ICSE Class 8 Chemistry Sample Paper with Solutions Set 03

Question 1

Choose the correct answer out of the four available choices given under each question. [15]

1. The following diagram shows the various shells of electrons. The maximum number of electrons which can be accommodated in the M shell is ________. [1 Mark]
[Figure: Diagram showing atomic structure with concentric shells labelled K (1), L (2), M (3), N (4) around a positively charged nucleus]
(A) 2
(B) 8
(C) 18
(D) 32

Answer: (C) 18

The maximum capacity of electrons in any shell is given by the formula \( 2n^2 \), where \( n \) is the shell number. For the M shell, \( n = 3 \), so maximum electrons = \( 2(3)^2 = 18 \).

Teacher's Note:
a) Remember the formula \( 2n^2 \) to calculate the maximum capacity of any shell where \( n \) represents the principal quantum number or shell index.
b) Students often confuse the capacity of the L shell (8) with the M shell (18).

2. Which of the following is incorrect about a heterogeneous mixture? [1 Mark]
(A) Constituents can be distinctly seen.
(B) Constituents are uniformly mixed.
(C) Different composition throughout.
(D) Sand in Water is an example of heterogeneous mixture.

Answer: (B) Constituents are uniformly mixed.

In a heterogeneous mixture, constituents are not uniformly mixed and have a non-uniform composition throughout.

Teacher's Note:
a) Homogeneous mixtures have a uniform composition, whereas heterogeneous mixtures have a non-uniform composition.
b) Read every option carefully before answering, as option (B) describes a homogeneous mixture.

3. Which of the following are combination reactions? [1 Mark]
i. \( 4\text{K} + \text{O}_2 \rightarrow 2\text{K}_2\text{O} \)
ii. \( \text{CuSO}_4 + 2\text{NH}_4\text{OH} \rightarrow (\text{NH}_4)_2\text{SO}_4 + \text{Cu}(\text{OH})_2 \)
iii. \( 2\text{ZnS} + 3\text{O}_2 \rightarrow 2\text{ZnO} + 2\text{SO}_2 \)
iv. \( \text{Na}_2\text{O} + \text{H}_2\text{O} \rightarrow 2\text{NaOH} \)
(A) Reactions (i) and (ii)
(B) Reactions (ii) and (iii)
(C) Reactions (ii) and (iv)
(D) Reactions (i) and (iv)

Answer: (D) Reactions (i) and (iv)

Reactions (i) and (iv) involve two or more reactants combining to form a single product, which defines a combination reaction.

Teacher's Note:
a) A combination reaction is a reaction where two or more substances combine to form a single new substance.
b) Reaction (ii) is a double displacement reaction, and reaction (iii) is a substitution or roasting reaction.

4. The following table gives the steps we use in writing the formulae of compounds. What is the correct formula of aluminium chloride? [1 Mark]
[Figure: Table showing Aluminium with symbol Al, Valency 3+, and Chlorine with symbol Cl, Valency 1-, with cross arrows showing interchanging valencies 1 and 3]
(A) \( \text{AlCl}_3 \)
(B) \( \text{Al}_3\text{Cl} \)
(C) \( \text{Al Cl} \)
(D) \( \text{Cl}_3\text{Al} \)

Answer: (A) \(\text{AlCl}_3\)

By criss-crossing the valencies of Aluminium (3+) and Chlorine (1-), we get \( \text{Al}_1\text{Cl}_3 \), which is written as \( \text{AlCl}_3 \).

Teacher's Note:
a) The criss-cross method involves writing the symbols of elements and interchanging their numerical valencies as subscripts.
b) Subscript '1' is omitted in chemical formulae.

5. Given the symbol \( ^\text{A}_\text{Z}\text{C} \), what are the values of A and Z for carbon? [1 Mark]
(A) A = 12, Z = 6
(B) A = 8, Z = 4
(C) A = 6, Z = 6
(D) A = 10, Z = 2

Answer: (A) A = 12, Z = 6

For carbon, the atomic number (Z) is 6 and the mass number (A) is 12.

