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Chemistry > 2 Chemical tests > Flashcards

2 Chemical tests Flashcards

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1
Q

How do you test for hydrogen?

A

A lighted splint is held to the mouth of the tube. The hydrogen explodes with a squeaky pop.

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2
Q

How does hydrogen combine with oxygen?

A

Explosively.

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2
Q

What is the equation for hydrogen reacting with oxygen?

A

2H2 + O2 —> 2H2O

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3
Q

How do you test for oxygen?

A

A glowing splint is put into the tube containing the gas. Oxygen relights a glowing splint.

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4
Q

How can you test for carbon dioxide gas?

A

The carbon dioxide is bubbled through limewater. Carbon dioxide turns limewater milky/chalky/cloudy.

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5
Q

What is another word for limewater?

A

Calcium hydroxide.

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6
Q

What does limewater (calcium hydroxide solution) react with carbon dioxide to form?

A

A white precipitate of calcium carbonate.

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7
Q

What is an equation that represents limewater reacting with carbon dioxide?

A

Ca(OH)2 + CO2 —> CaCO3 + H2O

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8
Q

How do you test for chlorine gas?

A

A piece of damp litmus paper or universal indicator paper is put into the test-tube or held over its mouth.

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9
Q

What does chlorine gas do to the indicators?

A

Chlorine is a green gas that bleaches (turns white) the damp litmus paper or universal indicator.

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10
Q

What exactly happens to these papers?

A

If blue litmus paper or universal indicator paper are used for this test, they go red first (the chlorine dissolves in the water to form an acid solution) and then white.

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11
Q

How can you test for ammonia gas?

A

Hold a piece of damp universal indicator or red litmus paper at the mouth of the test tube.

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12
Q

What does ammonia do to the universal indicator paper/litmus paper?

A

It turns it blue.

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13
Q

What is special about ammonia?

A

It is the only alkaline gas that we will come across in this syllabus.

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14
Q

How could we test for water?

A

With anhydrous copper sulfate a it turns it blue.

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15
Q

How can we tell that there is water?

A

Anhydrous copper sulfate lacks water of crystallisation and is white. Dropping water onto it replaces the water of crystallisation and turns it blue.

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16
Q

What is the word equation for the anhydrous copper sulfate?

A

Anhydrous copper sulfate + water —> Hydrated copper sulfate.

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17
Q

What could this test also be done on?

A

Anything that contains water, so it would work with sodium chloride solution or sulfuric acid. It does NOT show that the water is pure.

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18
Q

How can you find out if water is pure?

A

By measuring the freezing (melting) point or boiling point.

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19
Q

How could you use the freezing or boiling point to show purity?

A

Pure water freezes at exactly 0C and boils are exactly 100 at 1 atmospheric pressure.

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20
Q

How could we use this information for impurities?

A

If the water is impure, it will usually freeze at a lower temperature and boil at a higher temperature.

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21
Q

What do all salts have in relation to ions?

A

One cation and one anion.

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22
Q

What is a cation?

A

A positive ion.

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23
Q

What is an anion?

A

A negative ion.

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24
Q

What do flame tests show?

A

The presence of certain metal ions - cations - in a compound.

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25
Q

What is step 1 of the procedure for flame tests?

A

A platinum or nichrome wire is dipped into concentrated hydrochloric acid.

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26
Q

What is step 2 of the procedure for flame tests?

A

And then into the salt you want to test, so that some salt sticks on the end.

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27
Q

What is step 3 of the procedure for flame tests?

A

The wire and the salt are then held just within a roaring bunsen burner and their colours are observed.

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28
Q

What ion is red in flame tests?

A

Lithium.

29
Q

What ion is yellow-orange in flame tests?

A

Sodium.

30
Q

What ion is lilac in flame tests?

A

Potassium.

31
Q

What ion is orange-red in flame tests?

A

Calcium.

32
Q

What ion is blue-green in flame tests?

A

Copper (II).

33
Q

What’s another way to test for cations?

A

You dissolve the salt in distilled water and put about 1cm3 in a test-tube. Add about the same volume of dilute sodium hydroxide solution. What you are looking for in most of these tests is the formation of a precipitate; the solution will go cloudy as a solid forms.

34
Q

What are the only common hydroxides that dissolve in water?

A

Sodium, potassium, and ammonium hydroxides dissolve in water to any extent. Most metal hydroxides are insoluble.

35
Q

So what happens if you add sodium hydroxide to a solution containing metal ions?

A

You will get a precipitate of the metal hydroxide.

