CIE iGCSE Co-Ordinated Science C10.2 Air quality and climate Exam Style Questions Paper 3
Question
(a) The proton numbers and nucleon numbers of carbon and hydrogen are shown in Table 11.1
i) State the number of neutrons in a carbon atom and the number of neutrons in a hydrogen atom.
(ii) State the number of electrons in a carbon atom.
(b) (i) State the difference between a saturated hydrocarbon and an unsaturated hydrocarbon.
(ii) Aqueous bromine is used to show the difference between a saturated hydrocarbon and an unsaturated hydrocarbon.
Describe the result of the test with a saturated hydrocarbon.
(c) Fig. 11.1 shows the apparatus used for the catalytic cracking of a saturated hydrocarbon.
(i) State the purpose of the catalyst in the cracking of a saturated hydrocarbon.
(ii) Cracking is an endothermic reaction.
State what is meant by an endothermic reaction.
(d) The complete combustion of hydrocarbons produces the greenhouse gas carbon dioxide.
(i) Name one other greenhouse gas.
(ii) Identify the gas in the air that reacts with hydrocarbons during combustion.
(iii) Carbon dioxide is one of the gases found in clean air.
State the name of the two gases found in clean air in the greatest proportions.
▶️Answer/Explanation
Ans : 11(a)(i) carbon = 6 ;
hydrogen = 0 ;
11(a)(ii) 6 ;
11(b)(i) unsaturated has a C=C double bond ;
11(b)(ii) aqueous bromine remains orange ;
11(c)(i) speed up the reaction ;
11(c)(ii) temperature decreases / thermal energy is absorbed (from the surroundings) ;
11(d)(i) methane ;
11(d)(ii) oxygen ;
11(d)(iii) oxygen ;
nitrogen ;
Question
(a) Choose from the following substances to answer the questions.
Each substance may be used once, more than once or not at all.
(i) Used as a chemical test for water.
(ii) Used as a solvent.
(iii) Used in aircraft parts.
(iv) Used in the manufacture of sulfuric acid.
(v) Used in the treatment of the water supply.
(vi) Used in the treatment of acidic soil.
(b) Sulfuric acid has the formula H$_{2}$SO$_{4}$.
(i) State the total number of atoms in one molecule of sulfuric acid.
(ii) State the total number of different elements found in one molecule of sulfuric acid.
(c) A student reacts magnesium with dilute sulfuric acid.
The equation for the reaction is shown.
Mg(s) + H$_{2}$SO$_{4}$(aq) MgSO$_{4}$(aq) + H$_{2}$(g)
(i) State the names of the two products of the reaction.
(ii) State the separation technique used by the student to remove any unreacted solid magnesium from the reaction mixture.
▶️Answer/Explanation
Ans : 2(a)(i) copper sulfate ;
2(a)(ii) ethanol ;
2(a)(iii) aluminium ;
2(a)(iv) sulfur ;
2(a)(v) chlorine ;
2(a)(vi) limestone ;
2(b)(i) 7 ;
2(b)(ii) 3 ;
2(c)(i) magnesium sulfate ;
hydrogen ;
2(c)(ii) filtration ;
Question
(a) Air is a mixture of gases.
Fig. 2.1 shows two pie charts representing samples of air, A and B.
(i) State which sample, A or B, represents clean air.
Explain your answer.
sample ……………………………………………………………………………………………………………….
explanation …………………………………………………………………………………………………………
………………………………………………………………………………………………………………………….
(ii) Identify gas X.
…………………………………………………………………………………………………………………….
(b) Sulfur dioxide is a common pollutant found in air.
(i) State one adverse effect of sulfur dioxide on the health of humans.
………………………………………………………………………………………………………………………….
…………………………………………………………………………………………………………………….
(ii) State one source of sulfur dioxide in the air.
…………………………………………………………………………………………………………………….
(iii) State one other common pollutant gas found in the air.
…………………………………………………………………………………………………………………….
(iv) Sulfur dioxide dissolves in rainwater to make acid rain.
