Mixtures & separation techniques

Figure 4 shows the changes of state between ice, water and steam.

What is the name of the change of state labelled P in Figure 4?

Which of the following shows the arrangement of particles in ice?

Figure 5 shows the apparatus that can be used to separate a mixture of ethanol and water by fractional distillation.

Name the piece of apparatus labelled R in Figure 5.

Explain the purpose of the piece of apparatus labelled Q in Figure 5.

A student investigates the food dyes used to make the colours W, X, Y and Z in four different sweets.

Figure 6 shows the chromatogram produced.

The solvent front shows how far the solvent moves during the investigation.

Using the information in Figure 6, state which colour is made of only two food dyes.

The R_f value of a spot on the chromatogram can be calculated using the equation R distance moved by the food dye ^f distance moved by solvent front

Using the information in Figure 6, calculate the R_f value of the food dye labelled S.

Give your answer to 2 decimal places.

R_f value of food dye S =_........

Silver nitrate and potassium chloride are ionic compounds that are soluble in water.

When potassium chloride solution is added to a solution of silver nitrate, a white precipitate forms.

Complete the equation for this reaction, including the missing state symbol.

Give the name of the white precipitate that forms.

State and explain how the white precipitate can be separated from the reaction mixture.

Silver nitrate solution conducts electricity. Solid silver also conducts electricity.

Describe how silver nitrate solution conducts electricity and how solid silver conducts electricity.

A bottle containing an unknown solution is labelled with the two hazard symbols shown in Figure 16.

What are the meanings of these two symbols?

State a safety step that should be taken when using the unknown solution.

Silver nitrate and potassium chloride are ionic compounds that are soluble in water.

When potassium chloride solution is added to a solution of silver nitrate, a white precipitate forms.

Complete the equation for this reaction, including the missing state symbol.

Give the name of the white precipitate that forms.

State and explain how the white precipitate can be separated from the reaction mixture.

Silver nitrate solution conducts electricity. Solid silver also conducts electricity.

Describe how silver nitrate solution conducts electricity and how solid silver conducts electricity.

Nitrogen, N₂, oxygen, O₂ and fluorine F₂ are all gases.

Nitrogen, oxygen and fluorine are in the same period of the periodic table.

Explain, in terms of electronic configuration, in which period of the periodic table nitrogen, oxygen and fluorine are located.

Fluorine has a boiling point of –188 °C.

Explain why fluorine has a low boiling point.

A mixture of nitrogen and oxygen becomes a liquid when cooled to –200 °C.

Which row shows the changes in energy and movement of the molecules as they are cooled from room temperature to –200 °C?

The mixture of liquid nitrogen and liquid oxygen can be separated.

Which process could be used to separate a mixture of liquid nitrogen and liquid oxygen?

At high temperatures, nitrogen, N₂, can react with oxygen, O₂, to form nitrogen dioxide only.

Write the balanced equation for this reaction.

Water treatment is needed to make most sources of water suitable for drinking.

Water treatment includes the processes of chlorination, filtration and sedimentation.

Place these processes in the order that they take place during water treatment.

Some tap water contains chloride ions.

Explain, in terms of electrons, how a chlorine atom, C l, forms a chloride ion, Cl^–.

Why is chlorine added to water during water treatment?

State why tap water is not suitable for use in chemical analysis.

A student was asked to distil a sample of tap water. Figure 5 shows the apparatus the student used.

The student made an error when setting up the apparatus in Figure 5.

This error meant that pure water could not be collected in the test tube.

Explain what the student needs to change so that pure water can be collected in the test tube.

State what the student should use to heat the water.

A 250 cm³ solution of copper sulfate contains 6.52 g of dissolved solid.

Calculate the concentration of this copper sulfate solution in g dm^–3.

concentration =_........ g dm

Sodium hydroxide solution and copper sulfate solution were reacted together completely.

The result was a mixture of a precipitate of copper hydroxide in a solution of sodium sulfate.

