Correct option: (C) Antoine Lavoisier.

Lavoisier proposed the Law of Conservation of Mass in 1789. According to this law, matter can neither be created nor destroyed during a chemical reaction. Therefore, in a properly closed system, the total mass of reactants equals the total mass of products.

Correct option: (B) Carbon dioxide escapes.

The reaction produces carbon dioxide gas. In an open arrangement, some carbon dioxide leaves the system and is therefore not included in the final weighing. The apparent loss of mass does not violate conservation of mass because the escaped gas still possesses mass.

Correct option: (B) 1 : 8.

The hydrogen-to-oxygen mass ratio in pure water is always 1:8, irrespective of whether the water originally came from a river, borewell, ocean or another source. This illustrates the Law of Constant or Definite Proportions.

Correct option: (C) 71 g.

Doubling the amount of sodium from 23 g to 46 g requires the chlorine mass to double from 35.5 g to 71 g so that the fixed composition remains unchanged.

Correct option: (B).

Dalton proposed that all matter is made up of very small particles called atoms. He also stated that atoms participate in chemical reactions, are not created or destroyed in such reactions and combine in simple whole-number ratios.

Correct option: (B).

A molecule is electrically neutral, consists of more than one atom, can exist independently and exhibits the properties of the substance. H₂, O₂, HCl and H₂O are examples.

Correct option: (B).

Each hydrogen atom has one electron and requires one more to complete its K shell. Each contributes one electron to a shared pair. Therefore H₂ contains a single covalent bond: H—H.

Correct option: (B).

Oxygen has six valence electrons and requires two more for an octet. Two oxygen atoms therefore share two electrons each, producing two shared electron pairs and a double covalent bond, O=O.

Correct option: (C) N≡N.

Each nitrogen atom requires three additional electrons to complete its octet. Two nitrogen atoms share three pairs of electrons. Therefore a triple covalent bond is formed.

Correct option: (B).

Neon has atomic number 10 and electronic configuration 2,8. Its valence shell is already complete, so it normally has no tendency to gain, lose or share electrons merely to attain an octet.

Correct option: (C) dinitrogen tetroxide.

The prefix di- indicates two nitrogen atoms. Four oxygen atoms are indicated by tetra-, but before “oxide” the final vowel is dropped, producing tetroxide, not tetraoxide.

Correct option: (C).

The first element retains its normal name. The second element is changed to fluoride and the prefix hexa- shows that six fluorine atoms are present. “Mono-” is normally omitted for the first element.

Correct option: (B) 10.

A sodium atom forms Na⁺ by losing one electron. The number of protons remains 11 while electrons decrease from 11 to 10, giving a net charge of +1.

Correct option: (C) 18 electrons.

Neutral chlorine has 17 electrons. It gains one electron to complete its octet, forming Cl⁻. It then contains 17 protons and 18 electrons.

Correct option: (C) 6.

Sodium chloride does not normally exist as separate NaCl molecules. The ions form a repeating three-dimensional crystal structure in which each sodium ion is surrounded by six chloride ions, and vice versa.

Correct option: (D) S²⁻.

Sulfur requires two additional electrons to complete its octet. On gaining two electrons, it acquires two units of negative charge and forms the sulfide ion S²⁻.

Correct option: (C) nitrate, NO₃⁻.

A polyatomic ion contains two or more atoms acting together as a charged unit. Nitrate contains one nitrogen and three oxygen atoms and carries a net charge of −1.

Correct option: (B) MgCl₂.

Magnesium forms Mg²⁺ while chlorine forms Cl⁻. Two chloride ions are required to balance one magnesium ion:

Correct option: (C).

Aluminium forms Al³⁺ and oxygen forms O²⁻. The smallest charge-balanced combination contains two aluminium ions (+6) and three oxide ions (−6), hence Al₂O₃.

Correct option: (B).

OH⁻ is a polyatomic ion. Mg²⁺ requires two hydroxide ions for charge balance. The subscript 2 must apply to the entire OH group, so brackets are necessary: Mg(OH)₂.

Correct option: (D).

Al³⁺ and SO₄²⁻ must combine so that total positive and negative charges are equal. The LCM of 3 and 2 is 6, requiring two aluminium ions and three sulfate ions.