Suggest a pH value for acid rain.
pH = …………………………………………………
(v) Farmers need to treat acidic soil to neutralise the acidity.
State the chemical substance used by farmers to treat soil acidity.
…………………………………………………………………………………………………………………….
(c) An atom of sulfur has an electronic structure 2, 8, 6.
(i) On Fig. 2.2, complete the electronic structure for this atom of sulfur.
(ii) Suggest how many electrons this sulfur atom gains to become a sulfide ion S$^{2-}$.
…………………………………………………………………………………………………………………….
▶️Answer/Explanation
Ans: 2(a)(i) sample A (no mark)
idea that proportion of gases (other than Nitrogen) is correct ;
ORA about sample B
2(a)(ii) nitrogen ;
2(b)(i) respiratory problems ;
2(b)(ii) combustion of fossil fuels (containing sulfur) / volcanoes ;
2(b)(iii) carbon monoxide / nitrogen oxides ;
2(b)(iv) 3-6 ;
2(b)(v) lime(stone) / CaCO$_{3}$ / CaO / Ca(OH)$_{2}$ ;
2(c)(i) 2 electrons in first shell and
8 electrons in 2nd shell and
6 electrons in 3rd shell ;
2(c)(ii) 2 ;
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) Limestone is insoluble in water.
Limestone is mixed with water and poured through filter paper.
(i) Explain how filter paper separates limestone from water.
(ii) The filtrate is tested to find its pH number.
One way of finding the pH number is to use a pH meter.
Describe one other way to find the pH number of the filtrate.
(b) Limestone is sometimes added to soil to reduce soil acidity.
Limestone is mainly calcium carbonate which reacts with acid.
(i) This soil treatment adds carbon dioxide to the atmosphere.
State one other reason why the amount of carbon dioxide in the atmosphere is increasing.
(ii) Describe how an increase in carbon dioxide in the atmosphere affects the environment.
(c) A student investigates the rate of reaction between excess dilute hydrochloric acid and a piece of limestone.
Fig. 11.1 shows the apparatus used.
The student measures the volume of carbon dioxide in the measuring cylinder every 20 seconds for 280 seconds.
A graph of the student’s results is shown in Fig. 11.2.
(i) Use Fig. 11.2 to find the total volume of carbon dioxide released.
volume = ………………………………………….. cm3
(ii) Use Fig. 11.2 to find the time when the reaction finished.
time = ……………………………………………… s
(iii) The student repeats the experiment at a higher temperature.
On Fig. 11.2, sketch a line to show the results.
(iv) Increasing the temperature increases the rate of reaction.
State two other ways the student can increase the rate of the reaction.
1 ……………………………………………………………………………………..
2 …………………………………………………………………………………….
▶️Answer/Explanation
Ans : 11(a)(i) limestone does not pass through (the holes in) the filter paper and water does pass through (the holes in) the filter paper ;
limestone particles are larger / water particles are smaller (than the holes in the filter paper) ;
11(a)(ii) use universal indicator (paper / solution) ;
use colour to determine pH from chart ;
11(b)(i) combustion of carbon containing fuels ;
11(b)(ii) global warming / climate change ;
11(c)(i) 45 (cm3) ;
11(c)(ii) 190 (s) ;
11(c)(iii) steeper line ;
but levelling off at 45 (cm3) ;
11(c)(iv) decrease particle size / use powder / increase surface area ;
increase acid concentration ;
Question
(a) The list gives the names of six compounds.
aluminium oxide
ammonium nitrate
carbon dioxide
lead bromide
sodium chloride
sulfur dioxide
Answer the questions about these compounds.
Each compound may be used once, more than once or not at all.
State which compound:
(i) has the formula PbBr2.
(ii) is a salt from which ammonia can be displaced.
(iii) is an acidic oxide.
(iv) is a greenhouse gas.
(v) is the main constituent of bauxite.
(b) Aluminium, copper and iron are all solid metals.
State three general physical properties of solid metals.
1 …………………………………………………………………………
2 …………………………………………………………………………
3 …………………………………………………………………………
(c) (i) Duralumin is an alloy of aluminium.
