C12- Chemical analysis

Question 1

What is a pure substance?
Example?
What is a mixture?
Example?

Answer 1

A compound or element that only contains one substance
Pure water (just H2O)
Two or more substances that aren’t chemically combined
Air

Question 2

Key fact about pure substances?
Key fact about impure substances?

Answer 2

They have specific melting and boiling points and will melt/boil at exactly that temp
They will melt/boil over a range of temps

Question 3

What are fixed points?

Answer 3

The melting and boiling points of a substance

Question 4

How does rock salt help roads become less slippery?
How does salt help when cooking?

Answer 4

Lowers the melting point of water, the impurities disrupt the structure of ice, weakens intermolecular forces, less ice
Raises the boiling point of water, food cooks more quickly

Question 5

Formulation?
2 Examples?

Answer 5

A mixture designed to create a useful product. Measured in fixed proportions to create specific properties
Paint, washing up liquid

Question 6

Paper chromatography definition?

Answer 6

The separation of substances using a solvent which carries the mixture through paper

Question 7

Chromatography
Stationary phase?
Mobile phase?

Answer 7

The fixed surface where the process takes place (filter paper)
The solvent that carries the dissolves substances (solvent)

Question 8

From a chromatogram
What dyes are in the mixture?
How many substances it contains?

Answer 8

Contain dyes that travelled the same distance and have the same colour
The number of spots in the same column

Question 9

How to calculate the Rf value and what is it?

Answer 9

Retention factor = distance travelled by substance / distance travelled by solvent
The ratio of the distance a substance travels

Question 10

What does a higher Rf mean?
What does a lower Rf mean?
When will Rf change?

Answer 10

Substance is more attracted to solvent
Substance is more attracted to paper
The solvent and paper used

Question 11

Paper chromatography method?

Answer 11

-Draw pencil line 2 cm from bottom of the filter paper
-Put a small dot of each dye spaced apart on the pencil line and label
-Place small amount of water in a beaker
-Place filter paper in the solvent so the water is just below the dyes
-Leave for 5 mins, label solvent front and measure using a ruler, dry
-Calculate Rf values using Rf= distance moved by substance / distance moved by solvent

Question 12

Test for hydrogen?
Oxygen?
Carbon dioxide?
Chlorine?

Answer 12

Lit splint–>squeaky pop sound
Glowing splint–>splint relights
Bubble through limewater–>turns cloudy
Damp litmus paper–>turns white/bleached

Question 13

False positive

Answer 13

When a positive outcome of a test occurs but not from the substance you are testing for

Question 14

How to carry out flame test

Answer 14

Dip clean wire loop into compound being tested. Place it in blue bunsen flame. Observe and record flame colour

Question 15

How to identify positive (metal) ions of group 1 metals?

Answer 15

Dissolve in water, add sodium hydroxide solution. No precipitate will form. Carry out flame test. Lithium ions present = crimson flame. Sodium ions present = yellow flame. Potassium ions present = lilac flame.

Question 16

How to identify positive (metal) ions of Group 2 metals?

Answer 16

Dissolve in water, add sodium hydroxide solution. White precipitate forms. Add excess sodium hydroxide. Precipitate doesn’t dissolve. Carry out flame test. Calcium ions present = orange-red flame. Magnesium ions present = no colour.

Question 17

How to identify positive (metal) ions of a Group 3 metal?

Answer 17

Dissolve in water, add sodium hydroxide solution. White precipitate forms. Add excess sodium hydroxide. Precipitate dissolves. Aluminium ions present.

Question 18

How to identify positive (metal) ions of transition metals?

Answer 18

Dissolve in water, add sodium hydroxide solution. Forms coloured precipitate. Copper (II) ions present = blue precipitate, then green flame in flame test. Iron (II) ions present = green precipitate, turns brown if left in air. Iron (III) ions present = brown precipitate.

Question 19

How to identify negative (non-metal) ions of Group 7?

Answer 19

Dissolve in water, add nitric acid. No fizzing occurs. Add barium nitrate. No precipitate formed. Add acidified silver nitrate to new sample. Precipitate formed. Chloride ions present = white precipitate. Bromide ions present = cream precipitate. Iodide ions present = yellow precipitate.

Question 20

How to identify negative (non-metal) ions of carbonate and sulfate?

Answer 20

Dissolve in water, add nitric acid. Fizzing occurs. Carbonate ions present.
Dissolve in water. Add nitric acid. No fizzing. Add barium nitrate. White precipitate formed. Sulfate ions present.

Question 21

Ionic equations for positive ion precipitates formed? e.g Group 2 metals, Group 3 metal, transition metals
And the colour?

Answer 21

Mg2+ +2OH- –> Mg(OH)2 white
Ca2+ +2OH- –> Ca(OH)2 white
Al3+ +3OH- –> Al(OH)3 white
Fe2+ +2OH- –> Fe(OH)2 green
Fe3+ +3OH- –> Fe(OH)3 brown
Cu2+ +2OH- –> Cu(OH)2 blue

Question 22

Ionic equations for negative ion precipitates formed? e.g sulfate, group 7 non-metals
And the colour?

Answer 22

SO4 2- + Ba2+ –>BaSO4 white
Cl- + Ag+ –> AgCl white
Br- + Ag+ –> AgBr cream
I- + Ag+ –> AgI yellow

Question 23

Method to show a sample of calcium bromide solution. With ionic equations for precipitates formed.

Answer 23

Dissolve in water, split in three test tubes. In one, add sodium hydroxide, forms white precipitate. Ca2+ +2OH- –> Ca(OH)2. Add excess sodium hydroxide, precipitate doesn’t dissolve. Dip clean wire loop into solution, place in blue bunsen burner flame. Will turn orange-red, calcium ions present. In other test tube, add nitric acid, no fizzing will occur. Add barium nitrate, no precipitate will form. In a third test tube, add silver nitrate, cream precipitate forms. Br- + Ag+ –> BrAg. Bromide ions present.