The Nature of Substances & Chemical Reactions (WJEC GCSE Chemistry)

Paper chromatography can be used to identify plant leaf pigments. This process uses a chemical called acetone as the solvent instead of water.

The diagram shows the chromatogram of plant leaf extracts A, B, C and D in acetone.

All of the extracts contain a mixture of pigments with different R_f values.

For which plant leaf extract, A, B, C or D, is the highest R_f value 0.60?

Give your reasoning.

Extract ........................................................

Reasoning ........................................................................................................

Explain why the pigments travel different distances on the chromatogram

One of the extracts contains a pigment which is insoluble in acetone.

State the letter of this extract. Explain your choice.

Extract ........................................................

Explanation ...........................................................................................

The chemical formula of the solvent acetone is C₃H₆O. Calculate the percentage by mass of carbon in acetone.

The relative formula mass (M_r) of acetone is 58.

A_r (C) = 12

Percentage = ........................................ %

The table shows the solubility of potassium nitrate in water at different temperatures.

i) Choose a suitable scale for the y-axis and plot the data on the grid.

Draw a line of best fit.

[4]

ii) Calculate the mass in grams of potassium nitrate that would be crystallised if 250 cm³ of a saturated solution were cooled from 80°C to 30°C.

[3]

Mass = ................................... g

Ethanol can be separated from water by distillation using the apparatus shown.

Explain how ethanol and water are separated by distillation.

The diagrams represent five substances, A-E.

i) State which diagram, A-E, represents an element. Give a reason for your answer.

[2]

Element ………...............................................

Reason ...................................................................................

ii) State which diagram, A-E, represents carbon dioxide, CO₂.

[1]

iii) Give the formula of the substance represented by diagram C.

[1]

When copper(II) sulfate solution reacts with sodium hydroxide, it forms copper(II) hydroxide, Cu(OH)₂, and sodium sulfate, Na₂SO₄.

i) Write a word equation for the reaction taking place.

[1]

ii) Give the number of atoms of sulfur found in the formula Na₂SO₄.

[1]

iii) Give the total number of atoms in the formula Cu(OH)₂.

[1]

Iron(III) sulfate reacts with sodium hydroxide to produce iron(III) hydroxide.

Write the formula of iron(III) hydroxide.

A food company was accused of using a banned substance in its sweets. Scientists tested four dyes, A-D, to find out if this was true or not. The results are shown below.

Name the method used.

Dye B was found to have an R_f value of 0.428.

i) Use the following equation to calculate the distance moved by dye B.

[2]

distance moved by dye B = R_f × distance moved by solvent front

Distance moved = ........................................... cm

ii) Complete the diagram above to show the position of dye B.

[1]

Identify which dye, A-D, contains the banned substance. Give a reason for your answer.

Dye ............................................................

Reason ............................................................

The diagrams below show three methods of separating mixtures.

i) A student was given a solution of sodium chloride. State which method, A, B or C, he could use in order to obtain a sample of pure water from the solution. Explain how the method works.

[3]

Method ............................................................

Explanation ............................................................

ii) 500 g of solution was found to contain 43 g of sodium chloride. Calculate the percentage of sodium chloride in the solution.

[2]

Percentage = ........................................... %

Group 7 ions, chloride, bromide and iodide, can be identified using silver nitrate solution.

Complete the following table to show the colours of the precipitates produced by these ions.

The formula for calcium chloride is CaCl₂.

Give the names of the two elements present in this compound.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . .      and    . . . . . . . . . . . . . . . . . . . . . . . . . . .

Name a metal which is in the same period of the Periodic Table as argon.

i) A formula for nitrogen oxide is N₂O.

A molecule of nitrogen oxide can be drawn as follows.

Give the name of the element which is represented by 

ii) Draw your own key to represent hydrogen and carbon atoms. Use your key to draw a molecule of methane, CH₄.            

Key

Methane

Fizzy drinks such as lemonade contain carbonic acid. Each molecule of carbonic acid contains two atoms of hydrogen, one atom of carbon and three atoms of oxygen.

Give the formula for carbonic acid.   . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Complete the following table.

Explain how a sodium atom and a bromine atom form ions when they react to make sodium bromide.

Aluminium reacts with chlorine to form aluminium chloride. Complete and balance the symbol equation for the reaction taking place.

Al      +      Cl₂                    . . . . . . . . . .

Aluminium oxide, Al₂O₃, is found in bauxite.

i) Calculate the relative formula mass (M_r) of aluminium oxide, Al₂O₃.                    

[2]

A_r (Al) = 27        A_r (O) = 16

M_r (Al₂O₃) = . . . . . . . . . . . . . . . .  

ii) Using your answer from part (i) calculate the percentage of oxygen present in aluminium oxide, Al₂O₃.

