Revision Questions

The unit in which atomic weight is expressed is a.m.u.

The type of bond shared between sodium [²³₁₁Na] and sulphur [³²₁₆S] during the formation of the compound sodium sulphide is an ionic or electrovalent bond.

Sodium hydroxide is a deliquescent substance also used in the softening of hard water.

A liquid oxidising agent is hydrogen peroxide.
A gaseous reducing agent is hydrogen sulphide.

A salt that undergoes a photochemical reaction is silver chloride.

2AgCl $\xrightarrow{\text{sunlight}}$ 2Ag + Cl₂

A pair of gases that undergo a photochemical reaction are hydrogen and chlorine.

H₂ + Cl₂ $\xrightarrow{\text{diffused sunlight}}$ 2HCl

The salt formed when zinc reacts with caustic potash is potassium zincate.

The element having atomic number 12 is magnesium. It belongs to group 2 [IIA].

The subatomic particle carrying a unit negative charge and having negligible mass is the electron.

A metal other than iron and copper which shows variable valency is tin.

The two neutral gases are nitric oxide and oxygen. They combine to give the coloured acidic gas nitrogen dioxide.

2NO + O₂ → 2NO₂

The acid responsible for slight acidity in natural rain water is carbonic acid.

A gas which is combustible but a non-supporter of combustion is hydrogen.

A metal exhibiting valency 2⁺ and 3⁺ is iron.

The gas liberated when ammonium dichromate undergoes thermal decomposition is nitrogen.

A gas responsible for melting of ice-caps is carbon dioxide.

A salt whose solubility decreases with increase in temperature of water is calcium sulphate.

A compound which on thermal decomposition, leaves a metallic residue and a gas which relits a glowing splint is mercuric oxide.

The residue left on dehydration of glucose by concentrated sulphuric acid is carbon.

A hygroscopic liquid which acts as a drying and a dehydrating agent is sulphuric acid.

The covalent compound which has two shared pairs of electrons between the two atoms is oxygen, O₂.

The chemical other than chlorofluorocarbon, responsible for ozone depletion and global warming is carbon tetrachloride.

An exothermic reaction involving two neutral gaseous reactants is:

N₂ + 3H₂ ⇌ 2NH₃ + Δ

An element which does not lose, gain or share electrons and has two electrons in its valence shell is helium.

A non-metal in group 17 [VIIA] of the periodic table, which is a solid at ordinary temperatures is iodine.

When aluminium reacts with concentrated sodium hydroxide solution, the salt formed is sodium aluminate.

The process involving hydrogen used in the manufacture of vegetable fat is hydrogenation.

Amphoteric oxide Reason — The element in period 3 and group 13 [IIIA] is aluminium. It forms an amphoteric oxide, Al₂O₃.

The valency of calcium in calcium nitride is 2.

An element other than hydrogen and chlorine which exists in the isotopic form is carbon.

A lead salt which evolves reddish brown fumes on thermal decomposition is lead nitrate.

Oxygen is more electronegative element from the elements oxygen and sulphur present in group 16 [VIA].

The law is Boyle’s law.

The most reactive metal is sodium.
The least reactive metal is silver.

The element in group 17 [VIIA] of the periodic table which is a liquid at ordinary temperatures is bromine.

Period III contains 8 elements including the non-metal sulphur.

The element in group 17 [VIIA] of the periodic table which bleaches vegetable dyes by oxidation is chlorine.

A heptahydrate salt containing iron is ferrous sulphate heptahydrate. It is commonly known as green vitriol.

The most reactive element in group 16 [VIA] of the periodic table is oxygen.

The product obtained when nitrogen reacts with a neutral gas is nitric oxide.

A nitride of a divalent metal is magnesium nitride.

The displaced product when zinc reacts with copper sulphate solution is copper.

The first element of group 14 [IVA] is carbon and the second element of group 16 [VIA] is sulphur. They combine to form carbon disulphide.

The least reactive element in group 15 [VA] of the periodic table is nitrogen.

The oxidation number of nitrogen in nitric oxide is +2.

A soluble salt formed by synthesis, when a trivalent metal combines with a coloured acidic gas is ferric chloride.

The salt formed when zinc reacts with caustic potash solution is potassium zincate.

(a) The law is Boyle’s law.

(b) The law is Charles’ law.