Correct option: (B).

NaCl contains ions even in the solid state, but they are strongly held at fixed positions in its crystal lattice. Since charged particles cannot move freely through the solid, electric current cannot be carried through it.

Correct option: (B).

When an ionic compound dissolves in water, its ions become free to move through the solution. These mobile charged particles carry electric current.

Correct option: (B).

Sugar is a covalent substance. Its particles may disperse in water, but it does not produce the freely moving ions required for conduction. Solubility and electrical conductivity are therefore not the same property.

Correct option: (B).

Oppositely charged ions in an ionic crystal experience strong electrostatic attractions. Considerable energy is required to overcome these forces, so ionic compounds generally have high melting and boiling points.

Correct option: (C) 63 u.

Correct option: (C) 16 u.

Correct option: (C) 58 u.

Correct option: (B) Na⁺.

Eleven protons identify the element as sodium because atomic number = number of protons = 11. The species has one fewer electron than protons, so its charge is +1.

Correct option: (B) A₂B, ionic.

A has one valence electron and tends to lose it, forming A⁺. B has six valence electrons and tends to gain two, forming B²⁻. Two A⁺ ions are therefore required for every B²⁻ ion.

1. The reaction produces carbon dioxide gas.
2. In the open flask, CO₂ escapes and is not included in the final mass. In the closed arrangement, the balloon traps the gas, so the mass of the complete system remains constant.

Thus, the observations support the Law of Conservation of Mass.

In a compound, constituent elements are chemically combined in a definite fixed ratio by mass. A mixture is formed by physically combining substances, and its components may generally be present in variable proportions. Therefore the definite-proportion law is characteristic of compounds, not mixtures.

• Conservation of mass is explained by atoms being rearranged rather than created or destroyed during a chemical reaction.
• Constant proportions are explained by atoms of elements combining in fixed simple whole-number ratios to form a given compound.

An atom is electrically neutral because it contains equal numbers of protons and electrons. An ion is charged because an atom or group of atoms has lost or gained electrons. Loss produces a cation, while gain produces an anion.

Hydrogen has one valence electron and needs one more for a duplet. Chlorine has seven valence electrons and needs one more for an octet. Each contributes one electron to one shared pair.

Hence HCl contains a single covalent bond.

Each oxygen atom has six valence electrons and requires two more to complete an octet. Each oxygen therefore shares two electrons with the other, producing two shared electron pairs. Two shared pairs constitute a double bond, represented as O=O.

• Cation: Positively charged ion formed by loss of electron(s), e.g. Na⁺.
• Anion: Negatively charged ion formed by gain of electron(s), e.g. Cl⁻.

Sodium, with configuration 2,8,1, loses one electron to form Na⁺. Chlorine, with configuration 2,8,7, accepts this electron to form Cl⁻. Electrostatic attraction between the oppositely charged ions forms an ionic bond.

Sodium chloride forms a continuous three-dimensional lattice of Na⁺ and Cl⁻ ions rather than separate NaCl molecules. NaCl therefore represents the simplest whole-number ratio, 1:1, of sodium and chloride ions and is called a formula unit.

• The first element retains its normal name; the second generally ends in -ide.
• Prefixes such as mono-, di-, tri-, tetra-, penta- and hexa- indicate the number of atoms. Mono- is generally omitted for the first element.

Magnesium forms Mg²⁺ and oxygen forms O²⁻. Direct criss-crossing gives Mg₂O₂, but an ionic formula must represent the simplest whole-number ratio. Dividing both subscripts by 2 gives MgO.

OH⁻ is one complete polyatomic hydroxide ion. Three hydroxide ions are needed to balance Al³⁺. Brackets show that the subscript 3 applies to the entire OH group: Al(OH)₃. AlOH₃ would incorrectly suggest one oxygen and three separate hydrogen atoms.

In a solid ionic crystal, ions occupy fixed positions and cannot move through the lattice. On melting or dissolving in water, the ions become mobile. These moving charged particles carry electric current.

Answer: 74.5 u.

Any two suitable applications include:

• Generation of electricity in nuclear power plants.
• Medicine.
• Scientific research.
• Space exploration.

The chapter also highlights Raja Ramanna’s contribution to India’s nuclear energy programme and its peaceful development.

