Analysis

Question 1

define pure substance

Answer 1

A substance that consists of only one element or only one compound.

Question 2

how can you identify pure substances and mixtures from data about melting and boiling points?

Answer 2

• Pure substances have a sharp
  melting point but mixtures melt over a range of temperatures.
• This difference is most easily seen when the temperature of a liquid is measured as it cools and freezes
• a horizontal part of the graph shows that the substance has a sharp melting point, so it is pure.
• Impure substances would produce a gradual fall in temperature as it freezes (the graph goes from curved to a straight diagonal) (this is the melting range)

Question 3

define formulation

Answer 3

A mixture that is always made with the same proportions of the same substances that has been designed as a useful product

Question 4

what is chromatography?

Answer 4

used to separate mixtures (different substances dissolved in a liquid) of
soluble (able to dissolve in solvent) substances and to provide information on the possible identity of the substances present in the mixture

Question 5

what 2 different phases does chromatography rely on?

Answer 5

• mobile phase- phase in chromatography that moves, usually a solvent or mixture of solvents.
• stationary phase- phase in chromatography that does not move, for instance, the paper in chromatography.

Question 6

how can you identify pure substances from chromatography?

Answer 6

• a pure substance produces one spot on the chromatogram
• an impure substance produces two or more spots

Question 7

what does Rf value of a compound represent?

Answer 7

• the distance travelled by the substance divided by the distance travelled by the solvent (water)
• it’s always the same for a particular substance if run in the same solvent system

Question 8

what values does Rf vary between and what do they represent?

Answer 8

• Rf values vary from 0 (the substance is not attracted to the mobile phase)
• to 1 (the substance is not attracted to the stationary phase)

Question 9

what is the test for oxygen?

Answer 9

• Glowing splint held in a test tube
• Splint relights if present

Question 10

what is the test for hydrogen?

Answer 10

• Lighted splint held in a test tube
• Pop sound heard if present

Question 11

what is the test for carbon dioxide?

Answer 11

• Gas bubbled through limewater
• Limewater turns milky or cloudy white if present

Question 12

what is the test for chlorine?

Answer 12

• Damp litmus paper held in a test tube
• Paper turns white if present

Question 13

how do you carry out a flame test?

Answer 13

1. dip a clean wire loop into a solid sample of the compound being tested
2. put the loop into the edge of the blue flame from a Bunsen burner
3. observe and record the flame colour produced

Question 14

what ion does a crimson flame represent?

Answer 14

Lithium, Li+

Question 15

what ion does a yellow flame represent?

Answer 15

Sodium, Na+

Question 16

what ion does a lilac flame represent?

Answer 16

Potassium, K+

Question 17

what ion does an orange-red flame represent?

Answer 17

Calcium, Ca2+

Question 18

what ion does a green flame represent?

Answer 18

Copper, Cu2+

Question 19

what can sodium hydroxide be used to identify and how?

Answer 19

• Dilute sodium hydroxide solution is used in tests for some metal
  ions, which form metal hydroxides that are insoluble.
• This means that the metal hydroxides appear as precipitates.

Question 20

what 3 metal ions reacted with sodium hydroxide solution does a white precipitate represent?

Answer 20

• Aluminium, Al3+
• Calcium, Ca2+
• Magnesium, Mg2+

Question 21

what metal ion reacted with sodium hydroxide solution does a blue precipitate represent?

Answer 21

Copper(II), Cu2+

Question 22

what metal ion reacted with sodium hydroxide solution does a green precipitate represent?

Answer 22

Iron(II), Fe2+

Question 23

what metal ion reacted with sodium hydroxide solution does a brown precipitate represent?

Answer 23

Iron(III), Fe3+

Question 24

how do you write balanced equations for reactions between sodium hydroxide solution and and metal ions?

Answer 24

• the general equation is: metal sulfate + sodium hydroxide → sodium sulfate + metal hydroxide
• however the precipitation reactions can be represented using
  ionic equations
• these only include the ions which are involved in the formation of the precipitate. They ignore the
  spectator ions, which are present but not involved.
• these only represent the metal ion reacting with the hydroxide ion to make metal hydroxide
• they are structured like this: Fe3+(aq) + 3OH-(aq) → Fe(OH)3(s)

Question 25

how do you distinguish between aluminium ions, calcium ions and magnesium ions being present when reacting with sodium hydroxide since the precipitate is white for all of them?

Answer 25

* the aluminium hydroxide precipitate dissolves to form a colourless solution * the calcium hydroxide precipitate is unchanged * the magnesium hydroxide solution is unchanged

Question 26

how are do you test for negative carbonate ions?

Answer 26

* Carbonate ions, CO32-, are detected using a dilute acid. * Bubbles are given off when an acid, usually dilute hydrochloric acid, is added to the test compound. * The bubbles are caused by carbon dioxide. * Limewater is used to confirm that the gas is carbon dioxide. It turns milky/cloudy when carbon dioxide is bubbled through it. * Unlike the tests for other negative ions, the test for carbonate ions works whatever acid is added.

Question 27

how do you test for negative sulfate ions?

Answer 27

* Barium ions react with sulfate ions, SO42- to form insoluble white barium sulfate: * Ba2+(aq) + SO42-(aq) → BaSO4(s) * To test for sulfate ions: * add a few drops of dilute hydrochloric acid to the sample * add a few drop of dilute barium chloride solution * A white precipitate forms if sulfate ions are present.

Question 28

how do you test for negative halide ions?

Answer 28

* Silver ions react with halide ions (Cl-, Br- or I- ions) to form insoluble precipitates. * the colours of these silver halide precipitates: - Chloride, Cl- : White - Bromide, Br- : Cream - Iodide, I- : Yellow * For example, when silver nitrate is added to a solution containing chloride ions: * Ag+(aq) + Cl-(aq) → AgCl(s) * To test for halide ions: * add a few drops of dilute nitric acid to the sample * add a few drop of dilute silver nitrate solution * Observe and record the colour of any precipitate that forms.

Question 29

state 3 advantages of using instrumental (machine) methods to identify elements and compounds compared to chemical tests

Answer 29

Compared to simple laboratory tests, instrumental methods of analysis may give improved: * speed * accuracy * sensitivity (they can detect very small amounts of a substance in a small amount of sample)

Question 30

what is a flame emission spectroscope and what is it used for?

Answer 30

* The flame emission spectroscope is a scientific instrument based on flame testing. * Data from a spectroscope can be used to: - identify metal ions in a sample. - determine the concentration of metal ions in dilute solutions

Question 31

how can metal ions be identified using flame emission spectroscopy?

Answer 31

* In the flame emission spectroscope, the coloured light from a vaporised sample can be split to produce an emission spectrum. * The different lines in an emission spectrum look like a coloured barcode. * Each metal ion produces a unique emission spectrum. * The metal present in a sample is identified by comparing its spectrum with reference spectra. * These are emission spectra from known metal ions. * If two spectra match, they must be from the same metal ion.