A LEVEL CHEMISTRY - COLOURS OF COMPOUNDS

COLOURS OF COMPOUNDS

A few names of compounds, their colours, their special characteristics (if any), etc.
 You'll find this information useful in Acids, Bases, and Salts; Analytical Chemistry; and in the differentiate between two salt/acids/compounds questions.
 --> PbCl2 - white chalky ppt; insoluble in cold water, soluble in hot water.
 --> AgCl - white curdy ppt; insoluble in hot and cold water
 --> ZnO/PbO - yellow when hot, white when cold
 --> BaSO4 - white ppt; insoluble in all dilute mineral acids
 --> PbSO4 - white ppt; insoluble on heating (or when hot)
 --> Zn(NO3 )2 - white
 --> CuSO4 .5H2O - blue
 --> CuSO4 (after losing water of crystallization) - white amorphous
 --> CuCO3 - green
 --> Cu - reddish/pink deposit
 --> CuO - black
 --> MnO2 - black
 --> FeCl3 - reddish brown; NOTE: Name a reddish brown deliquescent salt (always FeCl3 )
 --> FeSO4 - dirty green
 --> KMNO4 - purple/violet
 --> K2Cr2O7 - orange
List of Gases:
 --> I2 - violet
--> Br2 - reddish/brownish gas
 --> NO2 - reddish brown gas
 --> Cl2 - greenish yellow gas; NOTE: NOT yellowish green
 --> SO2 - burning sulphur smell
 --> NH3 - green flame when burnt in oxygen
 --> NH3 , HCl, CO2 , SO2 - colorless gas with pungent odor
 --> HNO3 (pure) - colorless
 --> HNO3 (in sunlight) - yellow
List of Colored Ions:
 --> Cupric Cu2+ - Blue
 --> Chromium Cr3+ - Dark green
 --> Nickel Ni2+ - Green
 --> Ferrous Fe2+ - Light green
 --> Ferric Fe3+ - Brown
 --> Manganese Mn2+ - Light pink
 --> Cobalt Co2+ - Pinkish violet
 --> Permanganate MnO4 - - Pink
 --> Chromate CrO4 2- - Yellow
 --> Dichromate Cr2O7 2- - Orangish yellow NOTE: All salts of Na, K, NH4 are white

A LEVEL CHEMISTRY

LINK TO NOTES ON EXPERIMENTAL CHEMISTRY. THIS CAN HELP WITH PAPERS 3 AND 5.
http://www.teachifyme.com/experimental-chemistry/

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A Level Biology

TRANSMISSION OF ACTION POTENTIALS BETWEEN NERVE CELLS

How action potentials are transmitted:


o Action potential arrives at presynaptic knob, causing 
Ca2+ voltage gated channels to open


o Ca2+ ions cause exocytosis of Acetylcholine (ACh) 
vesicles (synapses that have acetylcholine are called 
cholinergic synapses)


o ACh moves to presynaptic knob temporarily binding to 
the receptor proteins that have complimentary shape

o This causes Na+ (chemical) channels in post synoptic 
knob to open, Na+ enter and cause depolarisation


o Ach is then recycled by enzyme acetylcholinesterase 
which breaks the it into acetate and choline which then 
recombined in the presynaptic knob


Note: If ACh is not broken down the Na+ channels will remain open so neurone will be permanently depolarised 


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A Level Physics

LIMITATIONS AND IMPROVEMENTS FOR A LEVEL PHYSICS PAPER 3

The List of Common Answers for the Limitations and Improvements Question of Paper 3 Question 2:

(a) Two/three readings are not enough to make a valid conclusion
- Take more readings and plot a graph
Note: This answer can be used for all experiments

(b) The value of (a physical quantity) is small so the percentage uncertainty of (the quantity) is large
- (The way to make the value of the quantity larger)

(c) The movement/oscillation of (something) is affected by wind movement
- Use a wind shield when carrying out the experiment

(d) Difficult to determine the start and end of oscillation/movement of (something) because it moves too fast
- Use a video camera with slow motion feature and timer to video the experiment with scale, then view the video playback frame by frame.

(e) Difficult to release (something) without applying a force
- Use a mechanical hand to release the (thing)

(f) Difficult to shape the plasticine into the shape of (something)
- Use a mould to shape the plasticine

(g) Heat loss through the sides and bottom of beaker/container
- Use polystyrene container or insulate the beaker/container
Note: This answer should be used for experiments involving the temperature of liquid.

(h) The (measuring instrument) is not precise enough
- Use another (instrument) with greater sensitivity and precision

Note: You should state in your answer the specific degree of precision for the limitation and the improvement.

(i) The length/diameter/thickness of (something) is not uniform
- Measure the length around/along the (thing) and calculate the mean

(j) Difficult to measure (something) due to (specific reason based on experiment)
- (Suggest a better way to measure it)

(k) Parallax error when measuring (something)
- (Suggest a better way to measure it, such as use mirror scale)
Note: This answer should only be used if the measurement is difficult to make and parallax error is very likely to occur.

(l) (Something) moves
- (Way to keep it in the original position)

(m) Oscillation does not occur in one plane only
Note: No possible improvement for this limitation. You should write in your answer the improvement for any other limitation.

(n) Difficult to maintain (something) at (a particular position) / maintain ruler vertical
- Use a clamp

(o) Difficult to zero the newton-meter when used horizontally
Note: No possible improvement for this limitation. You should write in your answer the improvement for any other limitation.

(p) Friction at pulley
- Apply oil to lubricate the pulley

(q) Resistance of contacts
- Clean the contacts
Note: This answer should be used for electric experiments.

(r) Difficult to determine when (something) reach the maximum height because it remains there for too short a time

(s) Difficult to take the reading of newton-meter immediately when (something) starts to move
because it moves suddenly

(t) Difficult to start or stop the stopwatch immediately when (something) passes through (somewhere)
- Use a video camera with slow motion feature and (the measuring device) to video the experiment with scale, then view the video playback frame by frame.
Note: The same improvement can be used for limitations r, s and t.

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