Chemical tests - TN

Question 1

test for hydrogen

Answer 1

burn it in oxygen
hear a squeaky pop

Question 2

why is a squeaky pop heard when hydrogen is burnt in oxygen

Answer 2

all the hot gas rushing out of the test tube

Question 3

test for oxygen and why

Answer 3

a glowing splint relighting - the oxygen in the tube is more concentrated than in normal air so the wood in the splint combusts faster

Question 4

how to test for the presence of water

Answer 4

anhydrous copper sulfate turning white to blue

Question 5

how to test the purity of water

Answer 5

measure its boiling point - 100*C

closer to 100*C = more pure

Question 6

test for carbon dioxide and why

Answer 6

bubble it through limewater - limewater is a saturated calcium hydroxide solution. it forms a white precipitate of calcium carbonate when it reacts with CO2

Question 7

test for chlorine

Answer 7

damp (the cl dissolves into the H2O) blue litmus turning blue to white - cl is a bleaching agent so it removes the colour.

Question 8

test for ammonia and why

Answer 8

damp red litmus turning red to blue

NH3 reacting with the H2O to form OH- ions which are alkaline = blue colour

Question 9

what is a cation

Answer 9

a positive ion

Question 10

what kind of flame is needed on a Bunsen burner in order to carry out a flame test

Answer 10

a non-luminous (blue) flame

Question 11

3 steps to conduct a flame test

Answer 11

1. dip a piece of nichrome/platinum wire into hydrochloric acid (these wires don’t colour the flame and the acid cleans the wire)
2. dip the wire into a sample of solid or solution
3. place the tip of the wire into a non-luminous bunsen flame

Question 12

flame test colour of lithium

Answer 12

red

Question 13

flame test colour of sodium

Answer 13

yellow

Question 14

flame test colour of potassium

Answer 14

lilac

Question 15

flame test colour of calcium

Answer 15

orange-red

Question 16

flame test colour of copper

Answer 16

blue-green

Question 17

how to test for copper without using a flame test

Answer 17

sodium hydroxide solution with a blue precipitate forming

Question 18

how to test for iron (II)

Answer 18

sodium hydroxide solution with a green precipitate forming

Question 19

how to test for iron (III)

Answer 19

sodium hydroxide solution with a brown precipitate forming

Question 20

how to conduct a sodium hydroxide test

Answer 20

1. dissolve the sample in water
2. add a few drops of NaOH

Question 21

how to test for ammonium (NH4)

Answer 21

sodium hydroxide solution - creates a gas that turns damp red litmus paper from red to blue

Question 22

chemical formula of ammonia

Answer 22

NH3

Question 23

chemical formula of ammonium

Answer 23

NH4

Question 24

what are the 3 steps for testing ammonium

Answer 24

1. dissolve the sample in water
2. add some NaOH and warm it
3. test the gas evolved with damp red litmus paper (ammonia test)

it NH4 ions are present the paper will turn blue

Question 25

what is an anion

Answer 25

a negatively charged ion

Question 26

how to test for halides

Answer 26

silver nitrate test

Question 27

what are the 3 steps of the silver nitrate test

Answer 27

1. dissolve the sample in water 2. add nitric acid 3. add silver nitrate solution is cl, br, or i ions are present, a precipitate forms with the Ag ions from the AgNO

Question 28

during a silver nitrate test for halide ions: what colour does Chloride turn

Answer 28

white precipitate

Question 29

during a silver nitrate test for halide ions: what colour does bromide turn

Answer 29

cream precipitate

Question 30

during a silver nitrate test for halide ions: what colour does iodide turn

Answer 30

yellow precipitate

Question 31

why do you add nitric acid first when performing a silver nitrate test

Answer 31

because the Ag ions that we need to react with the halides, also react and forms precipitates with other anions such as OH- and CO3. the HNO3 reacts with those anions preventing false positives

Question 32

how to test for carbonates

Answer 32

1. add hydrochloric acid, look for effervescence 2. if there is effervescence, bubble the gas through limewater if a carbonate is present, the limewater will turn cloudy

Question 33

why do we bubble the gas created (during a test for carbonates) through limewater

Answer 33

to test if the gas is actually CO2 because acids also evolve a gas when they react with some other things. this prevents false positives

Question 34

test for sulfates

Answer 34

1. add hydrochloric acid, look for effervescence 2. if there is no effervescence, add barium chloride solution is SO4 ions are present they will form a WHITE PRECIPITATE wiht the Ba ions from the BaCl

Question 35

during a test for sulfates, why do we first add hydrochloric acid

Answer 35

because carbonates also form a white precipitate with Ba ions. by only continuing to step 2 if there is no effervescence (the test for carbonates), we can prevent a false positive