chemical analysis

Question 1

define a pure substance

Answer 1

pure substance = a single element or compound not mixed with any other substance

Question 2

how can we distinguish pure substances in mixtures?

Answer 2

measure its melting point and boiling point as pure substances and compounds melt and boil at specific temperatures, and impure substances melt and boil over a range of temperatures

Question 3

define a formulation

Answer 3

formulation = a mixture that has been designed as a useful product

Question 4

how are formulations made?

Answer 4

formulations are made by mixing the components in carefully measures quantities to ensure that the product has the required properties

Question 5

give examples of formulations

Answer 5

formulations include fuels, cleaning agents, paints, medicines, alloys, fertilisers, and food

Question 6

what can chromatography be used for?

Answer 6

chromatography can be used to separate mixtures and can give infomation to help identify substances

Question 7

what is the test for hydrogen?

Answer 7

a burning splint held at the open end of a test tube of the gas. hydrogen burns rapidly with a pop sound

Question 8

what is the test for oxygen?

Answer 8

a glowing splint is inserted into a test tube of the gas. the splint relights in oxygen

Question 9

what is the test for carbon dioxide?

Answer 9

when carbon dioxide is shaken with limewater (calcium hydroxide) the limewater turns milky

Question 10

what is the test for chlorine?

Answer 10

when damp litmus paper is put into chlorine gas the litmus paper bleaches and turns white

Question 11

what are flame tests used to identify

Answer 11

metal ions (cations)

Question 12

what colour flame does lithium, sodium, potassium, calcium, and copper produce in a flame test?

Answer 12

• lithium = crimson flame
• sodium = yellow flame
• potassium = lilac flame
• calcium = orange-red flame
• copper = green flame

Question 13

why are the colour of some flames difficult to distinguish?

Answer 13

as if a sample containing some mixed ions is used some flame colours can be masked or if it only contains a low concentration of the metal compound

Question 14

what is flame emission spectroscopy?

Answer 14

an instrumental method used to analyse metal ions in solutions

Question 15

how does flame emission spectroscopy work?

Answer 15

a sample is put into a flame and the light given out is passed through a spectroscope. the output is a light spectrum that can be analysed to identify the metal ions in the solutions and measure their concentrations

Question 16

what are instrumental methods used

Answer 16

elements and compounds can be detected and identified using instrumental methods

Question 17

what are the advantages of instrumental methods compared with the chemical tests

Answer 17

• are more rapid
• are more sensitive so will work on even a tiny sample of metal compound
  -are more accurate so are more likely to identify a metal ion correctly than using a flame test

Question 18

what can sodium hydroxide solutions be used for?

Answer 18

sodium hydroxide solution can be used to identify some metal ions (cations)

Question 19

what do solutions of aluminium, calcium, and magnesium ions form when sodium hydroxide solution is added?

Answer 19

white precipitates

Question 20

what happens when we add excess sodium hydroxide solution to the aluminium precipitate?

Answer 20

the aluminium precipitate redissolves

Question 21

what colour precipitates do copper, iron(II), and iron(III) form?

Answer 21

• copper = blue precipitate
• iron (II) = green precipitate
• iron (III) = brown precipitate

Question 22

how do we test for carbonate ions?

Answer 22

add dilute acid to carbonate. it will react to produce carbon dioxide. then test for carbon dioxide by bubbling the gas through limewater, if it turns milky then carbon dioxide is present which means we started with a carbonate ion

Question 23

how do we test for halide ions?

Answer 23

add dilute nitric acid to the halide ion sample. then add dilute silver nitric acid. halide ions produce a precipitate of the silver halide. each halide makes a different coloured precipitate.

Question 24

what colour precipitate do chloride, bromide, and iodide ions form?

Answer 24

• chloride ions = white precipitate of silver chloride
• bromide ions = cream precipitate of silver bromide
• iodide ions = yellow precipitate of silver iodide

Question 25

how do we test for sulfate ions?

Answer 25

add dilute hydrochloric acid to the sulfate ion sample. then add barium chloride solution. if sulfate ions are present we will see a white precipitate