Teacher's Note:
a) The atomic number (Z) represents the number of protons, and the mass number (A) represents the total number of protons and neutrons.
b) Note the standard notation where mass number is superscripted and atomic number is subscripted before the element symbol.

6. Element __________ has symbol derived from its Latin name 'plumbum'. [1 Mark]
(A) Calcium
(B) Lead
(C) Carbon
(D) Hydrogen

Answer: (B) Lead

The chemical symbol for lead is \( \text{Pb} \), which is derived from its Latin name plumbum.

Teacher's Note:
a) Many elements have symbols derived from their Latin names, such as sodium (natrium), potassium (kalium), and iron (ferrum).
b) Learn the Latin names of common elements to avoid confusion with their English names.

7. Valency of iron in \( \text{FeO} \) is ______ of chlorine in \( \text{CaCl}_2 \) is ____ . [1 Mark]
(A) 1+, 2-
(B) 2+, 1-
(C) 2+, 2-
(D) 1+, 1-

Answer: (B) 2+, 1-

In \( \text{FeO} \), iron exhibits a valency of 2+ (since oxygen is 2-), and in \( \text{CaCl}_2 \), chlorine exhibits a valency of 1-.

Teacher's Note:
a) Valency is determined by balancing the charges of the combining atoms in a neutral compound.
b) Iron is a variable valency metal, showing 2+ in ferrous compounds and 3+ in ferric compounds.

8. Melting of ice is [1 Mark]
(A) Irreversible change
(B) Periodic change
(C) Chemical change
(D) Reversible change

Answer: (D) Reversible change

Melting of ice forms water, which can be frozen back into ice, making it a physical and reversible change.

Teacher's Note:
a) Physical changes do not result in the formation of a new chemical substance and are generally reversible.
b) Do not confuse physical phase transitions with chemical changes.

9. Which is the correct answer about the states of substances I, II and III with reference to the following picture? [1 Mark]
[Figure: Glass containing a carbonated drink with ice cubes at the top labelled I, froth/gas bubbles at the surface labelled II, and liquid drink at the bottom labelled III]
(A) I - Solid, II - Liquid, III - Gas
(B) I - Liquid, II - Gas, III - Solid
(C) I - Solid, II - Gas, III - Liquid
(D) I - Gas, II - Solid, III - Liquid

Answer: (C) I - Solid, II - Gas, III - Liquid

Ice cubes floating at the top represent the solid state (I), the gas bubbles or effervescence at the top represent the gaseous state (II), and the beverage represents the liquid state (III).

Teacher's Note:
a) Ice is the solid state of water, dissolved carbon dioxide forming bubbles represents the gaseous state, and the beverage is a solution in the liquid state.
b) Carefully observe the pointers in diagram-based questions before selecting the option.

10. When the temperature of water increases above \( 0^\circ\text{C} \) up to \( 4^\circ\text{C} \), its density _________. [1 Mark]
(A) decreases
(B) increases
(C) becomes zero
(D) remains unchanged

Answer: (B) increases

Water shows anomalous expansion; when heated from \( 0^\circ\text{C} \) to \( 4^\circ\text{C} \), its volume decreases and therefore its density increases, reaching a maximum at \( 4^\circ\text{C} \).

Teacher's Note:
a) Water has maximum density at \( 4^\circ\text{C} \) ($1\text{ g/cm}^3$).
b) This anomalous behavior of water is crucial for aquatic life survival during winters.

11. When an electric current is passed through acidulated water, _________ volume of hydrogen is formed at the cathode and __________ volume of oxygen is formed at the anode. [1 Mark]
(A) one, two
(B) three, one
(C) one, three
(D) two, one

Answer: (D) two, one

During the electrolysis of water, two volumes of hydrogen gas are liberated at the cathode for every one volume of oxygen gas liberated at the anode, reflecting the chemical formula \( \text{H}_2\text{O} \).

Teacher's Note:
a) The ratio of hydrogen to oxygen by volume in water is \( 2:1 \).
b) Cathode attracts positively charged hydrogen ions, whereas anode attracts negatively charged oxygen-containing ions.