Describe how to obtain

• a pure sample of solid copper hydroxide from the mixture

• a pure sample of solid sodium sulfate from the mixture.

Figure 9 shows the equipment used to electrolyse a sample of sodium sulfate solution.

Graphite electrodes are used in the electrolysis.

Give two reasons why graphite is a suitable material for the electrodes.

Sodium sulfate solution contains ions.

Which ions are attracted to the positive electrode during the electrolysis?

Draw one straight line from each electrode to the product formed at that electrode during the electrolysis of sodium sulfate solution.

Crystals of copper sulfate are prepared by reacting copper oxide, a base, with dilute sulfuric acid.

Name the other product of this reaction.

During the experiment, a spatula measure of copper oxide, a black powder, is added to warm, dilute sulfuric acid in a beaker.

When the mixture is stirred, the black powder disappears and the mixture turns pale blue.

The student then adds more copper oxide until the maximum amount of copper sulfate is formed without wasting copper oxide.

Explain how the student knows when to stop adding copper oxide.

The reaction produces an aqueous solution of copper sulfate.

What is the best way to obtain crystals of copper sulfate from an aqueous solution?

When some water is removed from the aqueous solution of copper sulfate, crystals of copper sulfate are made.

Describe how the arrangement and movement of the particles change as crystals are formed from a solution.

In this reaction, copper oxide, CuO, forms copper sulfate, CuSO₄.

Explain, in terms of electrons, whether the copper in copper oxide has been oxidised, has been reduced, or has not been oxidised or reduced.

In another experiment, a copper sulfate solution with a concentration of 39.875 g dm^–3 is used.

Calculate the mass of copper sulfate dissolved in 0.300 dm³ of this solution.

Figure 11 shows the structure of a molecule of compound S.

Use Figure 11 to deduce the empirical formula of compound S.

The melting points of three samples of S are shown in Figure 12.

State whether each of these samples, A, B and C, is pure or impure and justify your answers using the information in Figure 12.

A scientist uses chromatography in an investigation of compound S.

In the conditions used, compound S has an R_f value of 0.22.

Calculate the distance the spot of compound S moves if the solvent front has moved by 2.4 cm.

*(c) A solution of sodium chloride in water needs to be separated to obtain a sample of pure, dry sodium chloride and a sample of pure water.

Figure 13 shows the boiling points of sodium chloride and water.

Explain this difference in boiling points in terms of the structure and bonding of sodium chloride and water and how this difference is used to choose a method to separate sodium chloride solution into pure, dry sodium chloride and pure water.

Figure 12 shows the structure of the molecules of three organic compounds.

Each molecule in Figure 12 contains a different functional group.

Circle the alkene functional group in propene.

Propene reacts with bromine water.

Complete the equation for the reaction of propene with bromine by drawing the structure of a molecule of the product.

H H

H H

propene bromine product

Propanoic acid reacts with calcium carbonate, CaCO₃, to form calcium propanoate, Ca(C₂H₅COO)₂, and two other products.

Name the two other products.

*(b) Glucose, C₆H₁₂O₆, is a carbohydrate.

A dilute solution of ethanol can be produced from glucose by fermentation.

The dilute solution of ethanol can then be processed to form a concentrated solution of ethanol.

Describe how the fermentation of glucose is carried out and how the dilute solution of ethanol produced can then be processed to form a concentrated solution of ethanol.

You may include diagrams in your answer.

Potable water is water that is suitable for drinking.

River water can be treated to make it potable. Chlorination, filtration and sedimentation are three of the processes involved in making the river water potable.

Which row of the table shows these three processes in the order in which they are carried out?

State the reason why chlorine is added during the water treatment.

Describe how sedimentation is carried out.

Figure 5 shows the results of an analysis of a sample of potable water.

Using this information, explain why this sample of potable water is not the same as pure water.

