are painted with Aluminium paint.
Aluminium paint is anti-corrosive in nature hence it does not rust and is used to paint telegraph poles which are exposed to air and moisture.
Give a reason why oiling or greasing is done on the exposed moving iron parts of machinery and not preferred on heavy stationary iron parts.
Oiling or greasing is done on the exposed moving iron parts of machinery as it forms a coating which protects it from rusting. It smoothens the surface and reduces friction in the machinery which is its added advantage.
Being stationary, for heavy iron parts, red lead oxide paint is used as there is no use of reducing the friction.
State why galvanization, tinning and chromeplating are used as three different processes for coating specific articles for prevention of rusting.
Galvanization, tinning and chromeplating are used as three different processes because:
In galvanization, iron sheets are dipped in molten zinc. Zinc being more active than iron, protects iron by forming zinc ions more readily. It is used for iron articles exposed to heavy moisture like roof sheds, buckets, tubs, etc.
In tinning, iron sheets are dipped in molten tin. It is used for edible applications instead of galvanization because zinc dissolves in food acids and makes poisonous compounds.
In chromeplating, iron articles are electroplated with chromium, nickle, tin, copper etc. It is used for decorative purposes. It is costly and used exclusively.
Hence, the three processes are different ways of preventing articles from rusting depending on their use.
Give a reason why –
(a) rust turns moist red litmus blue
(b) a green deposit is seen on the surface of an article of copper
(c) iron objects rust easily but the iron pillar at Qutab Minar has not rusted over the years.
(a) Rust is hydrated iron [III] oxide. As all metallic oxides are basic in nature and rust is a metallic oxide of iron, hence, it turns moist red litmus blue.
(b) The green deposit on copper articles is caused by copper reacting with moisture and atmospheric gases to form copper carbonate [CuCO3], which is green, and copper hydroxide [Cu(OH)2].
(c) Iron pillar of Qutab Minar has not rusted over the years because
State a specific reason for using –
(a) steel in automobiles
(b) gold in jewellery
(c) copper in electrical cables
(d) zinc in galvanizing
(e) lead in bullets
(f) magnesium in fireworks.
(a) Steel in automobiles — It is malleable with high tensile strength.
(b) Gold in jewellery — It is expensive, lustrous, malleable and ductile.
(c) Copper in electrical cables — It is a good conductor of electricity.
(d) Zinc in galvanizing — It prevents rusting.
(e) Lead in bullets — It has high specific gravity.
(f) Magnesium in fireworks — It burns with a dazzling light.
Complete the statements given below, by filling in the blank with the correct word/s:
The ability of a metal to produce a resonant sound ............... [ductility/sonority/malleability].
A metal which floats on water ............... [zinc/potassium/magnesium].
A non-metal that leaves a mark on paper ............... [phosphorous/graphite/sulphur].
A non-metal which allows an electric current to pass through it ............... [graphite/sulphur/iodine].
A non-metal added to swimming pools to kill germs ............... [nitrogen/chlorine/oxygen].
The ability of a metal to produce a resonant sound sonority.
A metal which floats on water potassium.
A non-metal that leaves a mark on paper graphite.
A non-metal which allows an electric current to pass through it graphite.
A non-metal added to swimming pools to kill germs chlorine.
Give reasons for the following:
Metals form cations while non-metals form anions.
Potassium reacts vigorously with cold water while copper does not react with water or steam.
Rusting reduces the structural strength of iron.
A suspension of rust is basic in nature.
Galvanized steel is not used for articles used for edible applications.
Metals have 1, 2 or 3 valence electrons and tend to lose one or more electrons when supplied with energy and form cations whereas non-metals have 5, 6 or 7 electrons in the valence shells and tend to gain electrons to form anions.
Potassium is at the top of the metal activity series making it the most reactive metal. Its high reactivity is seen in its vigorous reaction with cold water. On the other hand, copper lies towards the bottom of the series at number 10, below hydrogen. Hence, it cannot displace hydrogen from water and therefore does not react with water or steam.
Rusting corrodes iron thereby reducing the structural strength of iron.
Rust is hydrated iron [III] oxide. As all metallic oxides are basic in nature and a suspension of rust is a metallic oxide of iron, hence, it is basic in nature.
Galvanized steel has zinc coating and so it is not used for food containers since food acids dissolve zinc forming poisonous compounds.
Name the following:
The most reactive halogen.
A metal used in packaging.
The chemical compound responsible for the green deposit on the surface of articles of copper.
A metal used in flexible pipes.
A halogen which is solid at room temperature.
Fluorine.
Aluminium.
Copper carbonate
Lead
Iodine
Match the correct answer from A, B, C, D and E – for each statement given below:
A : Boron, B : Chlorine, C : Potassium, D : Platinum, E : Radon
The element placed in the extreme left of the Modern Periodic Table.
The element which shows properties of both metals and non-metals.
The element from the halogen family which is most abundant.
The element which is noble gas and radioactive in nature.
The element which is a noble metal.
C: Potassium
A: Boron
B: Chlorine
E: Radon
D: Platinum
State whether the statements given below are true or false. If false write the correct statement.
Lead pencils contain lead.
The chemical name of rust is iron [III] oxide.
Silicon is an example of a metalloid.
Either oxygen or moisture is essential for rusting.
Graphite is a lustrous non-metal which conducts electricity.
False
Corrected Statement — Lead pencils contain graphite.
True
True
False
Corrected Statement — (Both oxygen and moisture are essential for rusting).
True