Mixtures & separation techniques

A student uses paper chromatography to investigate the dyes in five different inks, V, W, X, Y and Z.

The chromatogram shows the results of the investigation.

solvent front

V W X Y Z

Explain why the start line on the paper is drawn in pencil rather than in ink.

Explain which two inks contain the dye that is most soluble in the solvent.

Explain how the chromatogram shows that

• W contains only one dye

• V may contain more than one dye

Calculate the R_f value for the dye in W.

Give your answer to two significant figures.

This question is about a compound called potassium permanganate.

A teacher makes a saturated solution of potassium permanganate using this method.

The teacher adds 25 cm³ of water at 50 °C to a beaker.

The teacher then adds solid potassium permanganate one spatula at a time, while stirring, until the solution is saturated.

Give the meaning of the term saturated solution.

Give a reason why crystals form when the saturated solution cools from 50 °C to room temperature.

The teacher pours 25 cm³ of a saturated solution of potassium permanganate at room temperature into an evaporating dish.

The teacher leaves the solution until all the water has evaporated.

The teacher records the mass of the evaporating dish and the mass of the evaporating dish with the dry crystals.

mass of evaporating dish = 64.10 g

mass of evaporating dish with dry crystals = 65.35 g

Calculate the solubility of potassium permanganate in grams per 100 g of water.

[1.00 cm³ of water has a mass of 1.00 g]

solubility =_........ g per 100 g of water

The equation shows the thermal decomposition of potassium permanganate.

2KMnO₄ K₂MnO₄ + MnO₂ + O₂

Describe a test to show that oxygen gas is produced.

Calculate the M_r of K₂MnO₄

Potassium permanganate is an oxidising agent.

Give a reason why potassium permanganate is an oxidising agent.

A student does an experiment to identify the dyes in inks A, B, C, D and E.

The student uses this apparatus.

• a piece of chromatography paper

• a beaker

• a solvent

• inks A, B, C, D and E

Describe how the student should set up the apparatus and do the experiment.

You may use a diagram to help your answer.

The chromatogram shows the results for inks A, B, C, D and E.

Explain which ink contains a dye that is insoluble in the solvent.

Explain which inks contain the same dye.

Describe how the student can use the chromatogram to determine the R_f value for the dye in ink C.

You do not need to calculate the R_f value.

A teacher uses this apparatus to separate a mixture of ethanol and water.

Give the name of this method of separation.

Name the pieces of apparatus labelled X, Y and Z.

Give a physical test that the teacher could do to find out if the ethanol produced is pure.

This question is about elements, mixtures and compounds.

The box gives some methods used to separate mixtures.

Choose methods from the box to answer these questions.

Identify a method to remove sand from a mixture of sand and seawater.

Identify a method to separate a mixture of liquids with different boiling points.

The diagram shows part of the structure of silicon dioxide.

Explain why silicon dioxide is a compound.

The molecular formula of the compound insulin is C₂₅₇H₃₈₃N₆₅O₇₇S₆

Determine the number of different elements in C₂₅₇H₃₈₃N₆₅O₇₇S₆

Determine the number of atoms in a molecule of C₂₅₇H₃₈₃N₆₅O₇₇S₆

Magnalium is a mixture of magnesium atoms and aluminium atoms.

The diagram shows a sample of magnalium.

Calculate the percentage of magnesium atoms in this sample.

Chromatography is used to separate the components in a mixture.

Diagram 1 shows the apparatus used to separate the different dyes in a food colouring.

Diagram 1

Complete the diagram by adding the missing labels.

Give a reason why the baseline is drawn in pencil.

Diagram 2 shows a chromatogram produced from four different food colourings, W, X, Y and Z.

Which two food colourings contain the same dye?

Calculate the R_f value of the dye in food colouring W.

This is the equation for the decomposition of hydrogen peroxide.

The rate of reaction increases when a catalyst of manganese(IV) oxide is added.

Describe how a catalyst increases the rate of a reaction.

A student adds 50 cm³ of hydrogen peroxide solution to a glass container and then adds 1.0 g of manganese(IV) oxide.

The diagram shows the apparatus the student uses.

Name the glass container the student uses.

