(blue) Halogens Flashcards
(blue) Halogens (151 cards)
1
Q
what are the gp 7 elements in the periodic table
A
there called halogens , they are highly reactive , only exsits as compounds in nature
2
Q
how do gp7 elements for ions
A
they loose electrons the form halide ions
3
Q
what type of bond do they most likly form
A
covalent
4
Q
propertys of fluorine :
formula of molecule
A
F2
5
Q
propertys of fluorine :
Appearance and state at room temp
A
Pale yellow gas
6
Q
propertys of fluorine :
Electron
configuration
A
1s2 2s2 2p5
7
Q
propertys of fluorine :
Bleaching
ability
A
Very strong
8
Q
propertys of fluorine :
Solubility in
water and
appearance of
solution
A
Reacts with water to form HF and O2
9
Q
propertys of chlorine :
Formula of
molecule
A
Cl2
10
Q
propertys of chlorine :
Appearance
and state at
room temp
A
Green/yellow gas
11
Q
propertys of chlorine :
Electron
configuration
A
1s2 2s2 2p6 3s2 3p5
12
Q
propertys of chlorine :
Bleaching
ability
A
Strong
13
Q
propertys of chlorine :
Test for the gas
A
Damp blue litmus paper Turns red then bleached Or Damp starch iodide paper Turns blue
14
Q
propertys of chlorine :
Solubility in
water and
appearance of
solution
A
Slight solubility but also reacts to form chlorine water HCl and HOCl Pale green soln.
15
Q
Trend in
solubility in
H2O of Cl2 to I2
A
decreses in solubnility
16
Q
propertys of chlorine :
Solubility in
cyclohexane
and appearance
of solution
A
Soluble Yellow/green solution
17
Q
propertys of bromine :
Formula of
molecule
A
Br2
18
Q
propertys of bromine :
Appearance
and state at
room temp
A
Dark red/brown liquid
19
Q
propertys of bromine :
Bleaching
ability
A
Weak
20
Q
propertys of bromine :
Test for the gas
A
Damp starch + iodide paper Turns blue Bubble through KI solution with starch
21
Q
propertys of bromine :
Solubility in
water and
appearance of
solution
A
Slight solubility but also reacts to form bromine water HBr and HOBr Orange solution
22
Q
propertys of bromine :
Solubility in
cyclohexane
and appearance
of solution
A
Soluble red solution
23
Q
propertys of iodine :
Formula of
molecule
A
I2
24
Q
propertys of iodine :
Appearance
and state at
room temp
A
Shiny grey solid
25
propertys of iodine :
Electron
configuration
[Kr] 5s2 4d10 5p5
26
propertys of bromine :
Electron
configuration
[Ar] 4s2 3d10 4p5
27
propertys of iodine :
Bleaching
ability
none
28
propertys of iodine :
Test for the gas
Starch solution turns blue/black
29
propertys of iodine :
Solubility in
water and
appearance of
solution
Almost insoluble producing a very pale brown solution. Soluble in KI solution à dark brown solution.
30
propertys of iodine :
Solubility in
cyclohexane
and appearance
of solution
Soluble purple solution
31
describe the general trend in bleaching ability down the halogens
gets weaker
32
describe the general trend solubility on water down the group for halogens
F2 soluable , however rest is all silightly soluable
33
State, and explain the change in melting pts down .group 7
Increases, as the number of electrons increases the London forces also increase (IMF) requiring more energy to overcome.
34
State and explain the change in electronegativity down group 7.
Decreases, outer electrons are progressively more shielded from the nucleus as the atomic radius increases, also the distance between the bonding pair and nucleus increaces. (proton number increaes but this is more than offset by the other factors) So electrons in a covalent bond are attracted less to the halogen.
35
State and explain the change in reactivity of the Group 7 elements down group 7
As most reactions with halogens result in the halogen gaining an electron. They become less reactive on descending. Reactivity decreases as atomic radius increases and more shielding means electrons attracted less strongly to the nucleus.
36
What is the problem in distinguishing between bromine and iodine solutions?
Depending on concentration both could appear a brown/orange colour.
37
How could you show that you have a solution of iodine? Give two methods.
· Add an organic solvent and shake. Halogens are more soluble in organic solvents, Iodine —-> purple colour in upper, organic layer, bromine —-> red organic layer
· Add starch solution and shake. Iodine —->blue / black.