Table 2.1 shows the percentage composition of duralumin.
Calculate the mass of aluminium in 20 kg of duralumin.
mass of aluminium = ……………………………………………. kg
(ii) Table 2.2 shows the melting points of aluminium, copper, magnesium and duralumin.
Duralumin does not have a precise melting point but melts over a range of temperatures.
Explain why duralumin does not have a precise melting point.
▶️Answer/Explanation
Ans : 2(a)(i) lead bromide ;
2(a)(ii) ammonium nitrate ;
2(a)(iii) carbon dioxide / sulfur dioxide ;
2(a)(iv) carbon dioxide ;
2(a)(v) aluminium oxide ;
2(b) any three from:
conducts heat ;
conducts electricity ;
malleable ;
high melting/boiling point ;
2(c)(i) 95 / 100 × 20 = 19 (kg) ;
2(c)(ii) because it is not a pure substance / it is a mixture ;
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
(a) The list gives the names of seven elements.
aluminium carbon copper chlorine
helium phosphorus sulfur
State which of the elements in the list:
(i) forms diamond and graphite ……………………………………………………………………………
(ii) is a halogen …………………………………………………………………………………………………..
(iii) is electroplated onto another metal …………………………………………………………………..
(iv) is extracted from bauxite …………………………………………………………………………………
(v) is used in the manufacture of sulfuric acid …………………………………………………………
(vi) is used to sterilise drinking water ……………………………………………………………………..
(b) The formula of phosphorus oxide is P4O10.
State the number of atoms of phosphorus and of oxygen contained in one molecule of phosphorus oxide.
phosphorus ………………………………………………………………..
oxygen ………………………………………………………………………..
(c) Explain why phosphorus oxide makes an acidic solution when it dissolves in water.
(d) Phosphorus is contained in fertilisers.
State the two other elements commonly found in fertilisers.
1 …………………………………………………………………………
2 …………………………………………………………………………
▶️Answer/Explanation
Ans : 2(a)(i) carbon ;
2(a)(ii) chlorine ;
2(a)(iii) copper ;
2(a)(iv) aluminium ;
2(a)(v) sulfur ;
2(a)(vi) chlorine ;
2(b) phosphorus 4 oxygen 10 ;
2(c) non-metal oxides are acidic or non-metal oxides form acidic solutions ;
2(d) nitrogen ;
potassium ;
Question
(a) Fig. 5.1 is a chart showing the composition of clean air.
Use your knowledge of the composition of clean air to determine the percentage of other gases in clean air.
Show your working.
percentage = …………………………………………….. %
(b) (i) Argon is one of the other gases in clean air.
Use the Periodic Table on page 24 to determine the electronic structure of an argon atom.
(ii) Use your answer to (b)(i) to explain why argon is very unreactive.
(iii) State one use for argon gas that depends on it being unreactive.
(c) (i) Small quantities of carbon dioxide are found in the air.
State one source of the carbon dioxide found in the air.
(ii) Carbon dioxide is a compound formed from the elements carbon and oxygen.
Describe the difference between an element and a compound.
(iii) Carbon dioxide is a greenhouse gas.
State the name of one other greenhouse gas.
(d) (i) Water is neutral.
State the pH value of pure water.
pH = …………………………………………………
(ii) Carbon dioxide in the air dissolves in rainwater. This causes the rainwater to become slightly acidic.
Suggest a pH value for this rainwater.
pH = …………………………………………………
▶️Answer/Explanation
Ans : 5(a) 78% and/or 21% seen ;
other gases = 1% ;
5(b)(i) 2,8,8 ;
5(b)(ii) full outer shell ;
5(b)(iii) lamps;
5(c)(i) combustion of fossil fuels ;
5(c)(ii) any one from (molecule of an) element contains only one type of atom ;
compound consists of atoms of two or more elements (bound together) ;
5(c)(iii) methane ;
5(d)(i) 7 ;
5(d)(ii) 4–6 ;
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
CO
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)
(i) State the percentage of oxygen and of nitrogen in a sample of clean air.