The student waits until the hydrogen peroxide solution completely decomposes.

Describe how the student could then show that the manganese(IV) oxide was a catalyst and not a reactant.

This method is used in an experiment to produce hydrated zinc nitrate crystals.

• pour 50 cm³ of dilute nitric acid into a beaker

• add a spatula of zinc powder to the acid

• add more zinc until it is in excess

• filter the mixture

• obtain crystals of zinc nitrate from the filtrate

State why the mixture is filtered.

Describe how a pure, dry sample of hydrated zinc nitrate crystals could be obtained from the filtrate.

This equation represents the formation of hydrated zinc nitrate in the experiment.

In another experiment, 9.75 g of zinc is completely reacted with nitric acid.

Show that the maximum possible mass of hydrated zinc nitrate crystals that could be formed is approximately 45 g.

[for Zn(NO₃)₂.6H₂O, M_r = 297]

The actual yield of hydrated zinc nitrate crystals is 36.4 g.

Calculate the percentage yield of hydrated zinc nitrate crystals.

The box gives some methods that are involved in the separation of mixtures.

Use words from the box to identify the method involved in each of these separations.

Give the best method for obtaining gasoline from crude oil.

Give the best method for separating the dyes in black ink.

Give the best method for obtaining pure water from seawater.

A sample of solid hydrated copper(II) sulfate can be obtained from a mixture of copper(II) oxide and copper(II) sulfate.

Complete the passage by using words from the box.

The mixture of copper(II) oxide and copper(II) sulfate can be separated by first

_........ the copper(II) sulfate in distilled water.

The copper(II) oxide is then removed by

Some of the water from the copper(II) sulfate solution is then removed by

A pure sample of hydrated copper(II) sulfate is then obtained by

This question is about mixtures and compounds.

The box gives some techniques used to separate mixtures.

The table lists some substances and mixtures.

Complete the table using words from the box to show the best technique to obtain the named substance from each mixture.

Each technique may be used once, more than once or not at all.

State how the box represents a mixture.

Food colourings are mixtures of food dyes.

A student uses paper chromatography to separate the food dyes contained in food colouring D.

The student places spots of three food dyes A, B and C and food colouring D on chromatography paper.

The diagram shows the appearance of the paper after the experiment.

Describe the composition of food colouring D.

A compound has the formula Ca(HCO₃)₂

Determine the number of different elements in Ca(HCO₃)₂

Determine the number of atoms in the formula of Ca(HCO₃)₂

This question is about different hydrated forms of sodium sulfate.

A compound has the formula Na₂SO₄.7H₂O

How many different elements are there in the formula Na₂SO₄.7H₂O?

What is the total number of atoms in the formula Na₂SO₄.7H₂O?

Another hydrated form of sodium sulfate has the formula Na₂SO₄.xH₂O

A student uses this apparatus to find the value of x.

This is the student’s method.

• find the mass of an empty tube

• add solid hydrated sodium sulfate to the tube

• find the mass of the tube and hydrated sodium sulfate

• heat the tube for several minutes

• allow the tube to cool and find the mass of the tube and contents

Describe what the student should do next to make sure that all the water is removed from the hydrated sodium sulfate.

Explain the role of the ice in the beaker.

Describe how the student could prove that the liquid collected is pure water.

The table gives the student’s results.

Use the student’s results to calculate the value of x.

[for Na₂SO₄, M_r = 142 for H₂O, M_r = 18]

This question is about elements, mixtures and compounds.

Which of these is the formula of a molecule of an element?

Which method can be used to separate an insoluble solid from a liquid?

Give the name of a method used to separate a mixture of liquids with different boiling points.

A student adds a crystal of substance X to some water in a beaker and leaves the beaker for one day.

The diagram shows the beaker immediately after adding the crystal, and after

Which equation gives the correct state symbols for a process that occurs in the beaker?

Which other process occurs in the beaker?

After one day the student does two tests on the liquid in the beaker.

The table shows the student’s results.

Identify substance X.

A student uses this apparatus to investigate the dyes in a food colouring A.

Explain two mistakes that the student makes when setting up the apparatus.

The student repeats the experiment, but with no mistakes.

The table shows the R_f values for the two dyes in food colouring A.