38
how would you seperate bromine from iodine in a seperating funnel
add organinc solvent e.g hexane
shake and allow to settle
iodine moves to hexane layer
open tap and run off iodine layer
39
most metals react with halogens to make what
ionic compounds
40
Mg(s) + Cl2(g) -----> MgCl2(s)
which one has been oxidised and which has been reduced
What has been oxidised :Magnesium
What has been reduced? chlorine
41
when Cl , br and I react with AL to what charge does AL get oxidised to
Al ----> Al 3+ + 3e -
42
aluminium + bromine
what are the :
Observations
Oxidation equation
Reduction equation
Bromine liquid dropped on Aluminium foil. Sparks and glowed red Got hot and a lot of bromine vapour evolved. A white solid remained.
Oxidation equation Al(s) -----> Al3+ + 3e-
Reduction equation Br2 + 2e- ----> 2Br-
43
aluminium + iodine:
what are the :
Observations
Oxidation equation
Reduction equation
Vigorous reaction, clouds of purple I2vapour. Solid glowed red-hot Grey solid AlI3 remaine
Oxidation equation : Al(s) ----> Al3+ + 3e-
Reduction equation: I2 + 2e- ------> 2I-
44
how do halogens oxidise Fe 2+
Fe 2+ goes to Fe 3+
halogens are also reduced to halide ions
45
what is the Observation on adding Fe2+ to Cl2
also what is the reduction , oxi and redox reaction
Solution became very pale yellow (thiocyanate ----> red therefore Fe3+ present)
reduction : Cl2 + 2e- ----> 2Cl-
oxi : Fe2+-----> Fe3+ + e-
redox Cl2 + 2Fe2+ -----> 2Fe3+ + 2Cl-
46
what is the Observation on adding Fe2+ to Br2
also what is the reduction , oxi and redox reaction
Solution became very pale yellow (thiocyanate ------> red therefore Fe3+ present)
reduction : Br2 + 2e- -----> 2Br-
oxi : Fe2+ ------> Fe3+ + e-
redox : Br2 + 2Fe2+ ------> 2Fe3+ + 2Br-
47
what is the Observation on adding Fe2+ to I2
also what is the reduction , oxi and redox reaction
No change (thiocyanate -------> unchanged - no Fe3+)
reduction + oxi + redox : none as no reaction
48
what is a test to test for Fe 3+
Additional test for Fe3+ = add potassium thiocyanate (KSCN) a red colour indicates Fe3+
49
what is the colour of KCl
colourless solution
50
what is the colour of KBr
colourless solution
51
what is the colour of KI
colourless solution
52
what happens when you mix
chlorine with KCl
nothing
as Cl cannot oxidise its self
53
what happens when you mix
chlorine with KBr
colourless solution turns pale yellow
hexane ----> red
this is because Cl oxidises the Br
54
what happens when you mix
chlorine KI
colourless solution turn orange brown
henane ----> purple
this is because Cl oxidises the Br
55
what happens when you mix
bromine with chlorine
just a dilution of the orange colour
Br cannot oxidise Cl
56
what happens when you mix
bromine with Br
nothing as Br cannot oxidise its self
57
what happens when you mix
bromine with KI
turned dark orange ,
starch ----> blue/black
Br can oxidise I from KI
58
what happens when you mix
iodine with KCl / KBr /KI
nothing there will just be a dilution of the origonal colour as iodine cannot oxidise any of the halide ions
59
Does chlorine (Cl2) oxidise
Br- (aq)
I- (aq)
yes
yes
60
Does bromine (Br2) oxidise
Cl- (aq)
I- (aq)
no for Cl
yes for I
61
Does iodine (I2) oxidise
Cl- (aq)
Br- (aq)
no
no
62
give the oxidation half equation for when chlorine oxidises Br
then give a full redox equation
Oxidation of bromide ions
2Br-(aq) -----> Br2(aq) + 2e-
Oxidation of bromide ions by chlorine
Cl2(aq)+2Br-(aq) --------> Br2(aq)+2Cl-(aq)
63
give the oxidation half equation for when chlorine oxidises iodine
then give a full redox equation
Oxidation of iodide ions
2I-(aq) -----> I2(aq) + 2e-.