(ii) A sample of air collected near a busy road contains carbon monoxide. Carbon monoxide is made by car engines. Describe how carbon monoxide is made by car engines.
(iii) State one harmful effect of carbon monoxide on the health of a human.
▶️Answer/Explanation
Answer:
(i) Clean air contains approximately:
- 21% oxygen
- 78% nitrogen
(ii) Carbon monoxide is produced by the incomplete combustion of hydrocarbons in car engines. When there is insufficient oxygen, carbon reacts with oxygen to form carbon monoxide (CO) instead of carbon dioxide (CO₂).
(iii) Carbon monoxide binds to haemoglobin in red blood cells, forming carboxyhaemoglobin, which reduces the blood’s ability to carry oxygen. This can lead to oxygen deprivation, causing symptoms like dizziness, headaches, and even death in severe cases.
Explanation:
(i) Clean air is primarily composed of nitrogen (78%) and oxygen (21%), with trace amounts of other gases like argon and carbon dioxide.
(ii) Incomplete combustion occurs when there is not enough oxygen for complete combustion. Instead of producing carbon dioxide, carbon monoxide is formed, which is a toxic gas.
(iii) Carbon monoxide is dangerous because it binds to haemoglobin more strongly than oxygen, preventing oxygen from being transported around the body. This can lead to tissue damage and organ failure.
Question(b)
The atmosphere contains small quantities of sulfur dioxide. Sulfur dioxide causes acid rain.
(i) Suggest a pH for acid rain.
(ii) Suggest one adverse effect of acid rain on buildings.
(iii) Sulfur dioxide is made when sulfur is burned in oxygen. This reaction is called an oxidation reaction. State the meaning of the term oxidation.
▶️Answer/Explanation
Answer:
(i) The pH of acid rain is typically less than 7, often around 4-5.
(ii) Acid rain can cause the corrosion of building materials, such as limestone and marble, leading to the deterioration of structures over time.
(iii) Oxidation is the gain of oxygen or the loss of electrons by a substance during a chemical reaction.
Explanation:
(i) Acid rain has a lower pH than normal rainwater (which has a pH of around 5.6) due to the presence of acidic gases like sulfur dioxide and nitrogen oxides.
(ii) Acid rain reacts with calcium carbonate in building materials, forming soluble compounds that wash away, weakening the structure.
(iii) Oxidation involves the addition of oxygen or the removal of electrons from a substance. For example, when sulfur burns in oxygen, it forms sulfur dioxide (SO₂), which is an oxidation reaction.
Question (a)
Methane is a greenhouse gas.
Name one other greenhouse gas.
▶️Answer/Explanation
Answer: Carbon dioxide (CO2)
Explanation: Greenhouse gases trap heat in the Earth’s atmosphere, contributing to global warming. Carbon dioxide is one of the most common greenhouse gases, along with methane, water vapor, and nitrous oxide.
Question (b)
Methane is completely combusted in oxygen. Carbon dioxide and water are made.
Balance the symbol equation for this reaction.
▶️Answer/Explanation
Answer: (CH4 +) 2 (O2 → CO2 +) 2 (H2O) ;;
Explanation: The balanced equation for the complete combustion of methane is: \[ \text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O} \] This equation is balanced because there are equal numbers of each type of atom on both sides of the equation:
- 1 carbon atom on both sides
- 4 hydrogen atoms on both sides
- 4 oxygen atoms on both sides
Question (c)
Complete Fig. 11.1 to show the structural formula of methane.
▶️Answer/Explanation
Answer: The structural formula of methane is: \[ \text{H} – \text{C} – \text{H} \] \[ \quad \quad \text{H} \quad \text{H} \]
Explanation: Methane (CH4) consists of one carbon atom bonded to four hydrogen atoms in a tetrahedral arrangement. The structural formula shows the arrangement of atoms and bonds in the molecule.
Question (d)(i)
Methane is the main constituent of a fossil fuel.
Name this fossil fuel.