12. A soluble solid is separated from insoluble solid by __________ [1 Mark]
(A) Fractional crystallisation
(B) Solvent extraction
(C) sublimation
(D) Magnetic separation

Answer: (B) solvent extraction

A mixture of a soluble and an insoluble solid can be separated using a suitable solvent in which one component dissolves while the other remains insoluble (solvent extraction followed by filtration).

Teacher's Note:
a) Solvent extraction relies on the differential solubility of mixture components in a specific solvent.
b) Always choose a solvent that dissolves only one of the solid components.

13. The following picture represents Rutherford's gold foil experiment. The parts labelled A and B in the diagram represent _______ and _______, respectively. [1 Mark]
[Figure: Rutherford alpha scattering experiment showing gold atoms, incoming parallel rays, with label A pointing to an alpha ray passing straight and label B pointing to a deflected ray]
(A) light rays, electrons
(B) X-rays, nuclei of gold atoms
(C) alpha rays, electrons
(D) alpha rays, gold atoms

Answer: (D) alpha rays, gold atoms

In Rutherford's experiment, a beam of alpha particles (A) was bombarded on a thin sheet of gold atoms (B).

Teacher's Note:
a) Rutherford used radioactive polonium or radium as a source of high-energy alpha particles.
b) The deflection of alpha rays led to the discovery of the atomic nucleus.

14. Anthracite is [1 Mark]
(A) An inferior type of coal
(B) A superior type of coal
(C) A cheapest form of coal
(D) None of above

Answer: (B) A superior type of coal

Anthracite contains the highest percentage of carbon (92% to 98%) and burns with minimum smoke, making it the highest quality or superior type of coal.

Teacher's Note:
a) Coal grades in increasing order of carbon content are peat, lignite, bituminous, and anthracite.
b) Anthracite is hard, dense, and produces maximum heat.

15. Hydrogen is [1 Mark]
(A) Combustible
(B) Non-combustible
(C) Supporter of combustion
(D) Non-supporter of combustion

Answer: (A) Combustible

Hydrogen burns in air with a pale blue flame, making it a combustible gas, though it does not support combustion itself.

Teacher's Note:
a) A combustible substance catches fire easily and burns.
b) Oxygen is a supporter of combustion, whereas hydrogen is combustible.

Question 2

(A) State the electronic configuration of the following atoms? [5]

1. Atom 'A' (Atomic Number = 8) [1 Mark]

Answer:
Electronic configuration = 2, 6

Teacher's Note:
a) Distribute electrons into shells using the \( 2n^2 \) rule: K shell holds up to 2, L shell holds up to 6.
b) Ensure total electrons equal the atomic number.

2. Atom 'B' (Atomic Number = 18) [1 Mark]

Answer:
Electronic configuration = 2, 8, 8

Teacher's Note:
a) Atom 'B' is Argon, a noble gas with a completely filled valence shell.
b) The L and M shells accommodate 8 electrons each following octet stability rules.

3. Atom 'C' (Atomic Number = 11) [1 Mark]

Answer:
Electronic configuration = 2, 8, 1

Teacher's Note:
a) Atom 'C' is Sodium, belonging to Group 1 of the periodic table.
b) The single valence electron makes it highly reactive.

4. Atom 'D' (Atomic Number = 20) [1 Mark]

Answer:
Electronic configuration = 2, 8, 8, 2

Teacher's Note:
a) Atom 'D' is Calcium.
b) Notice that the M shell holds 8 electrons before filling the N shell with 2 electrons.

5. Atom 'E' (Atomic Number = 3) [1 Mark]

Answer:
Electronic configuration = 2, 1

Teacher's Note:
a) Atom 'E' is Lithium, an alkali metal.
b) The K shell is completely filled with 2 electrons, and the remaining 1 electron enters the L shell.

(B) Fill in the blanks and rewrite the sentences: [5]

1. Water reacts with metals to liberate __________ gas. [1 Mark]

Answer:
Water reacts with metals to liberate hydrogen gas.