Oxidation of iodide ions by chlorine
Cl2(aq)+2I-(aq) ------ >I2(aq)+2Cl-(aq
64
give the oxidation half equation for when bromine oxidises iodide
then give a full redox equation
and give reduction half equation
in KI
Oxidation half equation
2I-(aq) -----> I2(aq) + 2e-
Reduction half equation
Br2(aq) + 2e- ------> 2Br-(aq)
full redox equation of bromine and iodide
Br2(aq) + 2KI(aq) -----> I2(aq) + 2KBr(aq)
65
Put the halogens in order of their oxidising ability and explain the trend.
Cl2 > Br2 > I2 Chlorine is able to oxidise both bromide and iodide, it has fewest electon shells with least shielding and smallest radius so has the strongest ability to attract the electron from iodide or bromide.
66
give the definition of a disproportionation reaction
A disproportionation reaction is: the simultaneous oxidation and reduction of an atom of the same element in an ion or molecule.
67
though halogens are sparingly soluable , it can sometimes dissolve in water
give that equation
Cl2+ H2O -----> HCl + HOCl
68
Use oxidation numbers to show that this is a disproportionation reaction
Cl2+ H2O -----> HCl + HOCl
Chlorine is simmultaneously oxidised from 0 in Cl2 to +1 in HOCl and reduced from 0 in Cl2 to -1 in HCl
69
what is the observation of adding Naoh to Cl2
turned colourless, negative test for Cl2
70
what is the observation of adding Naoh to Br2
turned colourless
71
what is the observation of adding Naoh to I2
turned colourless
72
give the general formula for halogens reacting with cold alkali
X2 (g) + 2NaOH (aq) → NaX (aq) + NaXO (aq) + H2O
73
Write ionic equations for the disproportionation reactions of:- leave out spectator ions
bromine with cold alkali
Br2 + 2OH- ----> Br - + OBr - + H2O
74
Write ionic equations for the disproportionation reactions of:- leave out spectator ions
iodine with cold alkali
I2 + 2OH- ---> I- + OI- + H2
75
give the reaction of Cl with HOT alkali
also give the ionic equation
3Cl2 (aq) + 6NaOH (aq) → 5NaCl (aq) + NaClO3 (aq) + 3H2O (l)
ionic
3Cl2 (aq) + 6OH- (aq) → 5Cl- (aq) + ClO-3 (aq) + 3H2O (l)
Ionic equation (leaving out Na+ spectator ions).
76
Write full equations for the disproportionation reactions of iodine with hot sodium hydroxide (including spectator ions)
3I2 (aq) + 6NaOH (aq) → 5Nal (aq) + NalO3 (aq) + 3H2O (l)
77
Write full equations for the disproportionation reactions of bromine with hot sodium hydroxide (including spectator ions)
3Br2 (aq) + 6NaOH (aq) → 5NaBr (aq) + NaBrO3 (aq) + 3H2O (l)
78
Write ionic equations for the disproportionation reactions of iodine and bromine with hot alkali (leaving out spectator ions).
iodine : 3I2 (aq) + 6OH- (aq) → 5l- (aq) + lO3- (aq) + 3H2O (l)
Bromine: 3Br2 (aq) + 6OH- (aq) → 5Br- (aq) + BrO3- (aq) + 3H2O (l)
79
info card (read and recite)
Hydrogen chloride gas is very soluble in water, reacting with it to produce hydrochloric acid. The familiar steamy fumes of hydrogen chloride in moist air are caused by the hydrogen chloride reacting with water vapour in the air to produce a fog of concentrated hydrochloric acid.
80
give the equation for when HCL reacts with water
HCl(g) + H2O(l) → HCl (aq) → H+(aq) + Cl- (aq)
81
what are the observations of reacting conc ammonia and conc HCl
Observations
Hydrogen chloride gas forms steamy fumes in moist air due to HCl dissolving in water droplets and forming HCl(aq).
HCl(g) forms a white smoke with NH3(g), this is solid NH4Cl particles
82
what is the equation of reacting conc ammonia and conc HCl
NH3(g) + HCl(g) → NH4Cl (s)
83
give examples of metal halides
NaCl, NaBr, NaI
84
Two types of reaction can take place when metal halides (e.g.NaCl, NaBr, NaI) react with conc H2SO4
what are they
· Redox reactions forming Cl2, Br2 or I2
· Displacement reactions forming HCl, HBr or H
85
info card (read and recite)
We already know that the ability of the halogens Cl2, Br2 and I2 to gain electrons varies. The halogen which gains electrons most easily is chlorine This is, therefore the strongest oxidising agent.