Complete the chromatogram for food colouring A by adding and labelling the dyes.

Give a reason why the blue dye has a larger R_f value than the yellow dye.

This question is about the reaction between magnesium and dilute nitric acid.

A student reacts dilute nitric acid with an excess of magnesium powder as a first step in the preparation of dry crystals of hydrated magnesium nitrate.

This is the equation for the reaction.

Explain why it is important that magnesium is in excess.

The student adds 0.75 g of magnesium to 0.025 mol of nitric acid.

Calculate the mass of magnesium, in grams, that remains at the end of the reaction.

[for magnesium, A_r = 24]

Describe how the student can obtain dry crystals of hydrated magnesium nitrate from the mixture at the end of the reaction.

The student repeats the experiment and records the volume of hydrogen gas collected.

The graph shows the student’s results.

Use the graph to calculate the rate of reaction, in cm³/ s, at t = 40 s.

Show your working on the graph.

The box shows some changes of state.

The table lists some physical changes.

Complete the table using words from the box to show the change of state for each physical change.

A student plans to obtain salt crystals from a mixture of salt and sand.

The student adds pure water to the mixture to dissolve the salt.

State two things the student could do to make the salt dissolve quickly.

State what the student should do next to separate the sand from the salt solution.

Describe how the student can obtain pure dry crystals of salt from the salt solution.

This question is about apparatus used in the laboratory.

Complete the table by giving the name of each piece of apparatus and a unit used for the quantity it measures.

The diagram shows apparatus used to obtain pure water from sodium chloride solution by simple distillation.

Explain why it is necessary for water to flow continuously in and out of the apparatus.

Describe a chemical test to show that the sodium chloride solution contains chloride ions.

Describe a physical test to show that the liquid in the conical flask is pure water.

A student investigates how the electrical conductivity changes as dilute sulfuric acid is added to barium hydroxide solution.

This is the student’s method.

Step 1 =$~dd=RMKM cm³ of barium hydroxide solution to a beaker

Step 2 measure the electrical conductivity of the solution

Step 3 =$~dd=NMKM cm³ of dilute sulfuric acid to the beaker

Step 4 stir the mixture

Step 5 measure the electrical conductivity of the mixture

Step 6 repeat steps 3 to 5 until a total of 100 cm3³ of dilute sulfuric acid has been added

The table shows the student’s results.

Name a piece of apparatus the student could use to add 10.0 cm³ of dilute sulfuric acid to the beaker.

Plot the student’s results.

Ignoring the anomalous result, draw two lines of best fit, making sure that the two lines cross.

Total volume of acid added in cm³

Give the trend shown on the graph for the first 50 cm³ of acid added.

Suggest a mistake the student could have made to cause the anomalous result.

This is the equation for the reaction.

When 50 cm³ of dilute sulfuric acid have been added, only barium sulfate and water are present in the mixture.

Explain why this mixture does not conduct electricity. Refer to the type of bonding in barium sulfate and in water in your answer.

Name a technique the student could use to separate barium sulfate from the mixture after 100 cm³ of dilute sulfuric acid has been added.

This question is about mixtures and compounds.

The box gives some methods used to separate mixtures.

Choose methods from the box to answer the following questions.

Each method may be used once, more than once or not at all.

Identify a method to separate a single food dye from a mixture of food dyes.

Identify a method to separate gasoline from crude oil.

Identify a method to separate water from copper(II) sulfate solution.

The diagram represents a molecule.

Explain why this molecule is a compound.

The molecular formula of another compound is C₃H₅N₃O₉

State the number of different elements in C₃H₅N₃O₉

Determine the number of atoms in a molecule of C₃H₅N₃O₉

This question is about soluble and insoluble compounds.

A precipitate is an insoluble compound formed when solutions of soluble compounds react after mixing.

Different solutions are mixed in separate test tubes.

Tube 1 copper(II) sulfate solution and calcium chloride solution

Tube 2 magnesium nitrate solution and potassium sulfate solution

Tube 3 sodium carbonate solution and copper(II) sulfate solution

In which of the tubes will a precipitate form?

A student mixes solutions, containing equal amounts in moles, of silver nitrate and sodium chloride.

