CIE iGCSE Co-Ordinated Science C2.5 Simple molecules and covalent bonds Exam Style Questions Paper 3

Question

(a) Chlorine reacts with hydrogen to make hydrogen chloride.
(i) Construct the word equation for this reaction.
(ii) Hydrogen chloride gas is a covalent compound.
Explain why hydrogen chloride gas is a covalent compound and not an ionic compound.
(iii) Fig. 5.1 shows the electronic structure in atoms of hydrogen and chlorine.

Draw a dot-and-cross diagram to show the arrangement of outer-shell electrons in a molecule of hydrogen chloride gas.

(iv) Hydrogen chloride dissolves in water to make dilute hydrochloric acid.
The pH of the hydrochloric acid is found using a pH probe.
Describe one other way of finding the pH of dilute hydrochloric acid.

(v) Suggest the pH of the dilute hydrochloric acid.

(b) Table 5.1 shows information about three Group VII elements.

(i) Complete Table 5.1.
(ii) State the name given to Group VII of the Periodic Table.
(iii) Explain why chlorine, bromine and iodine are all in Group VII of the Periodic Table.
Use ideas about electrons in your answer.

▶️Answer/Explanation

Ans :

5(a)(i) hydrogen + chlorine → hydrogen chloride ;
5(a)(ii) two non-metals bonding ;
5(a)(iii) one pair of bonding electrons ;

all else correct ;

5(a)(iv) use universal indicator ;
colour indicates the pH ;

5(a)(v) any value between 1 and 3 ;
5(b)(i)

5(b)(ii) halogens ;
5(b)(iii) all have 7 electrons in their outer shell ;

Question

(a) (i) Complete the dot-and-cross diagram in Fig. 8.1 to show the bonding in a molecule of methane, CH4.
Show only the outer-shell electrons.

(ii) State the name of the type of chemical bonding present in a molecule of methane.

Explain your answer.
type of chemical bonding ……………………………………………..
explanation …………………………………………………………………….

(b) Complete the word equation for the complete combustion of methane.
methane + ……………………………………→ …………………………………… + …………………………………..
(c) (i) State the name of the pollutant gas produced during the incomplete combustion of methane.
(ii) State one adverse effect of the gas you have named in (c)(i) on the health of humans.

▶️Answer/Explanation

Ans : 8(a)(i) 1 shared pair of electrons ;
all else correct ;
8(a)(ii) covalent ;
bonding between 2 non-metals / shared electron pairs ;
8(b) oxygen ;
carbon dioxide and water ;
8(c)(i) carbon monoxide ;
8(c)(ii) poisonous ;

Question

A sample of clean air is a mixture of oxygen, nitrogen and small quantities of noble gases, water vapour and carbon dioxide.
(a) State the percentage of oxygen gas and nitrogen gas in clean air.
oxygen = ………………………………………………….. %
nitrogen = ………………………………………………….. %
(b) State the name of a noble gas and give a use for this noble gas.
name …………………………………………………………………………

use …………………………………………………………………………

(c) Water is made when hydrogen gas reacts with oxygen gas.
Look at the symbol equation for the reaction between hydrogen and oxygen.
This equation is not balanced.

H₂ + O₂ → H₂O
(i) Explain why this equation is not balanced.

(ii) Another way that water is made is by the decomposition of hydrogen peroxide, H₂O₂.
Oxygen is also made.
Balance the symbol equation for this reaction.
…………H₂O₂ → …………H₂O + O₂

(iii) Complete the dot-and-cross diagram in Fig. 5.1 to show the bonding in a molecule of water, H₂O.
Show only the outer-shell electrons.

(iv) Name the type of chemical bonding in a molecule of water.
(v) Describe a chemical test for water and give the positive result.
test ………………………………………………………………………………
positive result ……………………………………………………………..

▶️Answer/Explanation

Ans : 5(a) oxygen – 21% ;
nitrogen – 78% ;
5(b) named noble gas ;
correct use ;
5(c)(i) nos of oxygen atoms is different on LHS to RHS ;
5(c)(ii) 2 H₂O₂ → 2 H₂O × O₂ ;

5(c)(iii)

1 shared pair ;
all else correct ;
5(c)(iv) covalent (bonds) ;
5(c)(v) anhydrous copper sulfate ;
white to blue ;
OR
cobalt chloride (paper) ;
blue to pink ;

Question

Fig. 5.1 shows three hydrocarbon molecules, A, B and C.