(c) The law is the Law of Conservation of Mass.

The temperature scale with absolute zero as its starting point is Kelvin scale.

An element which is inactive and present in traces in the atmosphere is argon.

A secondary pollutant from the primary pollutant nitric oxide is nitrogen dioxide.

The gaseous product of boiling a sample of temporary hard water, containing soluble bicarbonates is carbon dioxide.

(a) A positive radical containing hydrogen is ammonium ion.

(b) A negative radical containing hydrogen is hydroxide ion.

(a) Copper

(b) Iron

(c) Tin

The source of inorganic material in sewage waste water is industrial effluents and domestic detergents.

A combustible neutral gas which is a non-supporter of combustion is hydrogen.

The oxidising agent is hydrogen peroxide.
The oxidised product is iodine.

This is a displacement reaction.

A trivalent metal which reacts with steam liberating hydrogen is aluminium.

The shell or energy level having a maximum of 8 electrons is the L-shell.

The atom which needs one electron to attain stable electronic configuration of the nearest noble gas helium is hydrogen.

An atmospheric radical present in the troposphere is hydroxyl radical.

Gram atomic mass is also considered as gram atom.

Decomposition of ammonium chloride by heat to give ammonia and chlorine is an example of thermal decomposition.

The valency of copper in CuCl₂ is 2⁺.

The law which relates between the pressure of a gas and the volume occupied by it, temperature remaining constant is Boyle's law.

The temperature -273°C is called absolute zero.

Oxygen in presence of U.V. light gives two oxygen atoms. This combines with oxygen molecule to give ozone.

Hydrogen and carbon monoxide are examples of combustible gases while chlorine and oxygen are examples of non-combustible gases.

An increase in pressure on the surface of water causes increase in the solubility of the gas dissolved in water.

Washing soda is an example of an efflorescent salt, while iron [III] chloride is an example of a deliquescent salt.

If an element 'X' has atomic number 18, then its valency is 0.

The maximum number of electrons in any shell of an atom is represented by 2n². The maximum number of electrons in the M shell is 18.

Isotopes of chlorine have the same atomic number but different number of neutrons.

An element 'Y' has atomic number 20. It finds position in period 4 and group 2.

The basis of classification of elements by Newland was based on arrangement of elements in increasing order of atomic weights in series of 7.

The period-1 contains 2 elements, and periods 2 and 3 8 elements each. There are 7 horizontal rows called periods, and 18 vertical columns called groups. An element in period-3 will have 3 electron shells or orbits and in group VA will have 5 valence electrons in its outermost shell.

Period-2 is a short period containing eight elements of which, carbon (at. no. 6) is placed in group 14 [IVA], nitrogen (at. no. 7) is placed in group 15, oxygen (at. no. 8) is placed in group 16, sulphur (at. no. 16) is placed in group 16 and chlorine (at. no. 17) is placed in group 17.

On moving from left to right in a period, the number of electron shells remain same, the number of valence electrons increase by one and the elements show a transition from metallic to non-metallic character. On moving down a group, the valence electrons remain same and the elements show a transition from non-metallic to metallic character.

A non-renewable source of energy fuel used instead of fossil fuel to reduce greenhouse gas pollution is C.N.G..

From the elements of group 15 [VA] nitrogen and phosphorus are non-metals and highly electronegative. The electronegativity decreases on moving down the group and so also the non-metallic character.

Hydrogen gas is lighter than air and collected in the laboratory by downward displacement of water.

Sulphur dioxide gas turns potassium permanganate solution from pink to colourless and potassium dichromate from orange to green.

Sodium chloride imparts a golden yellow colour on application of the flame test for identification of the metallic radical in the salt, calcium chloride a brick red colour and potassium chloride a lilac colour.

Hydrogen sulphide turns lead acetate paper silvery black.

Heat on copper nitrate gives a black residue, a coloured acidic gas and a colourless neutral gas.

A true solution is homogeneous in nature.

The atomic weight of chlorine is the weighted average of the atomic weight of its natural isotopes.

All temperatures on the Kelvin scale are in positive figures.

-273°C is equal to 0K.

Every substance has a stored amount of energy in it i.e. potential or latent, commonly called chemical energy.

Chlorine [³⁵₁₇Cl] needs one electron/s to attain stable electronic configuration of the nearest noble gas argon.

