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Cambridge IGCSE chemistry

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Lesson 3, Topic 4
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Diffusion past papers

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 May 2010 Q2 (i) 

Ozone is a form of oxygen. Ozone is present in the upper atmosphere and it prevents dangerous solar radiation from reaching the earths surface. Some of the chemicals that diffuse into the upper atmosphere decomposes ozone. Chemicals that have this effect are methane (CH4) , Chloromethane (CH3Cl) and an oxide of nitrogen (NO2)

(i) Which of these three chemicals diffuses the most slowly? Give a reason for your choice.


Chloromethane is the chemical which diffused the slowest because it has the largest molecular mass


Nov 2010 Q6 (b) 

The following apparatus can be used to measure the rate of diffusion of a gas

(i) Explain why nitrogen diffuses faster than chlorine

(ii) Explain why the nitrogen diffuses faster at the higher temperature


(i) Because nitrogen has a smaller molecular mass than chlorine

(ii) At higher temperatures nitrogen molecules move faster because they have more energy


May 2014 Q3 (a), (b), (c) 

(a) Different gases diffuse at different speeds

(i) What is meant by the term diffusion?

(ii) What property of a gas molecule affects the speed at which it diffuses?

(b) Helium is a gas used to fill balloons. It is present in the air in very small quantities . Diffusion can be used to separate it from the air. Air at 1000°C is on one side of a porous barrier. The air which passes through the barrier has a larger amount of helium in it.

(i) Why does the air on the other side of the barrier contain more helium ?

(ii) Why is it an advantage to have the air at a high temperature ?



(i) The movement of molecules from a region of high concentration to a region of low concentration through random motion

(ii) molecular mass


(i) because helium atoms are lighter than molecules in air

(ii) Because molecules of air move faster at high temperature


May 2015 Q6 (c) 

(c) Gases diffuse, which means that they move to occupy the total available volume

(i) Explain, using kinetic particle theory why gases diffuse

(ii) When the colourless gases hydrogen bromide and ethylamine come into contact, a white solid is formed

CH3CH2NH2(g) + HBr(g) → CH3CH2NH3Br(s) (white solid)

The following apparatus can be used to compare the rates of diffusion of the two gases ethylamine and hydrogen bromide

Predict at which position A, B or C the white solid will form. Explain your answer


(i) because particles collide with each other and move randomly in all directions

(ii) C, because ethylamine has a lower molecular mass than HBr so ethyl amine diffuse faster


May 2016 Q6 (a) 

Concentrated ammonia solution gives off ammonia gas. Concentrated hydrochloric acid gives of hydrogen chloride gas. Ammonia, NH3 and hydrogen chloride,  HCl are both colourless gases

Ammonia reacts with hydrogen chloride to make the white solid ammonium chloride

After 10 minutes a white solid forms in the tube where the gases meet

(i) Write the chemical equation for the reaction of ammonia with hydrogen chloride

(ii) Name the process by which the ammonia and hydrogen chloride gases move in the tube

(iii) At which point A,B,C or D does the white solid form. Explain.

(iv) The experiment was repeated at a higher temperature . Predict how the results of the experiment would differ. Explain.


(i) NH3 + HCl → NH4Cl

(ii) diffusion

(iii) Solid forms at A , because ammonia molecules have a smaller mass and so move faster

(iv) solid forms in less time, because molecules have more energy and so move (diffuse) faster


May 2017 Q1 (a),(b) 

(a) Dust particles in the air move around in  a random way.

(i) What term describes the random movement of the dust particles ?

(ii) Identify the particles in the air which cause the random movement of the dust particles

(iii) Explain why the dust particles move in this way

(b) When chlorine gas Cl2 is put into a gas jar, it spreads out to fill the gas jar. When bromine, Br2 is put into a gas jar it also spreads out to fill the gas jar.

The process takes longer for bromine gas than chlorine

(i) What term describes the way that the gas particles spread out?

(ii) Use data from the periodic table to explain why bromine gas takes longer to fill  gas jar than chlorine gas

(iii) Explain why increasing the temperature increases the rate that the gas particles spread out


(i) Brownian motion

(ii) molecules of nitrogen or oxygen


  • Nitrogen or oxygen particles collide with the dust particles

  • the bombarding particles move randomly


(i) diffusion


  • Bromine has an Mr of 160 and chlorine has an Mr of 71 

  • heavier bromine molecules diffuses more slowly

(iii) particles have more energy/move faster