CIE iGCSE Co-Ordinated Science C6.1 Physical and chemical changes Exam Style Questions Paper 3
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) (i) An atom of calcium has 20 protons and 20 neutrons.
State the number of electrons in this calcium atom.
(ii) State the number of electrons in one calcium ion, Ca2+.
(b) Limestone (calcium carbonate) and lime (calcium oxide) are both calcium compounds.
Fig. 11.1 shows a limekiln in which calcium carbonate thermally decomposes to make calcium oxide and carbon dioxide.
(i) Write the word equation for the thermal decomposition of calcium carbonate.
(ii) The mass of calcium oxide made in this reaction is always less than the mass of calcium carbonate used.
Suggest why.
(iii) The decomposition of calcium carbonate to calcium oxide is an endothermic reaction.
State the meaning of the term endothermic.
(iv) One use for limestone is in the production of lime.
State one other use of limestone.
(v) Suggest why the calcium carbonate is broken into small pieces before being thermally decomposed.
(c) Calcium carbonate has the formula CaCO3.
(i) State the number of different elements shown in this formula.
(ii) State the total number of atoms shown in this formula.
▶️Answer/Explanation
Ans : 11(a)(i) 20 (electrons) ;
11(a)(ii) 18 (electrons) ;
11(b)(i) calcium carbonate → calcium oxide + carbon dioxide ;
11(b)(ii) carbon dioxide released ;
11(b)(iii) (thermal) energy taken in ;
11(b)(iv) neutralising acidified soil ;
11(b)(v) to increase surface area ;
so that reaction is faster ;
11(c)(i) three ;
11(c)(ii) five ;
Question
(a) An isotope of magnesium has a proton number (atomic number) of 12 and a nucleon number (mass number) of 26.
Complete Table 5.1 to show the numbers of neutrons and electrons in an atom of this isotope.
(b) Fig. 5.1 shows part of the reactivity series of metals.
Magnesium reacts slowly with cold water.
Use the reactivity series to predict the result when calcium reacts with cold water.
Explain your answer.
prediction ………………………………………………………………………………………………………………….
…………………………………………………………………………………………………………………………………
explanation ……………………………………………………………………………………………………………….
…………………………………………………………………………………………………………………………………
(c) Magnesium reacts with carbon dioxide. Magnesium oxide and carbon are made.
(i) Write the word equation for this reaction.
(ii) The reaction between magnesium and carbon dioxide is exothermic.
State what is meant by the term exothermic.
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
(d) Platinum is a transition metal. Magnesium is not a transition metal.
State two properties of platinum that are not properties of magnesium.
1 …………………………………………………………………………………………………………………..
2 ………………………………………………………………………………………………………………….
(e) Table 5.2 shows the composition of an alloy of magnesium.
Complete the table with the % by mass of magnesium.
Calculate the mass of magnesium in 1.0kg of the alloy.
mass = ……………………………………………. kg
▶️Answer/Explanation
Ans: 5(a)
5(b) calcium reacts quickly/quicker ;
calcium is higher in reactivity series than magnesium ;
5(c)(i) magnesium + carbon dioxide → magnesium oxide + carbon ;
5(c)(ii) releases (thermal) energy ;
5(d) any two from:
forms coloured compounds ;
acts as catalyst ;
variable valency ;
5(e) 91.5 (%) ;
0.915 (kg) ;
Question (a)
The thermal decomposition of calcium carbonate makes calcium oxide and carbon dioxide.
(i) Write the word equation for the thermal decomposition of calcium carbonate.
(ii) The production of lime (calcium oxide) from limestone (calcium carbonate) is one use of limestone. State one other use of limestone.
(iii) Calcium carbonate has the formula CaCO3. State the number of different elements present in calcium carbonate.
(iv) Explain why the thermal decomposition of calcium carbonate is a chemical change and not a physical change.
(v) The thermal decomposition of calcium carbonate is an endothermic reaction. State what is meant by an endothermic reaction.
▶️Answer/Explanation
(i) The word equation for the thermal decomposition of calcium carbonate is:
calcium carbonate → calcium oxide + carbon dioxide
(ii) Another use of limestone is neutralizing acidic soil or as a building material.
(iii) Calcium carbonate contains 3 different elements: calcium (Ca), carbon (C), and oxygen (O).
(iv) The thermal decomposition of calcium carbonate is a chemical change because new substances (calcium oxide and carbon dioxide) are formed, and the change is irreversible.
(v) An endothermic reaction is a reaction that takes in thermal energy from the surroundings, resulting in a decrease in temperature.
Detailed Solution:
(i) The thermal decomposition of calcium carbonate involves breaking down the compound into calcium oxide and carbon dioxide when heated. The word equation represents this process.
(ii) Limestone is widely used in agriculture to neutralize acidic soil and in construction as a building material or to produce cement.
(iii) The formula CaCO3 indicates that calcium carbonate consists of calcium (Ca), carbon (C), and oxygen (O), totaling 3 different elements.
(iv) A chemical change involves the formation of new substances with different properties. In this case, calcium carbonate decomposes into calcium oxide and carbon dioxide, which are chemically different from the original compound.
(v) Endothermic reactions absorb energy from the surroundings, often in the form of heat. This energy is used to break chemical bonds during the reaction.
Question(b)
(i) Carbon dioxide is a compound, but carbon is an element. State the difference between an element and a compound.
(ii) Carbon is a solid, and carbon dioxide is a gas. Complete Table 2.1 to describe the differences between a solid and a gas.
Table 2.1
| Particle Separation | Particle Arrangement | Particle Motion | |
|---|---|---|---|
| Solid | Vibrate about a fixed point | ||
| Gas | Move rapidly in all directions |
▶️Answer/Explanation
(i) An element contains only one type of atom, while a compound contains two or more elements chemically combined.
(ii) The completed table is:
| Particle Separation | Particle Arrangement | Particle Motion | |
|---|---|---|---|
| Solid | Close together | Regular | Vibrate about a fixed point |
| Gas | Far apart | Random | Move rapidly in all directions |
Detailed Solution:
(i) An element is a pure substance made up of only one type of atom (e.g., carbon). A compound is a substance formed when two or more elements are chemically bonded together (e.g., carbon dioxide).
(ii) In solids, particles are closely packed in a regular arrangement and vibrate about fixed positions. In gases, particles are far apart, arranged randomly, and move rapidly in all directions.
Question(c)
Carbon exists in many forms, including diamond and graphite. Diamond and graphite are described as giant covalent structures. Fig. 2.1 shows three covalent structures, A, B, and C.
(i) State which structure represents diamond.
(ii) State which structure represents graphite.
(iii) State why the bonding between the carbon atoms in diamond is covalent and not ionic.
▶️Answer/Explanation
(i) Structure A represents diamond.
(ii) Structure B represents graphite.
(iii) The bonding between carbon atoms in diamond is covalent because the atoms share electrons to form strong covalent bonds, rather than transferring electrons as in ionic bonding.
Detailed Solution:
(i) Diamond has a tetrahedral structure where each carbon atom is bonded to four other carbon atoms, forming a rigid 3D network. This matches structure A.
(ii) Graphite has a layered structure where each carbon atom is bonded to three others, forming hexagonal rings. The layers are held together by weak forces, allowing them to slide over each other. This matches structure B.
(iii) Covalent bonding involves the sharing of electrons between atoms. In diamond, each carbon atom shares electrons with four neighboring carbon atoms, forming strong covalent bonds. Ionic bonding involves the transfer of electrons, which does not occur in diamond.