Hi everyone,
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Today i am going to discuss on some “tricky” exam-based questions that were sent in by my own student under our Chemistry Ultimate Leap Programme, for discussion below:
Hi Sean, I have a few chemistry questions which I am unsure about.
Question 1: on Properties of Gases
A sample of polluted air is bubbled through water. The pH of the solution is less than 7. What gas causes this?
A. Carbon dioxide
B. Carbon monoxide
C. Sulfur dioxide
D. AmmoniaSuggested Answer by Student X:
My answer is carbon dioxide because it is an acidic gas and would thus form an acidic solution. But the correct answer is C. Sulfur dioxide. Why?
&
Question 2: on Qualitative Analysis (QA)
Aqueous sodium hydroxide and aluminium foil are warmed with a substance Y. An alkaline gas is liberated. Deduce which anion Y contains.
A. Carbonate
B. Chloride
C. Nitrate
D. SulfateSuggested answer by Student X:
My answer is C, nitrate, which is correct. But I don’t understand why. My reason for putting C is because carbonate produces carbon dioxide which is acidic, which chloride produces chlorine which I know is not alkaline. Then I chose nitrate over sulfate through guess work, and hence I want to know why nitrate is correct.
Do you agree with the above answers by my Student X?
The best way to learn is to work the answers out on your own and prove to yourself that it is correct. I always tell my students: “Challenge me (nicely) if you think something i said is incorrect!” – cos i believe the best way to learn is to experience it yourself..do not believe everything others say.
Confucius famous quote: “I hear and I forget. I see and I remember. I do and I understand.”
PS: Work out the answers, and leave your answers (and reasonings) in the “LEAVE A REPLY” section below. I will come in to give my suggested answers soon.
Hi Moses im a maldivinan student
‘Moses’wAt ur telling is correct but by analysing the 4 options we know very well that its talking only about negative ions. As far as i know according to the curriculum the the answere should be Nitrate.As in the the questions its talking about the test of nitrates ions only.
Q2 Aqueous sodium hydroxide and aluminium foil are warmed with a substance Y. An alkaline gas is liberated. Deduce which anion Y contains.
The test for chloride ions is adding silver nitrate and the result will be white precipitate forming.
all in all without having any doubt im sure that this is not a question to be confused if u know the syllabus well.
Hi friends! I would like to comment on Q2..why is it that chloride is wrong?..
Ammonium Chloride can react with alkaline solution and liberate ammonia gas!
One of them is an AMPHOTERIC Oxide – it will react with both acids and alkalis.
Think about it.
I hope my answers are not too late. For question 1, CO2 will not be accepted because in our air, this is already a small percentage of CO2, so we do not consider it that acidic. SO2 is chosen because SO2 forms H2SO3 in water which is much more acidic than HCO3-, thus it is much safer to choose SO2.
As for question 2, I learnt it in class 2/3 years ago. It’s a reaction that is not required of O level candidates. (:
Hi Al Nasr Ahmed,
See my reply to Marcus & Jie below.
All the best to your Chemistry,
Sean Chua =)
Hi Marcus,
Your reasoning for the question on Zinc Oxide is correct.
Keep it up!
For the question on photography chemistry, your steps in analysis is also spot on. Answer would be options 2 in which Ag+ ions has been reduced to Ag after taking in electrons.
Hope this information helps.
To Your Academic Success in Chemistry,
Sean Chua =)
Well Done!
I like your explanations on this question posted by Al Nasr Ahmed.
Keep up the good work!
Sean Chua =)
Hi Shahbuz,
Please go ahead and send me an email if you have chemistry questions that you would like to discuss with readers in the humble chemistry blogsite.
Thanks,
Sean Chua
To Jie:
Yes, zinc oxide is an amphoteric oxide, and reacts with the sodium hydroxide to produce salt and water.
The other options are not acidic oxides or amphoteric oxides, so they cannot react with the sodium hydroxide.
