chapter 2 chemistry class 6

I. Select the correct option

Which is the symbol of mercury?
(b) Hg

Which of the following is a metal?
(c) Sodium

Which of the following is the most unreactive metal?

Which of the following is a soft metal?
(a) Sodium

Which of the following is a molecule of compound?
(c) Water

The positively charged particles in an atom are called
(c) protons

Which of the following non-metals is found in liquid state?
(c) Bromine

II. Assertion and Reasoning

1. The structure of atom was discovered by the British Chemist "J.J. Thomson".

Assertion is False, Reason is also False (protons are positively charged, electrons are negative, neutrons are neutral; assertion wrongly credits Thomson; he discovered the electron, not entire atomic structure):
(b) Both Assertion (A) and Reason (R) are false

2. Alchemists used pictorial symbols to represent different elements.

Assertion is True, Reason is False (because the symbols Ⓒ and ⓘ are not correct for copper and iron):
(d) Assertion (A) is true but Reason (R) is false

3. Metals are generally hard solid at room temperature except sodium.

Both are true:
(a) Both Assertion (A) and Reason (R) are true

4. Metalloids are elements that do not react chemically and are stable.

Assertion is False, Reason is True (Reason: boron, silicon, etc. are examples of metalloids, but assertion is not correct):
(c) Assertion (A) is false but Reason (R) is true

5. Aerated water is a mixture of gases and water, and is heterogeneous.

Assertion is true, Reason is false (aerated water is a homogeneous mixture):
(d) Assertion (A) is true but Reason (R) is false

III. State if the statements are True or False. Correct the false statement.

A molecule is the smallest particle of an element.
False. Correction: An atom is the smallest particle of an element.

A pure substance consists of particles of only one kind.
True.

Mercury is a non-metal.
False. Correction: Mercury is a metal.

Water is a compound made up of hydrogen and oxygen gas.
True.

Constituents of a mixture are present in a fixed proportion.
False. Correction: Constituents of a mixture are not present in a fixed proportion.

In a homogeneous mixture, constituents are uniformly mixed throughout the mixture.
True.

Air is a heterogeneous mixture.
False. Correction: Air is a homogeneous mixture.

IV. Fill in the blanks

Substances which can be beaten into sheets are called malleable.

Metals are good conductors of heat and electricity.

Elements cannot be broken down into two or more simpler substances by chemical methods.

The smallest particle of an element or a compound that can exist on its own is called a molecule.

Atoms join in whole numbers to form molecules.

The molecules of a compound contain atoms of the different kind.

Chemical symbols represent one atom of an element.

V. Match the following

Column A	Column B	Answer
1. Smallest unit of matter which may or may not have an independent existence	(d) Atom	1 → d
2. A non-metal that conducts heat and electricity	(e) Graphite	2 → e
3. Molecule	(a) Smallest unit of a compound	3 → a
4. A noble gas used for filling weather observation balloons	(b) Helium	4 → b
5. A liquid metal at room temperature	(c) Mercury	5 → c
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VI. Complete the following tables

1. Formula of compounds and their elements

Formula	Elements present in the compound
CO₂	Carbon and Oxygen
CaO	Calcium and Oxygen
PbO	Lead and Oxygen
PbS	Lead and Sulphur
CuS	Copper and Sulphur
FeS	Iron and Sulphur
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2. Complete the compound-elements table

Compound	Formula	Number of elements	Names of elements
Copper sulphate	CuSO₄	3	Copper, sulphur, oxygen
Zinc sulphide	ZnS	2	Zinc, sulphur
Ammonia	NH₃	2	Nitrogen, hydrogen
Sulphuric acid	H₂SO₄	3	Hydrogen, sulphur, oxygen
Hydrochloric acid	HCl	2	Hydrogen, chlorine
Table salt	NaCl	2	Sodium, chlorine
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VII. Answer in one word or one sentence

What is the atomicity of an ozone molecule?
Three

What does a chemical formula represent?
It represents one molecule of an element or a compound.