(a) (i) Complete the dot-and-cross diagram of molecule C. Show the outer shell electrons only.

(ii) Molecule C is a greenhouse gas.
State the name of one other greenhouse gas.

…………………………………………………………………………………………………………………….
(b) The reaction between molecule A and oxygen is exothermic.
Describe what is meant by an exothermic reaction.
…………………………………………………………………………………………………………………….
…………………………………………………………………………………………………………………….
(c) State and explain which molecule A, B or C represents a molecule of ethane.
molecule …………………..
explanation …………………………………………………………………………………………………..
…………………………………………………………………………………………………………………….

(d) (i) State and explain which molecule A, B or C is unsaturated.
molecule …………………..

explanation …………………………………………………………………………………………………….
……………………………………………………………………………………………………………………..
(ii) An orange solution is used to test if a hydrocarbon molecule is unsaturated or saturated.
State the name of this orange solution.

…………………………………………………………………………………………………………………….
(iii) Describe what is observed when the solution in (ii) is reacted with an unsaturated hydrocarbon molecule.
…………………………………………………………………………………………………………………….
…………………………………………………………………………………………………………………….

▶️Answer/Explanation

Ans: 5(a)(i) 1 shared pair ;
all else correct ;

5(a)(ii) water vapour ; carbon dioxide ;
5(b) (thermal) energy released ;
5(c) B – no mark and
has formula C$_{2}$H$_{6}$ ;

5(d)(i) A – no mark and

has two carbons but only four hydrogens / has general formula C$_{n}$H$_{2n}$ ;

5(d)(ii) (aqueous) bromine ;
5(d)(iii) decolourises ;

Question

(a) The list below shows the formulae of six molecules.

CH₄ Cl₂ CO N₂ NH₃ O₂

Identify which of the molecules from the list is:

(i) an ammonia molecule (Topic – C2.2)

▶️Answer/Explanation

NH₃

Explanation: Ammonia is a compound with the chemical formula NH₃, consisting of one nitrogen atom and three hydrogen atoms.

(ii) a molecule used in the treatment of water (Topic – C10.1)

▶️Answer/Explanation

Cl₂

Explanation: Chlorine (Cl₂) is commonly used in water treatment to kill bacteria and other pathogens.

(iii) a molecule formed during the incomplete combustion of carbon-containing substances (Topic – C6.2)

▶️Answer/Explanation

Explanation: Carbon monoxide (CO) is formed during the incomplete combustion of carbon-containing substances when there is insufficient oxygen.

(iv) the molecule of the gas that is 78% of clean dry air (Topic – C10.2)

▶️Answer/Explanation

N₂

Explanation: Nitrogen (N₂) makes up approximately 78% of the Earth’s atmosphere by volume.

(v) a molecule that contains four covalent bonds. (Topic – C2.5)

▶️Answer/Explanation

CH₄

Explanation: Methane (CH₄) contains four covalent bonds, one between the carbon atom and each of the four hydrogen atoms.

(b) Lime (calcium oxide) is made from limestone (calcium carbonate) by thermal decomposition.

(i) State the name of the other product of this reaction. (Topic – C6.1)

▶️Answer/Explanation

Carbon dioxide

Explanation: The thermal decomposition of calcium carbonate (CaCO₃) produces calcium oxide (CaO) and carbon dioxide (CO₂).

(ii) Calcium carbonate is not a fertiliser but is often added to soil. Explain why calcium carbonate is often added to soil. (Topic – C10.1)

▶️Answer/Explanation

To neutralize acidic soil

Explanation: Calcium carbonate is added to soil to neutralize acidity, improving soil pH and making it more suitable for plant growth.

(iii) List the three essential elements needed in a fertiliser. (Topic – C10.1)

▶️Answer/Explanation

1. Nitrogen
2. Phosphorus
3. Potassium

Explanation: These three elements are essential for plant growth and are commonly found in fertilizers to promote healthy development.

Question

(a) Orange bromine gas, Br$_{2}$, is put into the bottom of a gas jar which is immediately sealed.
After a short time, the bromine gas spreads out to fill the gas jar.
This process is called diffusion.
Fig. 11.1 shows the diffusion of bromine.