A student wanted to distil a sample of potable water. Figure 6 shows apparatus the student used.

Name the piece of equipment labelled X in Figure 6.

The student made an error when setting up the equipment in Figure 6. This error meant no water could be collected in the test tube.

Explain what the student needs to do so water can be collected.

A sample of potable water contains impurities.

Why is this sample of water potable even though it contains impurities?

Waste water can be used to produce drinking water. The processes used include sedimentation, filtration and chlorination.

What is sedimentation?

State why the waste water is filtered.

State the reason for chlorination.

Some salts can be added to waste water to remove impurities. In an experiment, different masses of salt A were added to 1000cm³ samples of waste water. The experiment was repeated with salt B. The percentages of impurities removed from the waste water are shown in Figure 10.

mass of salt in mg per 1000cm³ water

Figure 10

It was concluded that the best way to purify 1000cm³ of the waste water is to add 100 mg of salt B.

Use the information about salt A and salt B in Figure 10 to evaluate this conclusion.

*(d) A sample of water was contaminated with a dissolved solid.

Devise a plan to separate pure water from this mixture, including a test to show that the water obtained is neutral.

You may use some or all of the apparatus shown in Figure 11 and any other laboratory apparatus.

Copper carbonate reacts with dilute nitric acid.

During the reaction the copper carbonate powder completely disappears.

State what can be deduced about the amount of acid used.

During the reaction, the pH of the mixture changed from 2 to 6.

By what factor has the concentration of the hydrogen ions in the mixture changed?

Using different reactants, a solution of copper sulfate was prepared.

Describe what should be done to obtain copper sulfate crystals from this copper sulfate solution.

When chloride ions are added to a pale blue solution containing copper ions, the mixture turns yellow.

This is a reversible reaction.

pale blue solution + chloride ions yellow solution + water

What effect does the removal of chloride ions have on the colour of the yellow mixture?

Hydrated copper sulfate has the formula CuSO₄.5H₂O. The formula tells us that each mole of copper sulfate contains 5 moles of water.

A sample of CuSO₄.5H₂O was heated gently until all the water was removed to form anhydrous copper sulfate, CuSO₄.

The mass of water formed was 4.5 g.

Calculate the mass of hydrated copper sulfate that was heated.

(relative atomic masses: H = 1.0, O = 16.0; relative formula mass: CuSO₄.5H₂O = 249.5)

mass of CuSO₄.5H₂O =_........

The order of reactivity of copper, magnesium and zinc can be determined by the displacement reactions between these metals and solutions of their salts.

You are provided with

• samples of the three metals

• solutions of copper sulfate, magnesium sulfate and zinc sulfate.

Describe the experiments that can be done to determine the order of reactivity of these metals by displacement reactions.

Metals can be extracted from ores found in the Earth’s crust.

Explain why aluminium cannot be extracted from its ore by heating with carbon but can be extracted by electrolysis.

Titanium is extracted from its ore in several stages.

In the first stage, titanium chloride is formed as a gas.

The gas is cooled to form liquid titanium chloride containing dissolved impurities.

Suggest how pure titanium chloride could be separated from the impurities.

In another stage, the pure titanium chloride, TiCl₄, is reacted with 500 moles of magnesium, an excess.

Calculate the number of moles in 45 000 grams of titanium chloride.

(relative atomic masses: C l = 35.5, Ti = 48.0)

number of moles titanium chloride =_........

Show that the 500 moles of magnesium added is an excess.

After this reaction, there is a mixture of the solids magnesium, titanium and magnesium chloride.

Titanium does not react with dilute hydrochloric acid.

Suggest a simple method to separate titanium from the mixture.

A sample of rock salt contains a mixture of sodium chloride and some insoluble substances.

The rock salt is added to water and the mixture stirred.

The mixture is then filtered to obtain a filtrate of sodium chloride solution.

Draw a labelled diagram of the apparatus used to filter the mixture and collect the sodium chloride solution.