The valency of a metal is the number of electrons lost per atom of the metal.

A chemical equation does not tell about the concentration of both reactants and products.

The formula of aluminium sulphide is Al₂S₃.

Nitrogen [¹⁴₇N] needs three electrons to attain stable octet.

Thermal dissociation is a reversible reaction.

In the reaction: 2FeCl₃ + H₂S → 2FeCl₂ + 2HCl + S, the reduced product is FeCl₂

In the covalent compound carbon tetrachloride, chlorine needs one electron/s to attain stable octet.

The metal which is above hydrogen in the activity series, but does not react with water is Pb.

Ozone layer prevents harmful U.V. rays from reaching the earth's surface.

If an element has 6 electrons in its outermost 'L' shell its valency is -2.

Normal elements have atoms in which all shells are complete, except the outermost shell which is incomplete.

In Bosch process for the manufacture of hydrogen, the gas carbon dioxide is recovered by dissolving it in caustic potash soln..

In the manufacture of hydrogen, the reaction of water gas with excess steam in presence of a catalyst is an exothermic reaction.

Atoms are extremely small and very light, hence they cannot be weighed directly. Therefore, indirect methods are utilised to determine the absolute mass of an atom.

Silver salts like silver bromide are photosensitive, they decompose in the presence of light. Therefore, to prevent this photochemical decomposition, silver salts are stored in dark-coloured bottles, which block light.

4AgBr $\xrightarrow{\text{sunlight}}$ 2Ag₂Br + Br₂

When sodium is added to cold water, it reacts to form sodium hydroxide, which is highly soluble in water, so the solution remains clear and colourless. But when calcium is added to water, it forms calcium hydroxide which is sparingly soluble in water. The excess undissolved appears as a white precipitate, making the solution turbid.

Magnesium is above hydrogen in the activity series and displaces hydrogen from dilute hydrochloric acid. Lead also reacts with dilute hydrochloric acid initially, but it forms an insoluble coating of lead chloride on its surface. This prevents further reaction, so hydrogen is not liberated.

Hydrogen atom was initially chosen as a standard unit for determining the absolute mass of an atom because it is the lightest atom.

A displacement reaction involves an element replacing another element from its compound. Since one element is being substituted by another, a displacement reaction is also called a substitution reaction.

Isotopes have the same atomic number but different mass number. Isotopes of chlorine have the same atomic number (17), so they contain the same number of protons and therefore the same number of electrons in a neutral atom. They differ in their mass number because they have different numbers of neutrons in the nucleus.

Displacement reactions can be used to prepare soluble salts because metals above hydrogen in the electrochemical series i.e. active metals are seen to displace hydrogen from dilute acids.

Chlorine shows similarities with bromine and iodine due to their same valence shell electronic configuration. Chlorine, bromine, and iodine belong to Group 17 (halogens) of the Modern Periodic Table and have the same number of valence electrons (7 electrons in the outermost shell). Because of this similar electronic configuration, they show similar chemical properties.

Lime water reacts with both carbon dioxide and sulphur dioxide to form a white precipitate, so both gases produce a similar effect. Since both gases turn lime water milky due to formation of insoluble salts (carbonates and sulphites), lime water cannot distinguish between carbon dioxide and sulphur dioxide.

An exothermic reaction releases energy to the surroundings. This released energy is sufficient to keep the reaction going once it has started, so no continuous external supply of energy is needed. After initiation, the reaction becomes self-sustaining due to the heat evolved during the reaction.

Hydrogen shows dual character. It has only one electron in its outer shell, so it can either lose this electron to form H₊ (like an electropositive element) or gain one electron to form H_- (like an electronegative element).

Dobereiner’s law of triads was not accepted because it could be applied to only a few groups of elements and failed to classify all known elements.

In iron [II] sulphide, iron is chemically combined with sulphur and does not retain the properties of free iron. Hence, it does not displace hydrogen from dilute sulphuric acid. Instead, iron [II] sulphide reacts with dilute sulphuric acid to give hydrogen sulphide gas.

FeS + H₂SO₄ → FeSO₄ + H₂S

Copper shows variable valency because, besides the electron in its valence shell, an electron from the penultimate shell may also take part in bond formation. Hence, copper can show Cu¹⁺ and Cu²⁺ valencies. Sodium has only one valence electron which it loses to attain a stable configuration, so it shows Na¹⁺ valency only.

