Chapter 12 - Experiments and Tests in the Lab

Question 1

Test for cations: Ammonium NH4+
Give: test, positive test, and ionic equation

Answer 1

Test: Add a little dilute sodium hydroxide solution. Heat gently.
Positive Test: Ammonia gas given off (litmus paper turns from blue to red)

(see c19 f 1 for ionic equation)

Question 2

Test for cations: copper (II) Cu2+
Give: test, positive test, and ionic equation

Answer 2

Test: Add dilute sodium hydroxide or ammonia solution
Positive Test: A pale blue precipitate forms, but when adding more ammonia it creates a deep blue solution

(see c19 f 2 for ionic equation)

Question 3

Test for cations: iron (II) Fe2+
Give: test, positive test, and ionic equation

Answer 3

Test: Add dilute sodium hydroxide or ammonia solution
Positive Test: A pale green precipitate forms

(see c19 f 3 for ionic equation)

Question 4

Test for cations: Iron (III)
Give: test, positive test, and ionic equation

Answer 4

Test: Add dilute sodium hydroxide or ammonia solution.
Positive test: A red-brown precipitate forms

(see c19 f 4 for ionic equation)

Question 5

Test for cations: Aluminium Al 2+
Give: test, positive test, and ionic equation

Answer 5

Test: Add dilute sodium hydroxide or ammonia solution.
Positive test: A white precipitate forms
The precipitate will dissolve when excess sodium hydroxide, giving a colourless solution, but will not dissolve when ammonia is added instead

(see c19 f 5 for ionic equation)

Question 6

Test for cations: Zinc Zn2+
Give: test, positive test, and ionic equation

Answer 6

Test: Add dilute sodium hydroxide or ammonia solution.
Positive test: A white precipitate forms

It dissolves again when more sodium hydroxide or ammonia is added, giving a colourless solution

(see c19 f 6 for ionic equation)

Question 7

Test for cations: Calcium Ca2+
Give: test, positive test, and ionic equation

Answer 7

Test: Add dilute sodium hydroxide or dilute ammonia solution.
Positive test: When sodium hydroxide is added, a white precipitate is formed. It will not dissolve on adding excess sodium hydroxide. When ammonia solution is added, there is no precipitate, or a very slight white precipitate.

(see c19 f 6 for ionic equation)

Question 8

Test for anions: Test 1
Halide ions (Cl-, Br-, I-)

Answer 8

Method: To a small amount of the solution, add an equal volume of dilute nitric acid. Then add silver nitrate solution
Positive test: Silver halides are insoluble, so if halide ions are present a precipitate will form. The colour will tell you which one.

Chloride Ions (Cl-): white
Bromide Ions (Br-): cream
Iodide Ions (I-): yellow

(see c19 f 8 for ionic equations)

Question 9

Test for anions: Test 2
Sulfate ions (SO4^2-)

Answer 9

Method: To a small amount of the solution, add an equal volume of dilute hydrochloric acid. Then add barium nitrate solution
Positive test: Barium sulfate are insoluble, so if sulfate ions are present a white precipitate will form.

(see c19 f 9 for ionic equation)

Question 10

Test for anions: Test 3
Nitrate Ions (NO3^-)

Answer 10

Method: To a small amount of the unknown solid or solution, add a little sodium hydroxide solution. Then add some small pieces of aluminium foil, and heat gently.
Positive test: If ammonia gas is given off, the unknown substance contained nitrate ions.

(see c19 f 10 for ionic equation)

Question 11

Test for anions: Test 4
Carbonate Ions (CO3^2-)

Answer 11

Method: To a small amount of the unknown solid or solution, add a little dilute hydrochloric acid.
Positive test: If the mixture bubbles and gives off a gas that turns limewater milky, the unknown substance contained carbonate ions. The gas is carbon dioxide.

(see c19 f11 for ionic equation)