▶️Answer/Explanation
Answer: Natural gas
Explanation: Methane is the primary component of natural gas, which is a fossil fuel used for heating, cooking, and electricity generation.
Question (d)(ii)
Name one other fossil fuel.
▶️Answer/Explanation
Answer: Coal
Explanation: Coal is another fossil fuel, formed from the remains of ancient plants. It is primarily composed of carbon and is used for electricity generation and industrial processes.
Question (e)(i)
Methane is an alkane. Ethene is an alkene.
State the difference in structure between an alkane and an alkene.
▶️Answer/Explanation
Answer: Alkanes have only single bonds between carbon atoms, while alkenes have at least one double bond between carbon atoms.
Explanation: Alkanes are saturated hydrocarbons with single bonds (C-C), while alkenes are unsaturated hydrocarbons with at least one double bond (C=C). For example, methane (CH4) is an alkane, and ethene (C2H4) is an alkene.
Question (e)(ii)
A student uses a chemical reagent to show the difference between an alkane and an alkene.
Tick (✓) the correct box to show the chemical reagent used.
aqueous bromine
limewater
litmus
universal indicator
▶️Answer/Explanation
Answer: Aqueous bromine
Explanation: Aqueous bromine is used to distinguish between alkanes and alkenes. Alkenes react with bromine, causing the orange-brown color of bromine to disappear, while alkanes do not react. This is because alkenes have a double bond that can undergo addition reactions.
Question (f)
Ethene, C2H4, reacts with steam to produce ethanol, C2H5OH.
Ethene is a hydrocarbon. Explain why ethanol is not a hydrocarbon.
▶️Answer/Explanation
Answer: Ethanol contains oxygen, while hydrocarbons contain only hydrogen and carbon.
Explanation: Hydrocarbons are compounds that contain only hydrogen and carbon atoms. Ethanol (C2H5OH) contains an oxygen atom in addition to hydrogen and carbon, so it is not a hydrocarbon. It is classified as an alcohol.
Question
Impurities are unwanted substances in mixtures. Impurities can be removed using purification techniques.
(a) Some gaseous oxides are impurities which pollute air.
(i) Name the gaseous oxide that forms when sulfur burns.
▶️Answer/Explanation
Sulfur dioxide (SO2)
(ii) Describe how this gaseous oxide can damage the environment.
▶️Answer/Explanation
Sulfur dioxide can cause acid rain, which damages buildings, statues, and vegetation. It can also lead to respiratory problems in humans and harm aquatic life in rivers and lakes.
(iii) Identify two gaseous oxides, other than water vapour or carbon dioxide, that can form during the combustion of hydrocarbons in air.
▶️Answer/Explanation
1. Carbon monoxide (CO)
2. Oxides of nitrogen (NOx)
(b) A scientist uses the apparatus shown in Fig. 5.1 to test four food dyes, P, Q, R and S.
Fig. 5.2 shows her results for food dyes Q, R and S.
(i) Name this method the scientist uses to test the food dyes.
▶️Answer/Explanation
Chromatography
(ii) Describe what happens inside the container that produces the results shown in Fig. 5.2.
▶️Answer/Explanation
Water moves up the paper by capillary action. The dyes dissolve in the water and are carried along with it. Different dyes move at different speeds, causing them to separate and form distinct spots at different heights on the paper.
(iii) Food dye P is made by mixing together food dyes Q, R and S. Complete Fig. 5.2 by drawing the result for food dye P.
▶️Answer/Explanation
Food dye P will show three spots vertically in a line above the letter P, corresponding to the positions of the spots for dyes Q, R, and S.
(iv) Suggest why impurities in food dyes must be removed before the dyes are used to colour food.
▶️Answer/Explanation
Impurities in food dyes may be harmful or toxic, and their presence could affect the safety and quality of the food. Removing impurities ensures that the food dyes are safe for consumption and provide the desired color without adverse effects.
Question
Fig. 8.1 is a chart showing the composition of clean air.
(a) (i) Suggest the percentage of the section labelled other gases.
Use your knowledge of the amounts of nitrogen and oxygen in air.