36
Q

What does the formation of a blue precipitate show?

A

The presence of copper (II) oxide. The precipitate is copper (II) hydroxide.

37
Q

How do we know that this method is reliable?

A

Any copper (II) salt in solution will react with sodium hydroxide solution in this way.

38
Q

What does the formation of a orange-brown precipitate show?

A

The presence of iron (III) ions. The precipitate is iron hydroxide.

39
Q

How do we know that this method is reliable?

A

Any iron (III) compound in solution will give this precipitate

40
Q

What equations should we aim to use for precipitation reactions?

A

Ionic equations.

41
Q

What does the formation of a green precipitate show?

A

The presence of iron (II) ions. The precipitate is iron hydroxide.

42
Q

How can you test for ammonium?

A

Sodium hydroxide solution reacts with salts (either solid or in solution) to produce ammonia gas.

43
Q

How can you tell that there is the presence of ammonia gas in the cold?

A

In the cold, there is just enough ammonia gas produced for you to be able to smell it.

44
Q

How can you tell that there is the presence of ammonia gas after warming it?

A

If you warm it, you can test the gas coming off with a piece of damp red litmus paper or universal indicator paper.

45
Q

What colour would the ammonia turn the litmus/universal indicator papers?

A

Ammonia is alkaline and turns the litmus paper/universal indicator paper blue.

46
Q

What is special about this specific sodium hydroxide test?

A

This test works on a sample of a solid or a solution as you are not looking for a precipitate.

47
Q

What is step 1 of testing for carbonates?

A

Add a little dilute hydrochloric acid to your sample of salt and look for fizzing/bubbles of gas.

48
Q

What does fizzing show?

A

Fizzing indicated that a gas is given off.

49
Q

How can we test that the gas that is given off is carbon dioxide (indicating the presence of carbonates)?

A

You can test the gas by bubbling it through limewater to show that it is carbon dioxide.

50
Q

What is common to carbonates?

A

Most carbonates are insoluble in water and you will usually be doing this test on a sample of a solid.

51
Q

What are the soluble carbonates?

A

Sodium carbonate, potassium carbonate, and ammonium carbonate.

52
Q

Can the test be done on both soluble and insoluble carbonates?

A

The test also works on a solution of soluble and insoluble carbonates.

53
Q

What is the chemical equation that shows zinc carbonate and dilute nitric acid?

A

ZnCO3+2HNO3 —> Zn(NO3)2 + H2O

54
Q

What is step 1 for testing for sulfates?

A

Make a solution of your suspected sulfate in distilled water.

55
Q

What is step 2 for testing for sulfates?

A

Add some dilute hydrochloric acid to this solution.

56
Q

What is step 3 for testing for sulfates?

A

Then add some barium chloride solution.

57
Q

What is step 4 for testing for sulfates?

A

A sulfate will produce a white precipitate of barium sulfate,

58
Q

Why do we add acid?

A

You add acid to the solution to react with and remove other anions which might also produce white precipitates when you add the barium chloride solution.

59
Q

What would happen if we didn’t add acid?

A

You would also obtain a white precipitate if there were carbonate ions present. This is because barium carbonate is also white and insoluble. the acid reacts with the carbonate ions and removes them.

60
Q

When must the acid be added?

A

BEFORE the barium chloride solution.

61
Q

Why must you never acidify the solution with sulfuric acid?

A

Because sulfuric acid contains sulfate ions. If you add those, you are bound to get a precipitate of barium sulfate because you have added sulfate ions.

62
Q

What is step 1 of testing for chlorides, bromides, and iodides?

A

Make a solution of your suspected chloride, bromide, or iodide.

63
Q

What is step 2 of testing for chlorides, bromides, and iodides?

A

Add enough dilute nitric acid to make it acidic.

64
Q

What is step 3 of testing for chlorides, bromides, and iodides?

A

Then, add some silver nitrate solution.

65
Q

What is formed to show the presence of chloride ions?

A

A white precipitate (of silver chloride).

66
Q

What is formed to show the presence of bromide ions?

A

A cream precipitate (of silver bromide).

67
Q

What is formed to show the presence of iodide ions?

A

A yellow precipitate (of silver iodide).

68
Q

Why is the acid added?

A

To react with and remove other anions which might also produce precipitates with silver nitrate solution, for example carbonate and hydroxide ions.

69
Q

Why should you not use hydrochloric acid?

A

As you will be adding halide ions and you are certain to obtain a precipitate.

Chemistry (22 decks)

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