The atomic mass of sulphur is a whole number because sulphur occurs mainly as a single isotope, ³²S, whose mass number is 32. Chlorine, however, exists naturally as a mixture of two isotopes, ³⁵Cl and ³⁷Cl in different proportions. Therefore, its atomic mass is the weighted average of the masses of these isotopes (35.5). Hence, sulphur has a whole-number atomic mass, while chlorine does not.

Zinc reacts with dilute sulphuric acid because it is more reactive than hydrogen and can displace hydrogen from the acid. Lead becomes covered with an insoluble layer of lead sulphate. This coating prevents further contact between lead and the acid, so the reaction stops and hydrogen is not effectively liberated.

Water contains dissolved gases which contribute to its taste. On boiling, the temperature of water increases and the solubility of dissolved gases decreases. These gases are expelled out, so boiled water loses its taste.

Zinc reacts with dilute sulphuric acid because sulphuric acid is a non-oxidising acid, so zinc displaces hydrogen from it. Dilute nitric acid, however, is a strong oxidising agent. Any hydrogen that would be formed is immediately oxidised to water, and instead oxides of nitrogen are produced. Therefore, hydrogen gas is liberated with dilute sulphuric acid but not with dilute nitric acid.

Hard water contains calcium and magnesium ions. Soap reacts with the calcium and magnesium salts present in hard water to form an insoluble curdy precipitate and therefore, does not produce lather readily, so a large amount of soap gets wasted before cleaning action begins.

The atomic number is equal to the number of protons in an element. In a neutral atom, the number of electrons equals the number of protons. Therefore, the atomic number is also equal to the number of electrons in an atom of the element.

Atoms of elements other than noble gases have incomplete outermost electron shells because their outermost electron shell is not completely filled. Because of this, they tend to gain, lose, or share electrons to attain a stable configuration like that of the nearest noble gas.

Anhydrous crystals have no definite geometric shape because they do not contain water of crystallisation. Water of crystallisation helps in maintaining the regular crystalline structure of a salt. When this water is removed, the crystals lose their regular arrangement and become a shapeless powder. Hence, anhydrous crystals do not have a definite geometric shape.

Absolute zero is the lowest possible temperature (-273.15°C). Absolute zero is only a theoretical concept and not a practical one because practically all gases liquefy or solidify before reaching this temperature.

Nitrogen has five electrons in its outermost shell and requires three more electrons to complete its octet. Each hydrogen atom has one electron and needs one more electron to attain a stable duplet configuration. Therefore, one nitrogen atom shares one electron each with three hydrogen atoms, forming three shared pairs of electrons and producing ammonia.

Chlorine is more reactive than bromine because it is higher than bromine in Group 17 (halogens). A more reactive halogen can displace a less reactive halogen from its salt solution. Therefore, chlorine displaces bromine from potassium bromide.

Atoms of elements other than noble gases have incomplete outermost electron shells, making them unstable. To attain a stable electronic configuration like that of the nearest noble gas, they lose, gain or share electrons.

Solubility curves enable a pharmacist to determine the amount of solid drugs that must be dissolved in a given quantity of solvent at different temperatures to prepare a prescribed drug solution.

According to Boyle’s law, for a fixed mass of gas at constant temperature, the volume of a gas is inversely proportional to its pressure. Therefore, when the pressure is doubled, the volume becomes half so that the product PV remains constant.

According to Gay-Lussac’s law, for a fixed mass of gas at constant volume, the pressure of a gas is directly proportional to its absolute temperature. Therefore, if the absolute temperature is doubled, the pressure also doubles, provided the volume remains constant.

In a direct combination reaction, two or more substances combine to form a single product. Since a new substance is synthesized (formed) from simpler substances, such reactions are also called synthesis reactions.

Cations are positively charged ions and anions are negatively charged ions. Opposite charges attract each other due to electrostatic force of attraction. This strong electrostatic attraction between the oppositely charged ions holds them together and forms an electrovalent (ionic) bond.

Valency is the combining capacity of an atom. Oxygen has six electrons in its outermost shell and needs two more electrons to complete its octet. In water, one oxygen atom shares electrons with two hydrogen atoms by forming two covalent bonds. Therefore, the valency of oxygen in water is 2.

