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 ;