And for Q9, this is just a guess: photographic film contains silver, and since the film and sunlight have no bromine whatsoever, options 3 and 4 are out. It would be options 1 and 2 left.
very well Jie
try out no 9
Hey sean, I wanna try one reply no. 8 – Al Nasr Ahmed’s qn.
Is the ans zinc oxide?
Because it is am amphoteric oxide, thus reacting with both acid and alkali to produce salt (and water)
The other three choices are basic oxides(aka metal oxides), which will only react with acids to form salt (and water)
Is this explanation correct?
p/s just trying out 🙂
Dears,here 1 more.
which equation represents chemical change caused by exposing a photographic film
to sun light?
1 Ag=Ag+ + e-
2 Ag+ + e- = Ag
3 AgBr=Ag+ +Br-
4 2Ag + Br2=2AgBr
Hi friends.Um a new student but facing same problems like u.
My question in which um struck is :
which of the following reacts with aqueous sodium hydroxide to produce salt?
1 calcium oxide
2 iron 2 oxide
3 copper 2 oxide
4 zinc oxide.
Plz solve it for me.
I hope you say yes
I read “Huan’s” comment after deciding my own answers and am glad to see that they did not differ from mine.
Sir, do u mind me asking a couple of questions?
Great work! I understood it very well… Thanks
Hi “huaan”,
This is a good set of explanations above.
Keep it up!
And i look forward to more participation from you.
Cheers,
To Your Academic Success in Chemistry.
Sean Chua
Master Trainer & Author
WINNERS Education Group
?Experience Learning with A Difference?
Hi “hina”,
Good that you want to know why!
Now, i want you to think why the answer is (C) sulphur dioxide and give me your suggested answers.
The best way to learn is to participate…to seek the truth with your own thoughts.
PS: Look forward to hearing from you.
To Your Academic Success in Chemistry.
Sean Chua
Master Trainer & Author
WINNERS Education Group
?Experience Learning with A Difference?
Hi “Dcma 1993”,
Question 1: Answer is (C) indeed. But your explanations is not correct. CO2 is soluble in water just like SO2. There is a “clue” in the questions. Read it again. And it is linked to the chapter of “ATMOSPHERE / AIR”.
Question 2: Answer is (C) indeed. That is the standard test for Nitrates Anion (NO3-). NH3(g) is produced which is an alkaline gas.
PS: Think about it, based on the comments i have written. Hear from you soon!
To Your Academic Success in Chemistry.
Sean Chua
Master Trainer & Author
WINNERS Education Group
?Experience Learning with A Difference?
Question 1 is simple.
Basically, there are 2 answers, that will indeed form acidic solution when bubbled in water. They are carbon diovide and sulphur dioxide. Both, will definitely form a solution with pH of less than 7. Then we look back at the question. it states “sample of polluted air “. We have to recognise the fact that carbon dioxide is only a greenhouse gas, and is NOT an air pollutant. However, sulphur dioxide, instead, is one of the air pollutant, that is poisonous, and can cause acid rain. there, since they wanted “polluted air”, it is therefore sulphur dioxide.
Question 2 the answer is C.
Radically, we have to know that this question is a test of what anion is present.
In here, we see that substance Y is our main test for this whole reaction, and sodium hydroxide is added. And Ammonia will be given out after the reaction is completed.
thereotically speaking, from all i know for OLVL syllabus, we just have to recognise this trend in our QA, and acknowledge, that this is a test of NITRATE ions.
However other than that, i would think that this is a redox reaction, as Nitrate ions will be reduced to nitrogen by aluminium, which will react with sodium hydroxide, to form NH3. This is only a suggested reason.
i read that air pollution question whose answer is “c” “sulphurdioxide” i want
to know why??
For question 1, my answer is also C. Is it because Carbon Dioxide is only weakly soluble in water, unlike sulphur dioxide, which is vry soluble in water? I think question 1 is about choosing the best correct answer and not just the correct answer.
Question 2 definitely C. Ammonia gas would be given off. Ammonia is an alkaline gas.