Teacher's Note:
a) Reactive metals like sodium and potassium react vigorously with water to form metal hydroxides and release hydrogen gas.
b) Ensure the spelling of the chemical name is correct.

2. The process of change from the _________ state to the _________ state at a particular temperature is called liquefaction. [1 Mark]

Answer:
The process of change from the gaseous state to the liquid state at a particular temperature is called liquefaction.

Teacher's Note:
a) Liquefaction is also known as condensation.
b) Both states of matter must be specified correctly in order.

3. Atoms of the same elements differing in the number of ________ in their nuclei are known as isotopes. [1 Mark]

Answer:
Atoms of the same elements differing in the number of neutrons in their nuclei are known as isotopes.

Teacher's Note:
a) Isotopes have the same atomic number but different mass numbers due to a varying number of neutrons.
b) Hydrogen isotopes (protium, deuterium, tritium) are classic examples.

4. The gas which has now replaced hydrogen in air balloons is ___________ [1 Mark]

Answer:
The gas which has now replaced hydrogen in air balloons is Helium.

Teacher's Note:
a) Hydrogen is highly combustible, making it dangerous for weather and air balloons.
b) Helium is non-flammable and lightweight, making it a safe alternative.

5. The crystal of __________ is opaque to light and is good conductor of heat [1 Mark]

Answer:
The crystal of graphite is opaque to light and is good conductor of heat.

Teacher's Note:
a) Graphite is an allotrope of carbon that conducts heat and electricity due to free electrons.
b) Unlike diamond, graphite crystals are opaque and dark grey.

Question 3

(A) State whether the following statements are true or false. Rewrite the false statement. [5]

1. When potassium chlorate is heated strongly, potassium chloride is formed with evolution of carbon dioxide gas. [1 Mark]

Answer:
False. When potassium chlorate is heated strongly, it breaks to produce oxygen along with potassium chloride.

Teacher's Note:
a) Thermal decomposition of potassium chlorate produces potassium chloride and oxygen gas.
b) Always identify and correct the incorrect chemical product in false statements.

2. The maximum number of electrons which can be accommodated in the K shell is 8. [1 Mark]

Answer:
False. The maximum number of electrons which can be accommodated in the K shell is 2.

Teacher's Note:
a) For the K shell, \( n = 1 \), so \( 2n^2 = 2(1)^2 = 2 \).
b) Statement rewriting requires replacing the incorrect value with the correct one.

3. Mercuric oxide when heated gives mercury and oxygen. This is a displacement reaction. [1 Mark]

Answer:
False. A reaction in which a compound breaks up into two or more simple substances due to the application of heat is called a decomposition reaction. Mercuric oxide when heated decomposes to form two elements - mercury and oxygen.

Teacher's Note:
a) Single reactant breaking down into simpler products is a decomposition reaction, not displacement.
b) State both the correct classification and the correct products for full credit.

4. Balanced chemical equation shows both the number of molecules and the number of atoms involved in the reaction. [1 Mark]

Answer:
True.

Teacher's Note:
a) A balanced equation obeys the law of conservation of mass, showing exact molecular and atomic proportions.
b) True statements do not need rewriting.

5. The positive charge radicals are called as anions.. [1 Mark]

Answer:
False. The positive charge radicals are called as cations. The negative charge radicals are called as anions.

Teacher's Note:
a) Cations are positively charged ions formed by loss of electrons.
b) Anions are negatively charged ions formed by gain of electrons.

(B) State a method to separate the following mixtures: [5]

1. Two solid mixtures, one of which sublimes. [1 Mark]

Answer:
Sublimation

Teacher's Note:
a) Sublimation is used when one component changes directly from solid to gas upon heating (e.g., ammonium chloride and salt).

b) The sublimable component is recovered on a cool surface.

2. A solid - liquid mixture containing an insoluble solid in a liquid component. [1 Mark]

Answer:
Filtration

Teacher's Note:
a) Filtration separates insoluble suspended solids from liquids using filter paper.
b) The insoluble solid remains on the filter paper as residue.