Are metalloids a mixture of metals and non-metals?
No.

How many atoms are there in polyatomic molecules?
More than two.+1

What does nucleus of an atom contain?
Protons and neutrons.

VIII. Define the following terms

Atom:
The smallest particle of an element.

Molecule:
The smallest unit of a compound, capable of independent existence.+1

Heterogeneous mixture:
A mixture in which the constituents are not uniformly distributed.

IX. Answer the following questions in short

What is an element? Name any five elements.
An element is a pure substance which can neither be broken down into simpler substances nor formed from two or more simpler substances.
Five elements: Iron, gold, copper, hydrogen, oxygen.

Name any two:
(a) Malleable substances: Gold, silver.

(b) Ductile substances

Gold

Silver

Copper

Platinum

3. What are non-metals? Name any four non-metals.

Non-metals are substances which do not have lustre, cannot be beaten into sheets, cannot be drawn into wires, have low melting/boiling points, and are generally bad conductors of heat and electricity.
Examples: Hydrogen, Oxygen, Sulphur, Nitrogen

4. What are compounds? Name any four compounds.

A compound is a chemical combination of two or more elements. In a compound, the properties of elements change, and a molecule is the smallest unit of a compound.
Examples: Water (H₂O), Carbon dioxide (CO₂), Sodium chloride (NaCl), Ammonia (NH₃)

5. Name the following:

(a) Two elements whose names start with 'C':

Carbon

Copper
(b) Two elements whose symbols have the first letter 'A':

Aluminium (Al)

Argon (Ar)

6. What is meant by valency? Write the valency of Na, Mg, Ca and Al.

Valency is the combining capacity of an element.

Na: 1

Mg: 2

Ca: 2

Al: 3

7. Write down the symbols for each of the following elements:

(a) Iron: Fe
(b) Gold: Au
(c) Sodium: Na
(d) Oxygen: O
(e) Carbon: C
(f) Sulphur: S
(g) Silver: Ag
(h) Calcium: Ca

8. Name the elements having the following symbols:

(a) Al: Aluminium
(b) Cu: Copper
(c) Mg: Magnesium
(d) N: Nitrogen
(e) Hg: Mercury
(f) P: Phosphorus
(g) Zn: Zinc
(h) Pb: Lead

9. Write down the molecular formula for each of these molecules:

(a) Oxygen: O₂
(b) Chlorine: Cl₂
(c) Nitrogen: N₂
(d) Hydrogen: H₂

10. Write down the formula for the following compounds:

(a) Zinc chloride: ZnCl₂
(b) Carbon dioxide: CO₂
(c) Potassium hydroxide: KOH
(d) Zinc sulphide: ZnS
(e) Magnesium oxide: MgO
(f) Dinitrogen oxide: N₂O

11. Name the following chemical compounds:

(a) ZnO: Zinc oxide
(b) CuO: Copper(II) oxide
(c) CaS: Calcium sulphide
(d) FeS: Iron sulphide

12. What do you understand by a pure substance? Is milk a pure substance?

A pure substance is a kind of matter that cannot be separated into components by any physical means and consists of particles of only one kind (element or compound). Milk is not a pure substance; it is a mixture containing fats, vitamins, carbohydrates, proteins, salt, and water.

Detailed Answers

X. 1. What are metals? State their four properties. Also name any four metals.

Metals are substances that usually have lustrous (shiny) surfaces, are hard solids at room temperature (except mercury), are malleable, ductile, have high melting and boiling points, and are good conductors of heat and electricity.

Four properties:

Lustrous surface

Malleable (can be beaten into sheets)

Ductile (can be drawn into wires)

Good conductors of heat and electricity

Four metals: Gold, Silver, Zinc, Iron

2. Draw the structure of an atom and explain it.

An atom consists of a nucleus (containing protons and neutrons) and electrons revolving around the nucleus in shells or orbits. The nucleus is centrally located, electrons are negative, protons are positive, and neutrons are neutral.

