Chemical tests (Edexcel IGCSE Chemistry (Modular): Unit 2)

The gas burned in a Bunsen burner is methane.

The equation for the complete combustion of methane is

CH₄ (g) + 2O₂ (g) → CO₂ (g) + 2H₂O (g)

Calculate the mass of oxygen required to react with 32 g of methane.

  [M_r of methane = 16]

 mass of oxygen = .............................. g

The diagram shows methane burning in air. It also shows how the two gases formed are collected and tested.

 i) Explain why water collects in the U-tube.

 (2)

 ii) Describe how anhydrous copper(II) sulfate is used to test for water.

 (2)

 iii) Explain the change in the appearance of the limewater.

 (3)

This question is about salts.

When solutions of salts are mixed together, precipitates sometimes form. The insoluble salt barium carbonate forms as a precipitate when solutions of the soluble salts ammonium carbonate and barium chloride react together. When solutions of the soluble salts potassium chloride and magnesium sulfate are mixed, no precipitate forms. Complete the table to show the results of mixing solutions of some soluble salts.

A student has four unlabelled beakers, each containing a colourless solution of a different salt. The four solutions are

• potassium carbonate

• potassium chloride

• potassium iodide

• sodium chloride

Describe a method to identify each solution. Do not refer to safety in your answer.

A compound is suspected of containing copper ions, Cu²⁺ and bromide ions, Br^-.

Which test would not help to positively identify copper ions or bromide ions?

This question is about elements in Group 7 of the Periodic Table and their compounds.

i) Give the name of this group of elements.

(1)

ii) State the colour of chlorine gas.

(1)

iii) Give a test for chlorine gas.

(2)

Give a test to show that a solution contains iodide ions.

Test 

Result

A student compares the reactivity of the elements bromine, chlorine and iodine. He mixes these pairs of solutions and observes the reactions that occur.

• chlorine solution and potassium bromide solution

• bromine solution and potassium iodide solution

Explain how the reactions can be used to show the order of reactivity of the three elements. Include the colour change that the student would observe in each reaction.

Ammonium ions, NH₄⁺, in a solution can be identified by a chemical test.

Which statement is incorrect about the test for the ammonium ion, NH₄⁺?

Hydrated ammonium iron(Ill) sulfate is a violet solid that has the formula (NH₄)₂SO₄.Fe₂(SO₄)₃.xH₂O

The table shows some tests done on three separate samples of the solid.

Complete the table to show the observation made in each test.

A student needs to find the value of x in the formula (NH₄)₂SO₄.Fe₂(SO₄)₃.xH₂O He uses this apparatus.

The hydrated solid decomposes when heated gently. The equation for the reaction is 

(NH₄)₂SO₄.Fe₂(SO₄)₃.xH₂O —> (NH₄)₂SO₄.Fe₂(SO₄)₃ + xH₂O

The table shows the student's results. 

(i) Calculate the mass of (NH₄)₂SO₄.Fe₂(SO₄)₃ produced by heating.

(1)

mass of (NH₄)₂SO₄.Fe₂(SO₄)₃ = .............................. g 

(ii) Calculate the mass of water produced.

(1)

mass of water = ..................................... g

(iii) Calculate the value of x.

[M_r of (NH₄)₂SO₄.Fe₂(SO₄)₃ = 532 and M_r of H₂O = 18]

Give your answer to the nearest whole number.

(4)

Value of x = .....................................

A scientist finds an unlabelled bottle on a shelf. 

She thinks the bottle contains a solution of ammonium sulfate, (NH₄)₂SO₄ 

Describe tests the scientist could do to show that the solution is ammonium sulfate.

Ammonium sulfate is often used as a fertiliser. 

It is prepared by reacting ammonia (NH₃) with sulfuric acid (H₂SO₄).

 i) Name the type of reaction that occurs between ammonia and sulfuric acid.

 (1)

 ii) Write a chemical equation for the reaction of ammonia with sulfuric acid.

 (1)

 iii) Draw a dot‐and‐cross diagram to show the bonding in a molecule of ammonia.

 Show outer electrons only.

 (2)

A student is given a mixture of two white solid compounds, and a colourless solution containing the same two compounds.

The student is told that one of the compounds is a halide and that the other compound is a carbonate.

Give two reasons why the student should know, without doing any tests, that one of the compounds cannot be copper(II) carbonate.

Describe tests the student could do to show that the mixture contains potassium carbonate and potassium iodide.

A technician finds a white solid in an unlabelled beaker. She knows that the solid is one of the following substances:

• potassium chloride

• potassium sulfate

• sodium chloride

• sodium sulfate.

Describe how the technician can use chemical tests to identify the solid in the beaker.

This question is about ammonium compounds.

The table shows the names and formulae of some ammonium compounds.

 i) Complete the table by giving the missing information.

 (2)

 ii) When ammonia reacts with nitric acid, ammonium nitrate is formed.

 Write a chemical equation for this reaction.

 (1)

When ammonium chloride is heated, it undergoes thermal decomposition to form two gaseous products. 

Describe the result of testing each product with damp universal indicator paper.

The diagram shows the apparatus used to heat ammonium chloride.

 i) Suggest why the damp litmus paper is two different colours.

 (1)

 ii) Explain what happens to the damp litmus paper as the ammonium chloride is heated. 

 (3)

Explain why a fine white powder formed on the inside of the neck of the conical flask in part (c).