Chemical tests (C16)

Question 1

what is the test for hydrogen gas?

Answer 1

burning splint is placed in a test tube containing hydrogen, the hydrogen burns to produce water. The reaction occurs as a small explosion which is heard as a ‘squeaky pop’ noise.

Question 2

what is the test for oxygen gas?

Answer 2

Testing for oxygen requires a glowing splint. In this test, a burning splint is blown out to leave red glowing embers. If this splint is added to a test tube containing oxygen, the splint relights.

Question 3

what is the test for carbon dioxide gas?

Answer 3

Limewater is a solution of calcium hydroxide, Ca(OH)2. When carbon dioxide is bubbled through limewater, it reacts to form insoluble calcium carbonate. The white calcium carbonate precipitates and the limewater is said to have gone cloudy.

Question 4

what is the test for chlorine gas?

Answer 4

a piece of damp blue litmus or indicator paper is held in the gas. Chlorine forms an acidic solution in water which turns the litmus red. Chlorine is also a bleach so the red and blue colours will then both fade to leave a white colour.

Question 5

how do you test for carbonate ions, CO3 2-?

Answer 5

Carbonate ions react with an acid to release carbon dioxide gas. Effervescence is observed and the production of carbon dioxide can be confirmed using limewater. A solution of a carbonate will also be alkaline, which can be shown using universal indicator or a pH probe.

Question 6

what is the ionic equation for the test for carbonate ions, CO3 2-?

Answer 6

2H+(aq) + CO32−(aq) → H2O(l) + CO2(g)

Question 7

how do you test for halide ions (chlorine, bromine, iodine)?

Answer 7

react with silver to form a precipitate.When testing for the presence of a halide ion, any other anions must be excluded. Hydroxide or carbonate ions can be removed from a solution by adding an excess of dilute nitric acid.

Question 8

what is the positive result for testing halide ions (chlorine, bromine, iodine)?

Answer 8

-silver iodide is yellow
-silver bromide is cream coloured
-silver chloride is white

Question 9

what is the ionic equation for halide ions?

Answer 9

Ag+(aq) + X−(aq) → AgX(s)

Question 10

how do you test for sulphate ions, SO42− ?

Answer 10

By adding a small quantity of dilute nitric or hydrochloric acid, any hydroxide or carbonate ions are removed. Any precipitate then formed by the addition of BaCl2(aq) or Ba(NO3)2(aq) will be barium sulfate, which is white.

Question 11

what is the ionic equation when testing the presence of sulphate ions?

Answer 11

Ba2+(aq) + SO42−(aq) → BaSO4(s)

Question 12

in what order should you test for the ions?

Answer 12

1) Test for the presence of carbonate ions using a dilute acid

2) Test for the presence of sulfate ions using HCl/BaCl2(aq)

3) Test for halide ions using HNO3/AgNO3(aq)

Question 13

how do you carry out a flame test?

Answer 13

In a flame test a thin wire is washed in a little acid. This clean wire is then dipped into the unknown substance. A small quantity of the substance sticks to the wire which is then placed in a hot (blue) flame of a Bunsen burner. Metal cations change the colour of the Bunsen flame.

Question 14

why is nichrome or platinum used for the wire?

Answer 14

they are both relatively inert and have high melting points. They therefore do not melt or give a visible colour of their own in a flame test.

Question 15

what is the positive flame colour result for lithium?

Answer 15

crimson red

Question 16

what is the positive flame colour result for sodium?

Answer 16

yellow-orange

Question 17

what is the positive flame colour result for potassium?

Answer 17

lilac

Question 18

what is the positive flame colour result for calcium?

Answer 18

red-orange

Question 19

what is the positive flame colour result for copper?

Answer 19

green

Question 20

describe how coloured precipitation occurs

Answer 20

Coloured precipitates are formed from transition metal ions and white precipitates from other metal ions.

Question 21

Group 2 metal hydroxide precipitation test?

Answer 21

same colour (white) and the precipitates are insoluble on the addition of excess sodium hydroxide.

Question 22

Aluminium hydroxide test?

Answer 22

also a white precipitate but this precipitate will dissolve in an excess of sodium hydroxide to leave a colourless solution

Question 23

test for iron hydroxide?

Answer 23

When testing for Fe2+, note that the green precipitate of Fe(OH)2 that initially forms will turn brown on standing as it oxidises in the air to form Fe(OH)3.

Question 24

what is the general ionic equation using Mn+ to represent the metal ion?

Answer 24

Mn+(aq) + nOH−(aq) → M(OH)n(s)

Question 25

test for water?

Answer 25

using solid copper(II) sulfate, CuSO4(s). This solid is white when anhydrous but blue when hydrated. As a result, a few drops of water on anhydrous copper(II) sulfate will produce a colour change from white to blue.

Question 26

equation for the process of testing water?

Answer 26

CuSO4(s) + 5H2O(l) → CuSO4·5H2O(s)