Chemistry P3 practical tips

7/27/2019 Chemistry P3 practical tips

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Titrations

1. No unnecessary titrations. If you have done two that are close enough,

that's already sufficient. Remember close enough means two titres within

o.10 cm^3. Example 21.50 and 21.60 is fine, 22.75 and 22.80 is fine but

23.25 and 23.40 is not enough. In such cases, perform another titration.

BURETTE: A burette has an accuracy of 0.05 cm^3. The graduations are

after every 0.10 cm^3 but you can read up to an accuracy of 0.05 cm^3. So

the maximum accepted error in the titre is 0.10 cm^3 and that's how close

the two readings we pick should be.

Graphs

1. The scale needs your values to cover at least half of the page, mind it.

2. Sharp your pencil, use 2H pencil(not HB) for graphs if available. You can

get in a good stationary shop.

3. Double check before plotting, but be real quick. One mistake in plotting

and you may mess with the whole graph.

4. Line of best fit doesn't strictly mean line only, sometimes(rarely) it could

be a curve too.

5. Always draw a triangle for gradient. The bigger the traingle, the more

accurate is your gradient.

6. Ask me now if you don't know ideas about gradient and y-intercept clearly.

7. Always write by the axis, what it represents along with its unit.

8. Independent on x-axis, dependent is y-axis. Independent is something you

are changing, depending is something you are measuring.

9. LABELLING!!!

10. Not more than 15 minutes on graph!!!

11. Even if graph is wrong, you won't lose *dherai* marks, just don't lose

hopes and work out on gradient and intercept stuffs, you will get method

markings. But never leave the questions, just fill the space even if you go

blank.

12. When they ask you to prove some relationship like T=kV or T is directly

proportional to V, write that since the graph is a straight line, the value of the

K is constant and therefore the relationship is true. DON'T FORGET IT!!!

13. If you don't have enough time, don't make all points. Make two or three


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points and work out on other stuffs... Believe me, it helps if you are running

out of time.

Salt analysis (Ion test)

1. There is a trick with aluminium and lead(II) ions. Both of them give same

observation with ammonia and sodium hydroxide. To distinguish them, you

need to add HCl . If precipitate forms, it is because of lead(II) ions, otherwise

it is aluminium ions. Lead chloride is insoluble.

2. Never spend more than 45 minutes on this section.

Gas test

1. You should write the OBSERVATION of the confirmatory test whenever

needed. But to save time, you DO NOT need to perform it. Just use your

common sense of chemistry.

Example, when ZnCO3 is reacted with HCl, you know that a gas is released.

What could that be? Just think...it is CO2...Now what you need to write is

something like this(already given in the booklet) -"Colorless gas which turns

lime water milky"

Already given in the booklet, but remember NO2 is brown in color and to myknowledge, it is very difficult to observe it. Common sense helps a lot.

Otherwise use a white paper or tile as background while observing.

2. Ammonia is pungent, very disturbing...sometimes they make you smell it.

YES, they do! Don't forget to use wet red litmus to test ammonia as well. Red

litmus turns into blue because ammonia is a base.

3. If you are not familiar with the smells of gases, you can confuse colorless

gases which smell differently. H2S, for example, smells like rotten eggs while

SO2 smells like the smoke just after you burn a match.

General Tips

1. Read carefully the names of all solutions..otherwise you will have to suffer.

Sometimes you might have to start from the beginning, it will kill your

confidence as well as time. Unlike physics practical, you need to be very


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fast...It is very unlikely to happen that you will finish before time.

2. Don't spend time washing burettes and all...do it real quick!!! Like 1

burette washing in 10 seconds.

3. If you think some solutions have been mixed or there is impurity or even if

your intuition says daal mey kuch kaaala hey, then ask them to replace it!

They will not scold you like Minnu mam.

4. You make get confused with droppers. It helps making marks on

them..whatever, don't use same dropper for two solutions. Believe me, it will

affect your results.

5. If strong heating is directed, you should STRONG heat it, not just

warming...don't just write the observation fast.

6. Some examiners might be strict, they may not provide you chemicals

again until empty. Be careful in spending.

7. Use rationally pipette, burette or measuring cylinder.

8. Quote from the syllabus:

"Candidates should normally record burette readings to the nearest

0.05 cm^3 and temperature readings to the nearest 0.5 C when

using thermometer calibrated in 1 C intervals, and to the nearest 0.1

C when the interval is 0.2 C"

9. Talking about errors, an example:

Error in any instrument is halfof the smallest unit you can read on it. For

instance, the stopwatch we had in the lab had the smallest unit as a 0.01s so

the error in the readings read would be +/- 0.005s.

Now you can find the uncertainty using the following formula, it might give

you an understanding why it depends on the size of the value you measure.

Ex. If I measure a value of 50s using the same stopwatch as mentioned

before with the error 0.005s. Then,

Uncertainty = [(error)/(my measured value)]*100

= [0.005/50]*100

= 0.01 percent.

But it is much more complex due to human reaction time in stopwatchs. They

won't ask you, hopefully. :)


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7. Lastly, even if the practical goes horrible(like with me), don't get your

hopes down. You can easily manage to get 'a' if you work good on P1 and P2.

I've seen students with 'U' on P3, still they managed to get overall 'a'. :)

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Chemistry P3 practical tips