Test for Ions

Question 1

what are the 2 tests for cations?

Answer 1

flame tests
Hydroxide precipitates

Question 2

what metals can be tested using flame test and what colour do they go

Answer 2

Lithium - red
sodium - yellow
potassium - lilac
calcium - orange/red
copper - blue/green

Question 3

how do you do the hydroxide precipitates test on a solution?

Answer 3

add a few drops of sodium hydroxide and check the colour

Question 4

what are the 5 metals that can be tested using the hydroxide precipitates test and what colour do they go?

Answer 4

copper - blue
Iron(II) - green
Iron(III) - brown
calcium - white
aluminium - white

Question 5

why do aluminium and calcium create a white precipitate and how do you identify each?

Answer 5

• they are not transitional metals so will not form coloured compounds
• add excess sodium hydroxide, the aluminium precipitate will dissolve and turn clear

Question 6

what other 4 ions can you test for (non-metals)

Answer 6

• sulfates
• carbonates
• halide ions
• ammonium ions

Question 7

what is the test for sulfate ions?

Answer 7

add dilute hydrochloric acid then barium chloride, if sulfate ions are present a white precipitate of barium sulfate will form

Question 8

what is the test for carbonate ions?

Answer 8

add hydrochloric acid, if there are carbonates bubbles of CO2 will form

Question 9

what is the test for halide ions?

Answer 9

add dilute nitric acid, then silver nitrate solution. then you can check the colour

Question 10

what 3 halides can be tested using the halide ion test, and what colour do they go

Answer 10

chloride - white
bromide - cream
iodide -yellow

Question 11

what is the test for ammonium ions?

Answer 11

add sodium hydroxide solution and heat, ammonia gas will be produced

Question 12

what are the 3 tests for ammonia gas?

Answer 12

• smell
• damp red litmus paper will turn blue
• hydrogen chloride gas can react with ammonia to form a white smoke of ammonium chloride

Question 13

why is nitric acid added before silver nitrate in a halide ion test?

Answer 13

nitric acid will react with the carbonate ions, if it was not added the carbonate would react with the silver nitrate and form a precipitate which can give a false-positive result

Question 14

why is hydrochloric acid not used in a halide ion test?

Answer 14

because it contains chloride ions, which will contaminate the solution and give a false-positive result

Question 15

what are instrumental methods?

Answer 15

use machines to detect and analyse substances

Question 16

what are the 3 benefits of instrumental methods?

Answer 16

sensitivity - they can detect and analyse very small amounts of different substances
accuracy - they measure the amounts of substances very accurately
speed- they can carry out each analysis quickly and machines can run all the time

Question 17

what does a flame photometer output

Answer 17

an emission spectra