Mixtures and separation techniques

Figure 3 shows the changes of state between ice, water and steam.

What is the name of the change of state labelled P in Figure 3?

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

Figure 4 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 4.

Explain the purpose of the piece of apparatus labelled Q in Figure 4.

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

Figure 5 shows the chromatogram produced.

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

Using the information in Figure 5, 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 5, 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 =_........

Nitrogen, N₂, oxygen, O₂and 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 4 shows the apparatus the student used.

The student made an error when setting up the apparatus in Figure 4.

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^–3

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 6 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.

Figure 5 shows the structure of a molecule of compound S.

Use Figure 5 to deduce the empirical formula of compound S.

The melting points of three samples of S are shown in Figure 6.

State whether each of these samples, A, B and C, is pure or impure and justify your answers using the information in Figure 6.

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.

distance =_........ 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 7 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.

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 3 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 4 shows apparatus the student used.

Name the piece of equipment labelled X in Figure 4.

The student made an error when setting up the equipment in Figure 4. 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 7.

mass of salt in mg per 1000cm³ water

Figure 7

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 7 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 8 and any other laboratory apparatus.

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 3 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.

R_f =_........

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.

Mixtures of substances can be separated using different techniques.

Which of the following is a mixture of substances?

Figure 3 shows the apparatus that a student set up to obtain pure water from ink.

There are three mistakes in the way the apparatus has been set up.

One mistake is that the bulb of the thermometer is too low.

The bulb of the thermometer should be level with the side arm.

Give a reason why the bulb of the thermometer should be level with the side arm.

State one other mistake in Figure 3.

Paper chromatography is used to separate the substances in five different food colourings, P, Q, R, S and T.

Figure 4 shows the chromatogram at the end of the experiment.

The steps needed to carry out the chromatography experiment are listed below. They are not in the correct order.

1 leave the solvent to rise up the paper 2 put solvent in the beaker 3 draw a start line on the piece of paper 4 place the paper in the beaker 5 remove the paper when the solvent is near the top 6 put small spots of the food colourings on the start line

List the steps in the correct order.

The first two steps have been done for you.

Explain, using Figure 4, which food colouring contains the greatest number of coloured substances.

During chromatography of the food colourings, the solvent front moved 8.00 cm and the food colouring R moved 2.30 cm.

Calculate the R_f value for food colouring R. Give your answer to two significant figures.

R_f value =_........

(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.

percentage by mass =_........

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 6 can be used to separate water from ink.

Cold water flows through the condenser.

On Figure 6 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 7.

In the boxes below, draw the arrangement of particles that would be expected at A and B shown in Figure 6.

Changes of state between the three states of matter are shown in Figure 8.

The changes shown are physical changes.

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