1. Place dilute HCl in a conical flask and zinc separately in a balloon or another compartment attached to the flask without mixing them initially.
2. Seal the system and record its total initial mass. Allow zinc to react with HCl while ensuring the produced hydrogen gas cannot escape.
3. After the reaction is complete, weigh the entire closed system again. The final mass should equal the initial mass within experimental uncertainty.

This demonstrates that mass is conserved even though a gas is produced.

Since total mass of reactants = total mass of products = 11.3 g, the reaction obeys the Law of Conservation of Mass.

S : O = 2 : 3

The fixed 2:3 ratio illustrates the Law of Constant Proportions.

Yes. X obtains a ratio of 4:1. For Y:

Both ratios reduce to the same value. Therefore both samples have the same composition by mass, supporting the Law of Constant Proportions.

Any three:

• All matter consists of very small particles called atoms.
• Atoms cannot be created or destroyed during a chemical reaction.
• Atoms of a given element are identical in mass and chemical properties according to Dalton’s original theory.
• Atoms of different elements have different masses and chemical properties.
• Atoms combine in simple whole-number ratios to form compounds.
• The relative numbers and kinds of atoms remain constant in a particular compound.

• Nitrogen has atomic number 7 and electronic configuration 2,5.
• Each nitrogen atom needs three more electrons for an octet.
• Two nitrogen atoms share three pairs of electrons, producing a triple covalent bond.

• Electronic configuration of F = 2,7.
• Each fluorine atom needs one electron to complete its octet.
• Two fluorine atoms share one electron each, producing one shared electron pair and therefore a single covalent bond.

• Oxygen has six valence electrons and requires two more.
• Each hydrogen atom has one electron and needs one more for a duplet.
• Oxygen shares one electron pair with each of two hydrogen atoms, forming two O—H single covalent bonds.

Carbon requires four electrons to complete its octet while each oxygen requires two. Carbon therefore shares two pairs of electrons with each of two oxygen atoms.

Thus one carbon atom combines with two oxygen atoms, giving CO₂ with two double covalent bonds.

• H valency 1, S valency 2 → H₂S.
• N valency 3, H valency 1 → NH₃.
• C valency 4, Cl valency 1 → CCl₄.

The valencies are crossed over as subscripts and reduced where required.

• CO – carbon monoxide
• CO₂ – carbon dioxide
• CS₂ – carbon disulfide
• PCl₃ – phosphorus trichloride
• SF₆ – sulfur hexafluoride
• N₂O₅ – dinitrogen pentoxide

Calcium loses two valence electrons to form Ca²⁺. Each chlorine atom accepts one electron to form Cl⁻. Therefore two chloride ions are necessary to balance the +2 charge of one calcium ion.

• Potassium loses one electron → K⁺. It combines 1:1 with Cl⁻ → KCl.
• Calcium loses two electrons → Ca²⁺. It requires two Cl⁻ ions → CaCl₂.

Each sodium atom loses one electron and forms Na⁺. Sulfur needs two electrons and forms S²⁻. Therefore two sodium atoms provide the two electrons required by one sulfur atom.

• Fe³⁺ + OH⁻ → Fe(OH)₃
• K⁺ + CO₃²⁻ → K₂CO₃
• Al³⁺ + SO₄²⁻ → Al₂(SO₄)₃

In each case, the total positive and negative charges in the compound are equal.

• NaCl and copper sulfate form mobile ions in aqueous solution, so their solutions conduct electricity.
• Sugar may dissolve but does not produce ions, so its solution does not conduct appreciably.
• Camphor and naphthalene are covalent substances and are generally insoluble in water and non-conducting.

Therefore conductivity depends on the presence of mobile charged particles, not simply on whether a substance appears to dissolve.

Formula unit mass = 164 u.

Negative charge means electron gain; positive charge means electron loss. Ion formation does not alter the number of neutrons.

1. Open arrangement: Vinegar is placed in a flask and baking soda is added. Brisk effervescence occurs as CO₂ forms.
2. The gas escapes into the surroundings, so the measured final mass of the apparatus may be smaller than the initial mass.
3. Closed arrangement: Baking soda is placed in a balloon attached securely to the flask containing vinegar. The system is weighed before mixing.
4. When the substances react, CO₂ inflates the balloon but cannot escape. The total final mass remains equal to the initial mass within experimental error.