3. To separate the mixture of an insoluble solid and a soluble solid. [1 Mark]

Answer:
Solvent extraction

Teacher's Note:
a) Solvent extraction uses a solvent that dissolves only one of the solids.
b) Followed by filtration and evaporation to isolate both components.

4. To separate the mixture of different solid constituents in a liquid component. [1 Mark]

Answer:
Chromatography

Teacher's Note:
a) Chromatography is used to separate different solutes dissolved in the same solvent based on differential adsorption.
b) Paper chromatography is commonly used for plant pigments and dyes.

5. To separate the mixture of a soluble solid from a liquid component. [1 Mark]

Answer:
Distillation

Teacher's Note:
a) Distillation involves vaporizing the liquid and condensing the vapors back to liquid to recover both solute and solvent.
b) Simple distillation is used when only the solvent needs to be recovered.

Question 4

(A) Describe the formation of coal. What are its four types? [5 Marks]

Answer:
[Figure: Diagram illustrating coal formation stages from huge forests 300 million years ago, vegetation/peat, lignite, bituminous, to anthracite]
1. Formation of Coal:
a. Coal was formed about 200 to 300 million years ago from the remains of plants such as ferns, mosses, and trees in swampy forests.
b. As plants died, their remains sank to the bottom of swampy areas and got covered with layers of mud, water, and soil over time, forming peat.
c. With increasing heat and pressure from top layers, alongside bacterial action, the material gradually transformed into coal through carbonisation.
2. Four Types of Coal:
a. Peat: First stage, light brown, contains 50 - 60% carbon; most inferior form.
b. Lignite: Second stage, brown, harder than peat, contains 60 - 70% carbon.
c. Bituminous: Third stage, black and hard, contains 80 - 90% carbon; most common household coal.
d. Anthracite: Purest variety, contains 92 - 98% carbon, hard, dense, burns with high heat and no ash.

Teacher's Note:
a) Carbonisation is the slow chemical process of conversion of dead vegetation into coal under high temperature and pressure.
b) Students must list all four types with their carbon content percentages for full marks.

(B) Define the following terms: [5]

1. Crystallisation [1 Mark]

Answer:
Crystallisation is the process of separation or deposition of crystals from a hot saturated solution on gentle cooling of the solution.

Teacher's Note:
a) It is a purification technique used to obtain pure solid crystals from impure solutions.
b) Mention cooling of a hot saturated solution for completeness.

2. Water of crystallisation [1 Mark]

Answer:
Water of crystallisation is the definite number of water molecules bound in loose chemical combination with a crystal when it separates from its hot saturated solution.

Teacher's Note:
a) Examples include copper sulfate pentahydrate (\( \text{CuSO}_4 \cdot 5\text{H}_2\text{O} \)).
b) It is responsible for the geometric shape and color of many hydrated crystals.

3. Hydrated crystal [1 Mark]

Answer:
Hydrated crystals are crystals that contain a definite number of water molecules in loose chemical combination with them.

Teacher's Note:
a) Hydrated crystals lose their water of crystallisation upon strong heating.
b) Washing soda crystals (\( \text{Na}_2\text{CO}_3 \cdot 10\text{H}_2\text{O} \)) are a common example.

4. Anhydrous crystal [1 Mark]

Answer:
Anhydrous crystals are crystals that do not contain any definite number of water molecules in loose chemical combination.

Teacher's Note:
a) Anhydrous substances are formed when hydrated crystals are heated until all water of crystallisation escapes.
b) Anhydrous copper sulfate is white, whereas hydrated copper sulfate is blue.

5. Crystal [1 Mark]

Answer:
Crystals are solids having a definite regular shape, symmetrical arrangements, and sharp edges.

Teacher's Note:
a) The regular arrangement of particles gives crystals their characteristic geometric form.
b) Mention regular shape and sharp edges in the definition.