According to the Law of Conservation of Mass, mass can neither be created nor destroyed during a chemical reaction. Therefore, the total number of atoms of each element must be the same on both sides of a chemical equation. Balancing an equation ensures that the mass of reactants equals the mass of products, thus obeying this law.

In hydrogen sulphide, sulphur has an oxidation state of −2. During the reaction with concentrated nitric acid, sulphur is converted into free sulphur (S), where its oxidation state becomes 0. Since the oxidation state of sulphur increases from −2 to 0, sulphur undergoes oxidation. Therefore, sulphur is the oxidised product.

Non-metallic atoms with five valence electrons need three more electrons to complete their octet. Therefore, two such atoms share three pairs of electrons with each other. This results in the formation of a triple covalent bond.

Atomic weights (atomic masses) are usually not whole numbers because most elements exist as a mixture of isotopes having different masses. The atomic weight of an element is the weighted average of the masses of its naturally occurring isotopes, based on their relative abundances.

The solubility of most solutes changes with temperature. A solution that is saturated at one temperature may become unsaturated or supersaturated at another temperature. Therefore, temperature must be considered while preparing a saturated solution to ensure that the maximum amount of solute dissolves correctly at that particular temperature.

The atomic number is equal to the number of protons and, in a neutral atom, also to the number of electrons. Thus, by knowing the atomic number and mass number, the numbers of protons, electrons, and neutrons can be determined, and hence the atomic structure of the element can be deduced.

Atomic number = No. of protons = No. of electrons

Mass number = No. of neutrons + No. of protons

Argon, a noble gas of Group 18, has a completely filled outermost shell (2,8,8), giving it a stable octet configuration. Chlorine of Group 17 has the electronic configuration (2,8,7). Since its outermost shell contains only 7 electrons, it is one electron short of a complete octet and therefore unstable. Hence, argon is stable while chlorine is not because argon has a complete valence shell whereas chlorine has an incomplete one.

Hydrogen is a combustible gas and burns in oxygen with a high calorific value, producing a large amount of heat. Coal gas and water gas contain hydrogen as one of their main constituents. Due to the combustible nature of hydrogen, these gases are used as fuels for heating and industrial purposes.

Elements if arranged in increasing order of atomic number, showed similarity in properties after regular intervals because the physical and chemical properties of elements are periodic functions of their 'atomic numbers'. When elements are arranged in the increasing order of their atomic numbers, elements with similar chemical properties appear again after regular intervals. This repetition is called the periodicity of properties.

Elements in the same subgroup have the same number of valence electrons, so they exhibit similar chemical properties. However, as we move down the subgroup, the number of electron shells increases, causing gradual changes in atomic size, metallic character, and reactivity. Therefore, the chemical properties remain similar but show a gradual variation down the group.

Granulated zinc contains traces of impurities which have a slight catalytic effect and increase the rate of reaction. Hence, granulated zinc is preferred to pure metallic zinc in the laboratory preparation of hydrogen using dilute hydrochloric acid.

Isotopes of chlorine have the same atomic number and therefore the same number of electrons and identical electronic configuration. Since chemical properties depend on the electronic configuration, their chemical properties are similar. However, isotopes differ in the number of neutrons and hence have different mass numbers. Physical properties such as density and diffusion rate depend on mass, so their physical properties are different.

Chlorine gains electrons and is reduced to chloride ions. Since chlorine causes oxidation of iron(II) chloride to iron [III] chloride, it acts as an oxidising agent.

An excessive increase of chemical nutrients such as nitrates and phosphates in water causes rapid growth of algae. When these algae die and decompose, dissolved oxygen in the water is consumed. The reduced oxygen level harms or kills marine organisms such as fish and other aquatic life. This process is called eutrophication.

A covalent bond is formed by the sharing of electron pairs between atoms. If one pair of electrons is shared, a single covalent bond is formed. If two pairs are shared, a double covalent bond is formed. If three pairs are shared, a triple covalent bond is formed.

A chemical equation represents a chemical reaction using symbols and formulae of substances. It shows the reactants, products, and the result of a chemical change.

Some substances dissolved in water, such as mineral salts (calcium, magnesium, fluoride, etc.), are beneficial in small amounts because they are essential for health and body functions but when present in excess, these same substances may become harmful, causing problems such as hard water, fluorosis, or other health issues.