It follows that the halide ions Cl-, Br- and I- will have differing ability to lose electrons. Which halide ion will lose electrons most easily? iodide. This is, therefore the strongest reducing agent.
Concentrated sulphuric acid contains the sulfate (VI) ion of formula/charge SO42- . The S in this ion has oxidation number +6
86
how do you test for HCl
use NH3
87
after adding KCl with H2SO4 what are the observations
also what are the gases evolved
Vigorous became hot (exothermic) Steamy fumes litmus à red. Acidic gas Fumes produced white smoke when tested with NH3 and white ppt with AgNO3
gases evolved
HCl
88
after adding KBr with H2SO4 what are the observations
also what are the gases evolved
Vigorous became hot (exothermic) Steamy fumes litmus ----> red. Acidic gas Fumes produced white smoke when tested with NH3 Orange vapour Gas turned orange dichromate(VI) paper or solution green.
gases evolved :
HBr Br2 and SO2 Cr(VI) has been reduced to Cr(III) by SO2 gas
89
after adding KI with H2SO4 what are the observations
also what are the gases evolved
Very Vigorous became very hot Steamy fumes -----> white smoke + NH3 Brown black deposits Acidified dichromate -----> green. Lead ethanoate à black
gases evolved:
HI(g) I2 , SO2(g) , H2S(g)
90
give the equation for the displacement reaction of conc. sulphuric acid with sodium chloride
H2SO4 + NaCl → NaHSO4 (s) + HCl(g)
NOTE: There is no change in the oxidation numbers of any species.
This is the only reaction for a metal chloride with concentrated sulphuric acid.
91
give the reaction of KBr with sulfuric acid
is HBr oxidising or reducing agents?
KBr + H2SO4 → KHSO4 + HBr displacement
2HBr + H2SO4 → Br2 + 2H2O + SO2 redox
reducing agent
92
give the reaction of KI with sulfuric acid
is HI oxidising or reducing agents?
KI + H2SO4 → KHSO4 + HI displacement
8HI + H2SO4 → H2S + 4H2O + 4I2 redox
reducing agents
93
Explain why iodides react differently from chlorides.
(hydrogen) iodide is more easily oxidized / loses electrons more easily than (hydrogen) chloride because it has a larger radius (than chloride).
94
which is stronger when reacting with sulfuric acid
HI / HBr / HCl
HI(g) can reduce S in H2SO4 from +6 to -2 in H2S
Whereas HBr only reduces S in H2SO4 from +6 to +4 in SO2.
95
what are the observations of adding silver nitrate and HNO3 to Cl
white ppt
96
what are the observations of adding silver nitrate and HNO3 to Br
cream ppt
97
what are the observations of adding silver nitrate and HNO3 to I
yellow ppt
98
what are the observations of adding excess dilute NH3 to a silvernitrate solution for Cl
Ppt. dissolved with dil.NH3
99
what are the observations of adding excess dilute NH3 to a silvernitrate solution for Br
Ppt. dissolved in CONC NH3
100
what are the observations of adding excess dilute NH3 to a silvernitrate solution for I
Ppt. did not dissolve
101
Write ionic equations for the reactions between silver ions and halide ions, including the states of matter.
Chloride: Ag+(aq) + Cl-(aq) --->AgCl(s)
Bromide: Ag+(aq) + Br-(aq) ---> AgBr(s)
Iodide Ag+(aq) + I-(aq) ---> AgI(s)
102
Suggest why dilute nitric acid was added to the silver nitrate solution. Think what other precipitates it might dissolve.
Prevents precipitation of silver hydroxide, oxide or carbonate which would interfere with the observations.