Question 5

(A) What is atom? State the main postulates of Dalton's atomic theory. [5 Marks]

Answer:
1. Atom: An atom is the smallest fundamental particle of matter that can take part in a chemical reaction.
2. Postulates of Dalton's Atomic Theory:
a. All matter is made up of very tiny, indivisible, and indestructible particles called atoms.
b. Atoms can neither be created nor be destroyed.
c. Atoms of a given element are identical in mass and chemical properties.
d. Atoms of different elements have different masses and chemical properties.
e. Atoms combine in the ratio of small whole numbers to form compounds.

Teacher's Note:
a) Dalton's atomic theory laid the foundation of modern chemical science.
b) Mention at least four to five key postulates for full marks.

(B) What is meant by the metal activity series? What are its important features? [5 Marks]

Answer:
1. Metal Activity Series: A list in which metals are arranged in the decreasing order of their chemical reactivity is called the metal reactivity series.
2. Important Features:
a. The ease with which a metal in solution loses electrons and forms positive ions decreases down the series from potassium to gold.
b. Hydrogen is included in the activity series because hydrogen, like metals, also loses an electron and becomes positively charged (\( \text{H}^+ \)) in most chemical reactions.
c. The series facilitates the comparative study of metals in terms of their reactivity.
d. Compounds of metals (oxides, carbonates, nitrates, hydroxides) can also be easily compared.

Teacher's Note:
a) Potassium is at the top (most reactive) and platinum or gold is at the bottom (least reactive).
b) This series helps predict displacement reactions between metals and their salt solutions.

Question 6

(A) Draw a diagram representing the atomic structures of the following: [5]

1. Hydrogen [1 Mark]
Answer:
[Figure: Diagram of Hydrogen atom showing 1 proton (1p) in nucleus and 1 electron in K shell, Elec. Config. 1, Valency = 1]
Teacher's Note:
a) Hydrogen has 1 proton and 1 electron with no neutrons in its common isotope (protium).
b) Label the nucleus and the single shell clearly.

2. Helium [1 Mark]
Answer:
[Figure: Diagram of Helium atom showing 2 protons and 2 neutrons (2p, 2n) in nucleus and 2 electrons in K shell, Elec. Config. 2, Valency = 0]
Teacher's Note:
a) Helium has a completely filled K shell, making it chemically inert.
b) Valency is zero as it does not need to gain or lose electrons.

3. Lithium [1 Mark]
Answer:
[Figure: Diagram of Lithium atom showing 3 protons and 4 neutrons (3p, 4n) in nucleus, K shell with 2 electrons, and L shell with 1 electron, Elec. Config. 2,1, Valency = 1]
Teacher's Note:
a) Lithium has 3 electrons distributed as 2 in K shell and 1 in L shell.
b) It exhibits a valency of 1 by losing its single valence electron.

4. Carbon [1 Mark]
Answer:
[Figure: Diagram of Carbon atom showing 6 protons and 6 neutrons (6p, 6n) in nucleus, K shell with 2 electrons, and L shell with 4 electrons, Elec. Config. 2,4, Valency = 4]
Teacher's Note:
a) Carbon has 6 electrons with 4 valence electrons in the L shell.
b) Its tetravalency allows it to form extensive covalent bonds.

5. Nitrogen [1 Mark]
Answer:
[Figure: Diagram of Nitrogen atom showing 7 protons and 7 neutrons (7p, 7n) in nucleus, K shell with 2 electrons, and L shell with 5 electrons, Elec. Config. 2,5, Valency = -3]
Teacher's Note:
a) Nitrogen has 7 electrons distributed as 2 in K shell and 5 in L shell.
b) It achieves stability by gaining 3 electrons, showing a valency of 3.

(B) Define the following terms: [5]

1. Atomic number [1 Mark]

Answer:
Atomic number is the number of protons or number of electrons in the atom of an element, denoted by 'Z'.
Atomic no. (Z) = Number of protons (p) = Number of electrons (e)

Teacher's Note:
a) Atomic number uniquely identifies an element.
b) In a neutral atom, proton count equals electron count.

2. valency [1 Mark]

Answer:
Valency is the number of electrons donated, accepted, or shared by an atom so as to achieve the stable electronic configuration of the nearest noble gas.

