Solubility & solubility curves

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.

The table shows the maximum mass of potassium nitrate (KNO₃) and the maximum mass of sodium nitrate (NaNO₃) that dissolves in 25 cm³ of water at different temperatures.

Plot the data for potassium nitrate and for sodium nitrate on the grid.

Draw and label a curve of best fit for KNO₃ and for NaNO₃

10 20 30 40 50 60 70 80

The graph shows the temperature at which the maximum mass dissolved is the same for each solute.

Determine this temperature.

Use your graph to calculate the solubility, in g per 100 g of water, of sodium nitrate at 30C.

[1.0 cm³ of water has a mass of 1.0 g]

25 cm³ of a saturated solution of potassium nitrate is cooled from 50C to 20C.

Use your graph to determine the mass, in grams, of potassium nitrate that crystallises.

Show your working on the graph.

The solubility of a solid depends on the solvent used.

State one other factor that affects the solubility.

The flowchart shows the correct order of some of the steps.

Complete the flowchart by putting the remaining steps in the correct order.

These are the student’s results.

mass of empty evaporating basin = 60.5 g

mass of evaporating basin and dry salt = 78.1 g

Calculate the solubility of the salt in grams per 100 g of water.

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

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

This question is about soluble salts and insoluble salts.

Which pair of solutions produces an insoluble salt when mixed?

When solutions of lead nitrate and sodium sulfate are mixed, one product is solid lead sulfate.

This is the equation for the reaction.

Describe how a pure, dry sample of solid lead sulfate can be obtained from the mixture.

The table gives the solubility of a salt in water at six different temperatures.

Plot the points on the grid.

Draw a curve of best fit.

A saturated solution of the salt in 100 g of water is cooled from 90C to 30C.

Use your graph to determine the mass of salt that will crystallise.

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 solubility.

The graph shows the solubilities of copper(II) chloride and sodium chloride at different temperatures.

Determine the temperature at which copper(II) chloride and sodium chloride have the same solubility.

Show on the graph how you obtained your answer.

A saturated solution of copper(II) chloride in 100 g of water is cooled from 40 °C to 10 °C.

Determine the mass, in grams, of copper(II) chloride that crystallises.

A student uses this method to determine the solubility of potassium chloride in water at room temperature.

• record the mass of an empty evaporating basin

• pour some saturated potassium chloride solution into the evaporating basin

• record the mass of the evaporating basin and saturated potassium chloride solution

• heat the evaporating basin to remove all the water

• record the mass of the evaporating basin and the dry potassium chloride

The table shows the student’s results.

Calculate the mass of dry potassium chloride obtained.

Calculate the mass of water removed.

Calculate the solubility of potassium chloride in grams per 100 grams of water.

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

Suggest why the student’s method is not suitable for determining the solubility of hydrated copper(II) sulfate.

The box lists words that may be used to explain the term saturated solution.

Explain, using all the words in the box, the term saturated solution.

The diagram shows the apparatus a student uses to make a saturated solution.

This is the student’s method.

Step 1 add 4.5 g of solid to a boiling tube

Step 2 measure exactly 10.0 cm3³ of pure water and pour into the boiling tube

Step 3 place the boiling tube in the beaker of water and heat gently, stirring the mixture continuously until all the solid dissolves

Step 4 r emove the boiling tube from the beaker and allow it to cool

Step 5 record the temperature when crystals start to form in the boiling tube

The recorded temperature shows when the solution becomes saturated.

Name the piece of apparatus that the student should use in Step 2 to measure exactly 10.0 cm³ of pure water.

Suggest why the boiling tube is not heated directly using a Bunsen burner in Step 3.

Suggest how the student could improve the reliability of her recorded temperature in Step 5.

In Step 5, crystals start to form at 26C.

Calculate the solubility of the solid, in g per 100 g of water, at 26C.

[1.0 cm³ of pure water has a mass of 1.0 g]

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

The solubility curves for two solids, A and B, are shown on the grid.

20 30 40 50 60 70 80 90 100

State the temperature when A and B have the same solubility.

Calculate the mass of B that will dissolve in 250 g of water at 60C.

Show your working.

Suggest why the values for the solubility of A and B may be less accurate at 95C than at lower temperatures.

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_........ .