3. Give at least four differences between metals and non-metals.

Metals	Non-metals
Shining (lustrous) surface	Dull surface (except graphite and iodine)
High melting & boiling points	Low melting & boiling points
Malleable and ductile	Brittle, non-malleable, non-ductile
Good conductors of heat & electricity	Bad conductors (except graphite)

4. Define atomicity. Write the atomicity of the following: Br₂, S₈, P₄, O₃.

Atomicity is the number of atoms in one molecule of an element.

Br₂: 2

S₈: 8

P₄: 4

O₃: 3

5. Differentiate between a mixture and a compound.

Mixture	Compound
Components mixed mechanically	Components combined chemically
Components in any proportion	Components in fixed ratio
Retains properties of constituents	Properties differ from constituents
Can be separated by physical processes	Cannot be separated by physical processes

6. Can a mixture be a solid, liquid or gas? Give one example of each.

Solid: Soil (mixture of sand, clay etc.)

Liquid: Milk (mixture of water, proteins, fat etc.)

Gas: Air (mixture of oxygen, nitrogen etc.)

HOTS and Critical Thinking

1. Pale blue (gas), deep blue (liquid), sharp odour, causes headache in humans.

Substance: Ozone

Chemical formula: O₃

Atomicity: 3

2. Correct symbol for sodium

The students said "So" for sodium. That is incorrect, the correct symbol is "Na", from its Latin name 'Natrium'.

3. Classify substances in Anamika's life:

Substance	Category	Reason
Black coffee	Mixture	Contains water, caffeine, other substances
Water	Compound	Chemical formula is H₂O
Gold earring	Element	Pure gold, one kind of atom
Sea water	Mixture	Contains water, dissolved salts, other substances

Formula Table (for Rashmi)

Compound	Formula	No. of elements	Name of elements
Sulphuric acid	H₂SO₄	3	Hydrogen, sulphur, oxygen
Table salt	NaCl	2	Sodium, chlorine
Zinc sulphate	ZnSO₄	3	Zinc, sulphur, oxygen
Ammonia	NH₃	2	Nitrogen, hydrogen
Nitric acid	HNO₃	3	Hydrogen, nitrogen, oxygen
Copper sulphate	CuSO₄	3	Copper, sulphur, oxygen

Diagram-based Mixture State

(a) Tree/plant: Heterogeneous mixture (air, particles)
(b) Beaker with layers: Heterogeneous mixture
(c) Uniformly mixed liquid: Homogeneous mixture
(d) Cloud: Heterogeneous mixture
(e) Metal objects: Element

Case-based/Passage-based Questions on Dalton

(a) Characteristics of atoms:

Atoms are very reactive and usually do not exist freely

Atoms of one element are alike, and differ in properties (shape, size, mass) from atoms of other elements

An atom maintains its properties during physical and chemical changes

Here are the answers to all the questions on the provided page, using only content from the visible text and textbook.

(b) Label the parts of the structure of atom

Referring to the diagram:

Nucleus

Proton

Neutron

Electron

Shell/Orbit

Iron, gold, and silver are metals. Metals have atoms that are closely packed in a regular arrangement, which allows them to vibrate freely and produce a ringing sound when struck. Sulphur and carbon are non-metals; their atomic arrangements do not support such vibrations, so they do not produce a ringing sound when struck.

2. Answer the following questions

(a) State the atomicity of the following elements:

(i) Phosphorous: 4 (Atomicity is 4; a molecule contains 4 atoms)
(ii) Sulphur: 8 (Atomicity is 8; a molecule contains 8 atoms)
(iii) Sodium: 1 (Atomicity is 1; exists as single atom)

(b) Give one example of each type of mixture:

(i) Gas in gas: Air (mixture of oxygen, nitrogen, carbon dioxide, etc.)
(ii) Solid in gas: Smoke (solid particles dispersed in air)

(i) Gold: Au
(ii) Oxygen: O