Teacher's Note:
a) Valency determines the combining capacity of an element.
b) Noble gases have a valency of zero.

3. Atomic mass number [1 Mark]

Answer:
The atomic mass number is defined as the sum of the number of protons and neutrons contained in the nucleus of an atom, denoted by 'A'.
Atomic mass number (A) = Number of protons (p) + Number of neutrons (n)

Teacher's Note:
a) Mass number represents almost the entire mass of an atom concentrated in its nucleus.
b) Electrons have negligible mass and are excluded from mass number calculations.

4. Valence shell [1 Mark]

Answer:
The outermost shell of an atom of an element is called the valence shell.

Teacher's Note:
a) Electrons present in the valence shell participate in chemical bonding.
b) Also known as the outermost energy level.

5. Periodic table [1 Mark]

Answer:
A periodic table is a tabular arrangement of chemical elements ordered by their atomic numbers, electronic configurations, and recurring chemical properties.

Teacher's Note:
a) Elements in the same group have similar chemical properties due to the same number of valence electrons.
b) It is an essential tool in chemistry for systematic study.

Question 7

(A) 1. Write the balanced chemical equations [3]
a. Lead + Carbon -> Lead + Carbon dioxide [1 Mark]

Answer:
\( \text{PbO} + \text{C} \rightarrow \text{Pb} + \text{CO}_2 \)

Teacher's Note:
a) Note that lead monoxide reacts with carbon to form lead and carbon dioxide.
b) Ensure the equation is balanced on both sides.

b. Calcium oxide + Water -> Calcium hydroxide [1 Mark]

Answer:
\( \text{CaO} + \text{H}_2\text{O} \rightarrow \text{Ca}(\text{OH})_2 \)

Teacher's Note:
a) This is an exothermic combination reaction producing slaked lime.
b) Formula for calcium hydroxide must include the correct bracket notation: \( \text{Ca}(\text{OH})_2 \).

c. Hydrogen + Chlorine -> Hydrochloric acid [1 Mark]

Answer:
\( \text{H}_2 + \text{Cl}_2 \rightarrow 2\text{HCl} \)

Teacher's Note:
a) Balance the chlorine and hydrogen atoms by placing coefficient 2 before \( \text{HCl} \).
b) Hydrogen and chlorine gases combine in the presence of sunlight.

2. State the formula of the following compounds: [2]
a. Calcium nitrate [1 Mark]

Answer:
[Figure: Criss-cross method showing Calcium (Ca²⁺) and Nitrate (\text{NO}_3^{1-}) combining to form \text{Ca}(\text{NO}_3)_2]
Formula: \( \text{Ca}(\text{NO}_3)_2 \)

Teacher'sNote:
a) Calcium has a valency of 2+ and the nitrate radical has a valency of 1-.
b) Enclose the polyatomic radical in parentheses before applying the subscript 2.

b. Sodium chloride [1 Mark]

Answer:
[Figure: Criss-cross method showing Sodium (\text{Na}^{1+}) and Chloride (\text{Cl}^{1-}) combining to form \text{NaCl}]
Formula: \( \text{NaCl} \)

Teacher's Note:
a) Sodium has a valency of 1+ and chloride has a valency of 1-.
b) The formula simplifies to \( \text{NaCl} \) as the valencies cancel out.

(B)

1. What are the types of mixtures? [2 Marks]

Answer:
1. There are two types of mixtures: homogeneous mixtures and heterogeneous mixtures.
2. Homogeneous mixture: A mixture which has uniform composition and properties throughout its mass (e.g., sugar solution, salt solution).
3. Heterogeneous mixture: A mixture which has different composition and properties in different parts of its mass (e.g., sand mixed with salt, sugar in oil).

Teacher's Note:
a) Homogeneous mixtures have no visible boundaries of separation.
b) Heterogeneous mixtures have distinct boundaries between components.

2. Differentiate between a compound and mixture. [3 Marks]

Answer:

Teacher's Note:
a) Questions asking for differences must always be answered in a tabular format.
b) Mention at least three to four clear points of distinction for full marks.