[1]

Percentage oxygen present = . . . . . . . . . . . . . . . . . ..  %

Chromatography can be used to separate the pigments in ink.

Describe how chromatography can be used to determine whether two inks contain the same pigments. 

Your answer should include

• a description of how chromatography is carried out

• a description of what happens during the process

• how the results would show whether the two inks contain identical or different pigments.

 You may include a diagram in your answer.

Higher tier only

When sodium hydrogencarbonate is heated, it decomposes to form sodium carbonate, water and carbon dioxide gas.

2NaHCO₃              Na₂CO₃     +    H₂O    +    CO₂

Calculate the mass of sodium carbonate that can be produced from 126 g of sodium hydrogencarbonate.

A_r(Na) = 23;      A_r(C) = 12;        A_r(O) = 16;          Ar(H) = 1

Mass of sodium carbonate = . . . . . . . . . .  . . .  g

It was found that the actual mass obtained was only 70 g. Calculate the percentage yield for this reaction and give your answer to two decimal places.

Percentage yield = . . . . . . . . . . . . .. . . . . .  %

The following diagrams represent atoms of 5 different elements, A, B, C, D and E.

A, B, C, D and E are not chemical symbols.

i) Give the electronic structure of E. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

[1]

ii) Which letter represents aluminium?  . . . . . . . . . . . . . . . . . . . . . . . . . .

[1]

iii) Give the letters of the two elements which are found in the same group of the Periodic Table and give a reason for your choice.

[2]

i) Calculate the relative formula mass (Mr) of sodium hydroxide, NaOH.

[1]

Ar(Na) = 23         Ar(O) = 16         Ar(H) = 1

Relative formula mass = . . . . . . . . . . . . . . . . . . .

ii) Using your answer to part (i), calculate the percentage by mass of oxygen in sodium hydroxide, NaOH.

[2]

Percentage by mass of oxygen = . . . . . . . . . . . . . . . . .  %

Give the observations made during the reactions of lithium, sodium and potassium with water. Explain the trend in reactivity in terms of electronic structure.

Higher tier only

Potassium reacts with oxygen to produce potassium oxide.

4K + O₂ → 2K₂O

i) Calculate the maximum mass of potassium oxide that could be produced when 15.6 g of potassium reacts completely with oxygen.

[3]

A_r(K) = 39 A_r(O) = 16

Mass = .......................... g

ii) Another reaction requires 0.050 mol of oxygen gas. Calculate the number of molecules in 0.050 mol of oxygen gas. Give your answer in standard form.

[2]

Avogadro’s number = 6.0 × 10²³

Number of molecules = ...............................

Sodium is extracted from sodium chloride.

The overall reaction taking place is shown in the equation below.

2NaCl 2Na + Cl₂

i) When carrying out the reaction 120 kg of sodium chloride was found to produce 38.05 kg of sodium.

Calculate the maximum possible mass of sodium that could be produced and use this figure to calculate the percentage yield of this reaction.

[4]

A_r(Na) = 23                A_r(Cl) = 35.5

Maximum possible mass = ............................................................ kg

Percentage yield = ............................................................ %

ii) Suggest a reason why the yield is less than 100%.

[1]

iii) Suggest why this reaction must be carried out under dry conditions.

[1] 

A sample of lithium is found to contain two isotopes.

i) Calculate the relative atomic mass (A_r) of lithium. Give your answer to three significant figures.

[3]

A_r = .........................................

ii) State the difference between the atomic structures of lithium-6 and lithium-7.

[1]

Many metal ores contain sulfides. Chalcocite is an important copper ore which contains copper(I) sulfide, Cu₂S.

Copper can be obtained from the ore by heating in air.

The equation for the reaction that takes place is as follows.

Cu₂S  +   O₂         2Cu   +   SO₂

Use the above equation to calculate the mass of copper produced on reacting 20.5 tonnes of copper(I) sulfide with an excess of oxygen.

Ar(Cu) = 64                Ar(S) = 32

Mass of copper = . . . . . . . . . . . . .  . . . . .  tonnes

When the extraction was carried out with 20.5 tonnes of chalcocite only 12.3 tonnes of copper was formed.

Calculate the percentage of impurity present in the ore.

Percentage of impurity = . . . . . . . . . . . . . . .  %

Higher tier only 

This question is about power stations.

Coal-fired power stations are being phased out as the supply of coal decreases and greater restrictions on emissions are implemented.

Coal typically contains between 0.2% and 5% sulfur by dry weight.

Calculate the average number of moles of sulfur in 75 tonnes of coal. 

Relative atomic mass (A_r):     S = 32

1 tonne = 1 x 10⁶ g

Higher tier only

Calculate the volume of sulfur dioxide released from the combustion of 75 tonnes of typical coal.