The experiment shows that an apparent loss in an open system results from matter leaving the measured system, not from destruction of matter.

1. Keep sodium sulfate solution in flask A and barium chloride solution in flask B and record the combined mass.
2. Transfer one solution into the other. A white precipitate of barium sulfate forms along with sodium chloride.
3. Place both flasks back on the balance. The total mass remains essentially unchanged, demonstrating conservation of mass.
4. Precaution: Both flasks should remain part of the weighing before and after transfer because traces of solution may remain stuck to the flask walls.

1. Law of Conservation of Mass:

Total mass before a chemical reaction equals total mass after the reaction. Example: if reactants have masses 4.0 g and 2.92 g:

If products are 1.76 g, 0.72 g and 4.44 g:

2. Law of Constant Proportions:

A given compound always contains its constituent elements in a fixed ratio by mass. In water H:O = 1:8. Thus 18 g water contains:

• A compound would no longer possess a definite composition.
• Samples obtained from different places could contain different mass ratios of their elements.
• The Law of Constant Proportions would not hold for that substance.
• Reliable chemical formulae representing definite numbers or ratios of atoms would lose their meaning for the supposed compound.

Fixed composition is therefore one of the key characteristics that distinguish a pure compound from a variable mixture.

In every case the atoms gain greater stability through electron sharing.

1. The first element keeps its ordinary name; the second generally ends in -ide.
2. Prefixes mono-, di-, tri-, tetra-, penta-, hexa-, etc. indicate atom numbers.
3. Mono- is usually omitted before the first element: CO is carbon monoxide, not monocarbon monoxide.
4. Where necessary, the final vowel of a prefix is dropped before “oxide”: N₂O₄ is dinitrogen tetroxide, while CO is monoxide.

Other examples: CO₂ carbon dioxide, PCl₃ phosphorus trichloride and SF₆ sulfur hexafluoride.

Magnesium chloride:

Calcium oxide:

Both contain oppositely charged ions held together by electrostatic attraction. This attraction constitutes the ionic bond.

• Ionic compounds consist of a regular three-dimensional arrangement of oppositely charged ions.
• Strong electrostatic attractions hold the ions together, so a large amount of energy is required to separate them. This gives generally high melting and boiling points.
• In the solid state, ions are fixed in position and cannot carry current.
• When molten or dissolved, ions become mobile and can carry electric charge, so the substance conducts.

• Magnesium hydroxide – Mg(OH)₂
• Calcium carbonate – CaCO₃
• Aluminium hydroxide – Al(OH)₃
• Aluminium sulfate – Al₂(SO₄)₃

Brackets are used when two or more of the same polyatomic ion are required. Thus (OH)₃ means three complete OH⁻ ions, and (SO₄)₃ means three complete sulfate ions.

1. Test each substance for solubility in water and in non-aqueous solvents such as petrol/kerosene using appropriate safety precautions.
2. NaCl and CuSO₄ are ionic examples and are generally water-soluble.
3. Their aqueous solutions conduct because mobile ions are present.
4. Camphor and naphthalene are covalent and generally dissolve better in non-aqueous solvents; sugar is a useful exception because it dissolves in water but does not ionise and therefore does not conduct.

Thus multiple properties must be considered rather than using water-solubility alone.

For C:O = 3:8:

The calculation applies both constant composition and conservation of mass.

• H₂O = 2(1) + 16 = 18 u
• CO₂ = 12 + 2(16) = 44 u
• Na₂O = 2(23) + 16 = 62 u
• Mg(OH)₂ = 24 + 2(16 + 1) = 58 u

• Atomic number = number of protons = 11.
• Mass number = protons + neutrons = 11 + 12 = 23.
• It has 11 protons but only 10 electrons, so charge = +1.
• Electronic configuration for 10 electrons = 2,8.
• Atomic number 11 corresponds to sodium, so the species is ²³Na⁺.

• A has five valence electrons and requires three more to complete an octet.
• B has seven valence electrons and requires one more.
• Both have more than four valence electrons, so electron sharing is expected rather than one readily donating several electrons to the other.
• A can share with three B atoms. The simplest formula is therefore AB₃, with covalent bonding.