103
How does adding NH3 allow you to identify the precipitate?
to Agcl
It allows confirmation of precipitate as AgCl is soluble in both dil and conc NH3, AgBr is only soluble in conc. NH3 and Agl is insoluble in NH3
104
what is the effect on silver nitrate solutions with the halogens
AgCl - Decomposes to silver (goes lilac)
AgBr - Decomposes slowly to silver(grey)
AgI - No change
105
Silver chloride and silver bromide react with ammonia solution to form a complex ion
give the equation
AgCl(s) + 2NH3(aq) ---> [Ag(NH3)2]+(aq) + Cl-(aq)
106
Silver chloride and silver bromide decompose on exposure to light
giev the equation
2AgCl(s) -----> 2Ag(s) + Cl2(g)
107
how do you test for a CO3 2- group
and also the observation you would see
add HCL , effervescence and colourless solution produced
108
how do you test for a HCO3 - group
and also the observation you would see
On heating / boiling the solution without acid a gas is produced.
109
what gas has evolved from CO3 2- when adding HCl , and/or boiling HCO3 -
and how would you test for this gas
Bubble through limewater which would give a white ppt./turn cloudy
110
Give the reaction including state symbols for the reaction between the carbonate and the acid
Na2CO3(aq) + 2HCl (aq) ------> 2NaCl(aq) + H2O(l) + CO2(g)
111
Give an ionic equation for the reaction between carbonate and hydrogen ions
CO32-(aq) + 2H+(aq) -------> H2O(l) + CO2(g)
112
Give the reaction including state symbols for the reaction between the hydrogencarbonate and the acid
NaHCO3(aq) + HCl (aq) ---------> NaCl(aq) + H2O(l) + CO2(g)
113
Give an ionic equation for the reaction between the hydrogencarbonate ion and hydrogen ions
HCO3-(aq) + H+(aq) ---------> H2O(l) + CO2(g)
114
what are the observation of adding SO4 2- and dil HCL
then after that adding BaCl2 (barium chloride)
colourkess solution produced
immediate thick white ppt
115
Give the equation for the reaction between barium chloride and sodium sulfate, including state symbols.
Na2SO4(aq) + BaCl2(aq) ------> BaSO4(s) + 2NaCl(aq)
116
Give an ionic equation for the reaction between sulfate and barium ions
SO42-(aq) + Ba2+(aq) -------> BaSO4(s)
117
info (read and recite)
Note. Addition of barium chloride to a carbonate will give a white precipitate of barium carbonate. However, if HCl is added to the anion solution before BaCl2(aq) then only the sulfate(VI) SO42- will form a precipitate.
118
after adding NaOH to NH4 + and heating gently what are the observations
Gas given off, smell of NH3
119
after adding NaOH to NH4 + and heating gently what are the observations when putting it infrom of DAMP red litmus paper
Turned blue
120
after adding NaOH to NH4 + and heating gently what are the observations when putting it infrom of DAMP Blue litmus paper
no change
121
give the equation between NH4 + and NaOH
NH4+ + OH- ----->NH3 + H2O
122
info (read and recite)
The warming (of NH4+ and NaOH) releases the ammonia gas, which is detected by the litmus paper. This is a example of an acid/base reaction which you will study next year.
123
give the test and observation for:
Li+
test :
Flame test Clean nichrome wire in HCl and use non-luminous flame Dip in HCl then solid. Hold at edge of non-luminous flame
obsvation :
Red
124
give the test and observation for:
Na+
test :
Flame test Clean nichrome wire in HCl and use non-luminous flame Dip in HCl then solid. Hold at edge of non-luminous flame
obsvation :
Persistent yellow orange
125
give the test and observation for:
K+
test :
Flame test Clean nichrome wire in HCl and use non-luminous flame Dip in HCl then solid. Hold at edge of non-luminous flame
obsvation :
lilac
126
give the test and observation for:
Ba 2+
test :
Flame test Clean nichrome wire in HCl and use non-luminous flame Dip in HCl then solid. Hold at edge of non-luminous flame
obsvation :
apple green
127
give the test and observation for:
Sr 2+
test :
Flame test Clean nichrome wire in HCl and use non-luminous flame Dip in HCl then solid. Hold at edge of non-luminous flame
obsvation :
red
128
give the test and observation for:
Ca 2+
test :
Flame test Clean nichrome wire in HCl and use non-luminous flame Dip in HCl then solid. Hold at edge of non-luminous flame
obsvation :
yellow - red
129
give the test and observation for:
Mg 2+
test :
To a solution add dil.NaOH
observation :
White ppt of Mg(OH)2(s) Mg2+(aq) + 2OH-(aq) ------> Mg(OH)2(s)
130
give the test and observation for:
NH4 +
test
To a solution add dil.NaOH, warm, and test gas with red litmus OR Heat the solid
observation
NH4+(aq) + OH-(aq) -------> NH3(g) + H2O(l) NH3 alkaline gas NH4+ sublimes producing a white solid further up the test tube.