Describe how a sample of pure, dry sodium chloride crystals can be obtained from the filtrate.

Inks contain coloured dyes. Samples of four inks, W, X, Y and Z, were separated using paper chromatography. Figure 5 shows the chromatogram obtained.

In the experiment, the solvent front moved 6.5 cm. Calculate the R_f value of the dye that is present in both inks X and Z.

State what could be changed in the experiment to make the R_f value more accurate.

In this experiment, ink sample Y did not move from the start line. Explain a change to the experiment that would be needed to separate the dyes in ink sample Y.

Figure 10 shows a flask fitted with a cotton wool plug. The flask contains an aqueous solution of a carbohydrate.

State two steps that need to be taken to turn the solution of the carbohydrate in the flask into a solution of ethanol.

The apparatus in Figure 11 is used to increase the concentration of the dilute solution of ethanol.

This apparatus did not produce a very concentrated solution of ethanol.

Describe how the apparatus can be altered to produce a more concentrated solution of ethanol.

The equation for the fermentation of a carbohydrate is

Calculate the maximum mass of carbon dioxide that could be produced if 135 g of this carbohydrate is fully fermented.

(relative formula masses: CO₂ = 44; C₆H₁₂O₆ = 180)

*(c) Figure 12 shows information about some compounds in the same homologous series.

Explain, using the data in Figure 12, why these compounds belong together in the same homologous series.

(a) One way to extract metals from land contaminated with metal compounds is phytoextraction. When plants grow they absorb metal ions through their roots. The plants are harvested, dried and burned forming an ash. The ash contains metal compounds.

Plants were grown in a piece of ground contaminated with nickel compounds.

1 kg of the ash from these plants contained 142.0 g of nickel compounds.

Calculate the percentage by mass of nickel compounds in the ash.

Nickel is extracted from nickel compounds.

State an advantage of extracting nickel by phytoextraction rather than from its ore.

Some nickel ores contain nickel sulfide.

In the first stage of extracting nickel from nickel sulfide, the nickel sulfide, NiS, is heated in air to form nickel oxide, NiO, and sulfur dioxide.

Write the balanced equation for this reaction.

In the final stage of the extraction process, a nickel compound is electrolysed to produce pure nickel.

An advantage of producing a metal by electrolysis is that

In a different method of obtaining nickel, the process produces a mixture of the liquids nickel tetracarbonyl and iron pentacarbonyl.

The boiling point of nickel tetracarbonyl is 43 °C. The boiling point of iron pentacarbonyl is 103 °C. These two liquids mix together completely.

Describe the process used to separate these two liquids.

An ink is a mixture of coloured substances dissolved in water.

Which method is used to separate the coloured substances in the ink?

The apparatus shown in Figure 9 can be used to separate water from ink.

Cold water flows through the condenser.

On Figure 9 use arrows to show where the water should flow in and where it should flow out.

Explain why a condenser is used.

The flask was heated with a Bunsen burner.

Give the name of an alternative piece of apparatus that could be used to heat the flask.

The particles in the ink in the flask can be shown as in Figure 10.

In the boxes below, draw the arrangement of particles that would be expected at A and B shown in Figure 9.

Changes of state between the three states of matter are shown in Figure 11.

The changes shown are physical changes.

Explain why these changes are called physical changes rather than chemical changes.

Which of the following substances will be a solid at 20 C and will melt when placed in a beaker of hot water at 80 C?

A student set up the apparatus shown in Figure 4 to obtain pure water from sea water by distillation.

Explain how the water in sea water separates to produce the pure water in this apparatus.

Explain how the apparatus could be improved to increase the amount of pure water collected from the same volume of sea water.

A substance is heated at a constant rate and its temperature is taken every minute. During the heating, the substance undergoes one change of state.

The results are shown on the graph in Figure 5.

Explain the shape of the graph in terms of the changes in the movement and arrangement of the particles as the substance is heated.