Higher tier only

Natural gas is replacing coal in some power stations. 

The typical sulphur content of natural gas is 5.5 x 10^–6 g / dm³. 

An articulated fuel tanker truck can carry up to 36000 dm³ of natural gas. 

Calculate the volume, in cm³, of sulfur dioxide that could be released by combusting the 36000 dm³ of natural gas carried in the tanker truck.

New evidence suggests that a food colouring that has a R_f value of 0.7 may cause dangerous side effects.

The chromatogram of two sweets, X and Y, were collected and the results are shown below.

For each of the sweets, state whether they should or should not be sold until further research into the food colouring has taken place.  

Give reasons for your answer.  

Use the scale on the chromatogram to help you answer this question.

Some food colouring additives are thought to cause hyperactivity in young children.

Suggest two reasons why manufacturers still use these additives in their products.

One of the additives thought to cause hyperactivity is the additive tartrazine, E102, and it is often added to orange drinks.

An investigation into the food colouring in an orange drink was carried out using paper chromatography.

The image shows the chromatogram produced.

 The manufacturer of the drink stated that their drink did not contain any additives that caused hyperactivity.

Is this claim correct? Explain your answer.

Use this chromatogram.

A student produced zinc chloride by reacting zinc oxide with dilute hydrochloric acid.

ZnO (s) + 2HCl (aq) → ZnCl₂ (aq) + H₂O (l)

Describe how a sample of zinc chloride crystals could be made from zinc oxide and dilute hydrochloric acid.

Higher tier only

A student wanted to make 15.0 g of zinc chloride using the method in part (a).

Relative atomic masses, (A_r):     H = 1,     Cl = 35.5,     Zn = 65,     O = 16 

Calculate the mass of zinc oxide that the student should react with dilute hydrochloric acid to make 15.0 g of zinc chloride.

The percentage yield of the student's reaction to produce zinc chloride using Method 1 was 67.4 %.

Calculate the mass of zinc chloride the student actually produced.

This question is about the reaction of iron and chlorine.

A teacher uses the apparatus shown below to react iron with chlorine.

This is the teacher's method.

• Record the mass of the weighing boat

• Add iron and record the mass again

• Heat the weighing boat and iron strongly for two minutes and then allow to cool

• Record the mass of the weighing boat and its contents

Give an addition to the method to check that the iron has fully reacted with the chlorine.

Explain one resulting safety precaution for this reaction.

The table shows the teacher's results.

Use the teacher's results to show that the empirical formula of iron chloride is FeCl₃.

Relative atomic masses (A_r):     Fe = 56,     Cl = 35.5

Write the balanced symbol equation, including state symbols, for this reaction of iron and chlorine.

The image below shows the displayed formula of a compound

i) State the meaning of the term compound

[2]

ii) Name the molecule.

[1]

State the empirical formula of the molecule.

Calculate the relative formula mass of the molecule.

(relative atomic masses: C = 12, H = 1, O = 16)

Higher tier only

Calculate the number of moles in 75.14 g of the molecule.

Give your answer to 3 significant figures.

Higher tier only

Iron (III) chloride is used in industrial waste treatment, in water purification and in the production of electronic circuit boards.

One method of producing iron (III) chloride is reacting iron with chlorine:

2Fe + 3 Cl₂ → 2FeCl₃

Calculate the maximum mass of FeCl₃ that can be produced from 14.0 g of iron. 

Relative atomic masses (A_r): Cl = 35.5; Fe = 56

Maximum mass of FeCl₃ = _______________ g

Higher tier only

A student wanted to make 12.0 g of copper chloride by reacting copper carbonate with hydrochloric acid. The equation for the reaction is as follows: 

CuCO₃ + 2HCl  →  CuCl₂ + H₂O + CO₂

Relative atomic masses, A_r: H = 1; C = 12; O = 16; Cl = 35.5; Cu = 63.5

Calculate the mass of CuCO₃ the student needs to react with dilute hydrochloric acid to make 12.0 g of copper chloride.

Mass of copper carbonate = _________________________ g

Black ink is made from other colour dyes.

A student investigated black ink using paper chromatography in a beaker with water as the solvent.

Figure 1 shows the chromatogram that the student obtained. It is not to scale.

Figure 1

The R_f value of the blue dye = 0.86
 
The distance moved by the solvent = 6.3 cm
 
Calculate the distance moved by the blue dye, in cm.
 
Give your answer to two significant figures.

The student took measurements from the chromatogram and calculated the R_f value of the red dye to be 1.3.

Explain why this value is not correct.

What does the R_f value of a dye depend on?

In Figure 1, the yellow dye has not moved from the start line.

Give a reason why it has not moved and suggest how the method could be altered to obtain a chromatogram in which all three dyes move up the paper.