131
give the test and observation for:
Cl -
test :
1)Acidify with dil HNO3 Add AgNO3 dropwise To the ppt. add dil NH3 followed by conc. NH3
2)Add a few drops of conc H2SO4 to the solid sample
3) To 2cm3 of the sample add chlorine water until no further changes.
observation :
1)White ppt of AgCl, Soluble in NH3
2) Steamy fumes HCl
132
give the test and observation for:
Br -
test :
1)Acidify with dil HNO3 Add AgNO3 dropwise To the ppt. add dil NH3 followed by conc. NH3
2)Add a few drops of conc H2SO4 to the solid sample
3) To 2cm3 of the sample add chlorine water until no further changes.
observation
1) Cream ppt of AgBr, Soluble in conc. NH3
2) Steamy fumes of HBr + Orange fumes of Br2
3) Solution turns orange
133
give the test and observation for:
I -
test :
1)Acidify with dil HNO3 Add AgNO3 dropwise To the ppt. add dil NH3 followed by conc. NH3
2)Add a few drops of conc H2SO4 to the solid sample
3) To 2cm3 of the sample add chlorine water until no further changes.
observation
1) Yellow ppt of AgI, insoluble in conc. NH3
2) Steamy fumes of HI + Black solid I2
3) Solution turns brown
134
give the test and observation for:
SO4 2-
test :
Add BaCl2(aq) dropwise followed by HCl(aq)
observation :
White ppt produced BaSO4. Ba2+(aq) + SO42-(aq) ----> BaSO4(s) Unchanged with HCl
135
give the test and observation for:
CO3 2-
test :
Add acid, test gas by bubbling through limewater.
observation :
Effervescence ----> colourless solution Gas turns limewater milky
136
give the test and observation for:
HCO3 -
test :
Heat aqueous solution
observation :
Effervescence, lime water ------> milky
137
give the test and observation for:
Oxygen
· Put a glowing spill into a test tube of the gas.
· It will relight
138
give the test and observation for:
carbon dioxide
· Bubble the gas through a solution of .limewater
· This will go cloudy / form a white ppt
139
give the test and observation for:
Ammonia
· Smelly gas. Damp red litmus goes blue
· With HCl gas white smoke is produced
140
give the test and observation for:
Nitrogen dioxide
· This is a brown coloured gas
· Damp blue litmus goes red
140
give the test and observation for:
HYdrogen
· Put a lighted spill into a test tube of the gas.
· It will give a squeaky pop
141
give the test and observation for:
HYdrogen
· Put a lighted spill into a test tube of the gas.
· It will give a squeaky po
142
give the test and observation for:
Hydrogen chlordie
· Steamy fumes. Damp blue litmus goes red
· With NH3 gas white smoke is produced
· When bubbled into AgNO3(aq) a white precipitate forms
143
give the test and observation for:
Hydrogen bromide
· Steamy fumes. Damp blue litmus goes red
· With NH3 gas white smoke is produced
· When bubbled into AgNO3(aq) a cream coloured precipitate
144
give the test and observation for:
Hydrogen iodide
· Steamy fumes. Damp blue litmus goes red
· With NH3 gas white smoke is produced
· When bubbled into AgNO3(aq) a yellow precipitate forms
145
give the test and observation for:
Cl2
· Smells of bleach
· Damp blue litmus paper goes red then bleached · Damp starch/iodide paper ----> blue
· Turns KBr (aq) from colourless to yellow / orange
146
give the test and observation for:
Br2
· Vapour is orange in colour.
· Turns KI (aq) from colourless to brown
147
give the test and observation for:
I2
· Vapour is purple in colour. Solid is gray
· Turns starch soln. from white to blue/black
148
give the test and observation for:
Water vapour
· Anhydrous copper(II)sulfate goes from white to blue
149
give the test and observation for:
Sulfur dioxide
· Acidified potassium dichromate solution goes from orange to green/blue
150
give the test and observation for:
Hydrogen sulfide
· Rotten egg smell.
· Lead(II) nitrate solution goes from colourless to black