Describe the process of diffusion in terms of the movement of particles.
…………………………………………………………………………………………………………………………………
…………………………………………………………………………………………………………………………………
…………………………………………………………………………………………………………………………………
(b) Chlorine and bromine are both halogens.
State the name of one other element that is a halogen.
…………………………………………………………………………………………………………………………………
(c) Chlorine is in Period 3 of the Periodic Table.
Describe the change in metallic character across Period 3.
…………………………………………………………………………………………………………………………………
…………………………………………………………………………………………………………………………………
(d) Explain why the drinking water for a city is treated with chlorine.
…………………………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………………

(e) Complete the dot-and-cross diagram to show the bonding in a molecule of chlorine Cl$_{2}$.

Only show the outer shell electrons.

(f) Hydrogen and chlorine combine to make hydrogen chloride, HCl.
(i) Balance the symbol equation for this reaction.
H$_{2}$ + Cl$_{2}$→ …….. HCl
(ii) Explain why hydrogen chloride is a covalent compound and not an ionic compound.
………………………………………………………………………………………………………………………….
…………………………………………………………………………………………………………………….

▶️Answer/Explanation

Ans: 11(a) gas particles in (constant) random motion ;

(particles move) from region of high concentration to region of low concentration ;

11(b) iodine / fluorine / astatine ;
11(c) metallic to non-metallic (from left to right) ;

11(d) to kill, microorganisms/bacteria/pathogens ;
11(e) shared pair ;

correct number of electrons on each atom / 14 electrons in total ;

11(f)(i) 2 HCl ;
11(f)(ii) two non-metals bonding / shared pair of electrons ;

Question

Fig. 11.1 shows the apparatus and reagents used to make a salt.

(a) (i) State the name of process A shown in Fig. 11.1. (Sub-topic – C7.3)

▶️Answer/Explanation

filtration

Explanation: Process A is filtration, which is used to separate solid particles from a liquid.

(ii) State the name of the salt made in this experiment. (Sub-topic – C7.3)

▶️Answer/Explanation

copper sulfate

Explanation: The salt made in this experiment is copper sulfate.

(iii) Carbon dioxide gas is also made.

Explain why the bonds between the carbon atoms and the oxygen atoms in carbon dioxide are covalent. (Sub-topic – C2.5)

▶️Answer/Explanation

two non-metals bonding / electrons are shared

Explanation: The bonds between carbon and oxygen in carbon dioxide are covalent because they involve the sharing of electrons between the carbon and oxygen atoms. Carbon dioxide is a molecule formed by two non-metals (carbon and oxygen), and non-metals typically form covalent bonds by sharing electrons to achieve a stable electron configuration.

(iv) The temperature of the reacting mixture increases during the reaction.

State the name given to all chemical reactions that release heat. (Sub-topic – C5.1)

▶️Answer/Explanation

exothermic

Explanation: Chemical reactions that release heat are called exothermic reactions.

(v) The experiment is repeated using large pieces of copper carbonate instead of powdered copper carbonate.

State what happens to the rate of reaction. (Sub-topic – C6.2)

▶️Answer/Explanation

(rate of reaction) decreases

Explanation: Using large pieces of copper carbonate instead of powdered copper carbonate decreases the rate of reaction. This is because the surface area of the reactant is reduced, leading to fewer collisions between reactant particles per unit time.

(b) Copper carbonate is green. Copper is a transition metal.

State two other properties of transition metals which are not properties of all metals. (Sub-topic – C8.4)

▶️Answer/Explanation

1. high density; 2. catalyst; 3. variable valency (any two)

Explanation:

Two other properties of transition metals are:

They form colored compounds.
They can act as catalysts in chemical reactions.

(i) State the name given to a mixture of metals. (Sub-topic – C9.3)

▶️Answer/Explanation

alloy

Explanation: A mixture of metals is called an alloy.

(ii) State one advantage of brass compared to copper. (Sub-topic – C9.3)

▶️Answer/Explanation

stronger / any valid point

Explanation: One advantage of brass compared to copper is that it is stronger and more resistant to corrosion.

Question

8. (a) (i) Identify two molecules from Table 8.1 that are elements.