The equation for the reaction between silver nitrate solution and sodium chloride solution is

AgNO₃(aq) + NaCl(aq) AgCl(s) + NaNO₃(aq)

State the colour of the precipitate of silver chloride.

The student wants to obtain pure, dry crystals of sodium nitrate.

Crystals of sodium nitrate decompose at temperatures above 300 °C.

Describe a method the student could use to obtain pure, dry crystals of sodium nitrate.

Give an advantage of mixing solutions containing equal amounts, in moles, of silver nitrate and sodium chloride.

A student uses this apparatus to heat crystals of hydrated zinc sulfate and collect the liquid produced.

Describe a chemical test to show that the colourless liquid contains water.

Describe a physical test to show the colourless liquid is pure water.

The equation for the decomposition of hydrated zinc sulfate is

The student records these masses.

mass of boiling tube = 41.64 g

mass of boiling tube + ZnSO₄·7H₂O = 54.46 g

Calculate the maximum volume, in cm³, of pure water that could be produced.

Give your answer to 1 decimal place.

[1.00 cm³ of pure water has a mass of 1.00 g]

[M_r of ZnSO₄·7H₂O = 287 M_r of H₂O = 18]

In an experiment using a different mass of ZnSO₄·7H₂O the maximum volume of pure water that could be produced is 8.5 cm³.

The student collected the pure water and calculated the percentage yield to be 20.3 %.

Calculate the volume, in cm³, of pure water collected.

Explain an improvement to the apparatus that would increase the percentage yield of pure water.

The box gives some methods used in the separation of mixtures.

Use words from the box to answer these questions.

Identify the method used to obtain pure water from sea water.

Identify the method used to separate the dyes in a food colouring.

Identify the method used to obtain ethanol from a mixture of ethanol and water.

Complete the sentences by writing a suitable word in each blank space.

When salt is added to water and stirred until no more will_........ a saturated solution forms.

The salt is the_........ .

The water is the_........ .

A student uses the reaction between zinc and dilute sulfuric acid to prepare some zinc sulfate crystals.

Complete the equation for this reaction by giving the correct state symbols.

State what would be observed during this reaction.

The student adds excess zinc to a beaker of dilute sulfuric acid.

Explain why it is necessary to add excess zinc.

Draw a diagram of the apparatus the student should use to remove the unreacted zinc and collect the zinc sulfate solution.

The student obtains a pure, dry sample of zinc sulfate crystals.

The formula of zinc sulfate crystals is ZnSO₄.7H₂O

Calculate the relative molecular mass (M_r) of zinc sulfate crystals.

The student uses 0.0200 mol of dilute sulfuric acid in her preparation.

Show that the maximum mass of zinc sulfate crystals that the student could obtain is about 6 g.

The student obtains a mass of 4.28 g of zinc sulfate crystals.

Calculate the percentage yield of the zinc sulfate crystals.

Give your answer to three significant figures.

A student is given a mixture of salt solution and sand.

She wants to obtain pure water from the mixture.

She separates the sand from the salt solution.

Which method of separation should she use?

The student then uses this apparatus to obtain pure water from the salt solution.

Name the pieces of apparatus labelled X, Y and Z.

State what remains in the flask when the separation is complete.

This question is about elements, compounds and mixtures.

Name the element that burns with a lilac flame.

Name the technique used to separate the mixture of colours in black ink.

The box gives the names of some substances.

Choose substances from the box to answer these questions.

Identify the compound.

Identify the mixture.

Identify the non‑metal element that is a solid at room temperature.

Chromatography is used to analyse mixtures.

A student does a chromatography experiment to analyse the composition of green food colouring in sweets.

She places four known dyes, A, B, C and D, and the green food colouring on chromatography paper.

The diagram shows the student’s apparatus at the start of her experiment.

The diagram shows that the student makes two mistakes when setting up her apparatus.

State the two changes that the student should make so that her experiment works.

Another student does the chromatography experiment correctly.

The diagram shows her chromatogram at the end of the experiment.

Explain what the chromatogram shows about the composition of the green food colouring.

The distance between the start line and the spot for dye C is 6.2 cm.

Calculate the R_f value of dye C.

Suggest why dye A does not move.