▶️Answer/Explanation
1%
Explanation: Clean air is composed of approximately 78% nitrogen and 21% oxygen. The remaining 1% consists of other gases such as argon, carbon dioxide, and trace amounts of other gases.
(ii) The other gases in Fig. 8.1 include very unreactive elements.
State one of these unreactive elements.
▶️Answer/Explanation
Argon
Explanation: Argon is a noble gas and is very unreactive due to its full outer electron shell.
(iii) State, in terms of electronic structure of its atoms, why the element in (a)(ii) is unreactive.
▶️Answer/Explanation
It has a full outer shell of electrons.
Explanation: Noble gases like argon have a complete outer electron shell, making them stable and unreactive.
(iv) State two harmful gaseous oxides that are present in polluted air.
▶️Answer/Explanation
1. Sulfur dioxide
2. Nitrogen oxides
Explanation: Sulfur dioxide (SO2) and nitrogen oxides (NOx) are harmful gaseous oxides commonly found in polluted air, contributing to acid rain and respiratory problems.
(b) Fig. 8.2 shows apparatus used in the electrolysis of dilute sulfuric acid.
(i) State the term used to describe the dilute sulfuric acid in this electrolysis experiment.
▶️Answer/Explanation
Electrolyte
Explanation: The dilute sulfuric acid acts as an electrolyte, allowing the conduction of electricity through the movement of ions.
(ii) Identify the gas produced at the cathode.
▶️Answer/Explanation
Hydrogen
Explanation: During the electrolysis of dilute sulfuric acid, hydrogen gas is produced at the cathode.
(c) Fig. 8.3 shows apparatus a student uses to investigate the rate of reaction between calcium carbonate and dilute hydrochloric acid.
(i) Complete the word equation for this reaction.
▶️Answer/Explanation
hydrochloric acid + calcium carbonate → calcium chloride + carbon dioxide + water
Explanation: The reaction between hydrochloric acid and calcium carbonate produces calcium chloride, carbon dioxide, and water.
(ii) It takes 50 seconds for 80 cm3 of gas to collect in the gas syringe.
Calculate the rate at which the gas is produced in cm3/s.
▶️Answer/Explanation
1.6 cm3/s
Explanation: The rate of gas production is calculated by dividing the volume of gas by the time taken: 80 cm3 / 50 s = 1.6 cm3/s.
(iii) State one change to the reaction conditions that reduces the rate of reaction.
▶️Answer/Explanation
Decrease the temperature
Explanation: Lowering the temperature reduces the kinetic energy of the particles, leading to fewer effective collisions and a slower reaction rate.
Question
(a) State the names and percentages of the two main gases in air.
▶️Answer/Explanation
Answer: The two main gases in air are nitrogen (78%) and oxygen (21%).
(b) Fig. 11.1 shows the composition of two gas mixtures Y and Z.
(i) Deduce which gas mixture, Y or Z, is natural gas. Explain your answer.
▶️Answer/Explanation
Answer: Mixture Y is natural gas because natural gas is mainly composed of methane, and mixture Y has a high percentage of methane compared to mixture Z.
(ii) Describe how gas mixture Z is tested to show it contains carbon dioxide.
▶️Answer/Explanation
Answer: To test for carbon dioxide in gas mixture Z, bubble the gas through limewater. If carbon dioxide is present, the limewater will turn milky or cloudy due to the formation of calcium carbonate.
(c) Methane and propane are used as fuels.
(i) Fig. 11.2 shows a molecule of propane.
State why propane is described as:
- a hydrocarbon
- a saturated compound.
▶️Answer/Explanation
Answer:
- Propane is described as a hydrocarbon because it consists only of hydrogen and carbon atoms.
- Propane is described as a saturated compound because it contains only single bonds between carbon atoms, meaning it has the maximum number of hydrogen atoms possible.
(ii) Identify the compounds that are produced by the complete combustion of methane.
▶️Answer/Explanation
Answer: The compounds produced by the complete combustion of methane are carbon dioxide (CO2) and water (H2O).