Note: For Singapore GCE O-Level Pure Chemistry Combine Science (Chemistry) students that are taking exams this coming week

Following are some quick questions that my O-Level Chemistry students asked me last two days as they revise their work and need to clarify some doubts before their exams.

I thought it would be great to share with all my readers (especially if you are one of those going for the exam also). Questions are posted in BLACK while my suggested answers / comments are posted in BLUE.

If you find the post useful to your friends, feel free to share the link with them. =)

Questions from Elizabeth:

Hi Mr Chua, I have 21 questions for you, could you kindly respond accordingly? Thanks ((:

1) What is the precision of measuring cylinder & pipette?

Measuring cylinder is to 1 cm³ accuracy.

Pipette normally is not used for measurement. Instead it is used to transfer specific volume of liquid/solution.
E.g 10 cm³, 25 cm³, 50 cm³, etc.

2) What exactly is calcium chloride?

It is an ionic compound / salt.

3) Is metal sulphate a salt?

Yes. Metal sulfate is a general term. So many examples you can easily think of are: Sodium sulfate, calcium sulfate, magnesium sulfate, etc.
They are all ionic compound / salt.

4) During O’s, for the test for sulphate, do I write:
-add nitric acid, then barium nitrate
OR
-add acidified barium nitrate?

Both can. And you will see either form in the questions in O-Levels. Adding nitric acid means to acidify barium nitrate solution.

5) To test for chlorine, do I use damp litmus paper OR damp blue litmus paper?

Given a choice, we would prefer to write damp blue litmus paper.
It turns from blue to red and then bleached.

6) What exactly happens when liquid freezes?

Liquid state changes to solid state.

Refer to Kinetic Particle Theory topic to understand how the ARRANGEMENT and MOVEMENT changes from L to S state.

7) What is the definition of ‘volatile’?

It refers to liquid that easily changes to gaseous state at room temperature.
E.g. Ethanol in perfume.

Sometimes, they apply it to solids that sublimes also. E.g. Solid iodine sublimes to become gaseous iodine at room temperature.

9) Does concentration affect cations?

Don’t understand this question.

10) For O’s,are these the only soluble hydroxides we need to know- Zinc, Aluminium & Potassium?

We talked about this before. I disagree with your answer above.
We usually remember Group I metal hydroxides are soluble in water (NaOH, KOH, LiOH)
as well as Ba(OH)₂ for Group II metal hydroxides. Ca(OH)₂ is sparingly soluble.

Zinc hydroxide and Alumnium hydroxide are INSOLUBLE! Refer to QA, test for Cations.

11) What does energy level on the energy profile diagram represent?

It shows 3 things.

1. Reactants higher or lower energy than Products
2. Enthalpy change is negative or positive
3. Presence of Activation Energy

12) In exo reactions, why do products have lower energy?

In EXO, enthalpy change is NEGATIVE.

Refer to the formula of:  Enthalpy Changes = H_B-B + H_B-M

Use the formula and reason out the thought process.

H_B-B is endo (+ve) whereas H_B-M is exo (-ve).

You will get it. Refer to textbook if you still dont. They give very good examples to understand.

13) Which is the definition of catalyst?

- Provide an alternative route for the reaction with lower activation energy?
- substance which speeds up the rate of reaction

Yes. Combine both sentence into one answer.

14) For the oxidation of ethanol, for the chemical equation, do I write 2[O] or O₂?

If due to bacterial oxidation, we should write O₂.

If used acidified potassium dichromate (VI) in lab, we normally write as 2[O].

Check textbook to re-affirm.

15) Is Group 1 hydroxides soluble?

Answer above.

16) For Group 1(Apart from SPA) salts, do I use titration or Excess method?

Theoretically, use TITRATION.

17) What are found ‘free’ in the ground?

The more common elements are Gold, Platinum and maybe silver.

18) For reduction by carbon, when do you know it will give Carbon Monoxide/ Carbon Dioxide?

I mentioned this before.

The equation required in your O-Level Pure Chemistry (code: 5072) is showing the use of carbon monoxide as the reducing agent.

Thus, the products from the reduction of iron (III) oxide in Blast Furnace should be MOLTEN IRON and CARBON DIOXIDE.

Forget about the carbon monoxide that your teacher told you it will happen for the prelim papers.

It is not in syllabus.

If O-Level it comes out, they will give you alot of clues if they want you to write the product as MOLTEN IRON and CARBON MONOXIDE.

19) Can Silver and Gold be reduced by Carbon and Hydrogen?

You meant Silver oxide? They can, but it is not necessary. You can simply heat it to get Silver metal.

Gold usually are found uncombined in the earth.

20) Do I write ‘Calcium Sillicate’ OR ‘slag’ for my answer during O level?

It is Calcium Silicate. You have made a spelling error.

Both should be fine if they asked for substance.

But if they ask for chemical name, then you should write Calcium Silicate.

If they asked for Chemical Formula, then you write it as CaSiO₃.

21) When I add bromine to an AlkAne, do I say it decolourise slowly OR there is no observation?

I supposed you meant adding aqueous bromine, Br₂(aq).

Alkanes are saturated hydrocarbons and will not react with aqueous bromine. Thus, no changes. Aqueous bromine remains reddish-brown.

Only alkenes will decolourised the reddish-brown aqueous bromine since they are unsaturated.

Thanks in advance for answering (:

Sent from my iPhone

Questions from Lay Teng:

Her questions are in fact from the Pass With Distinction book that i wrote for Shinglee Publisher.

1) When questions ask about compare & contrast, do we have to mention both Similarity & Differences?

Yes. You need to. Let’s say questions ask us to compare and contrast Evaporation & Boiling process, you are required to mention the similarity and differences.

Similarity: Both involves the change of state from liquid to gas of water.

Differences:

A. Boiling is fast while evaporation is slow

B. Boiling is throughout the whole liquid while evaporation only at the surface

C. Boiling occurs at 100 ^oC (BP of water) while evaporation occurs below the boiling point

2) How come the book state Caesium atom and Caesium ion has different number of shells? And why the atom and ion has the same mass?

The answers given in the book is correct.

Caesium atom has 6 electron shells. When it gives away one electron, it forms Caesium ion which has only 5 electron shells.

Mass number is only determined by the sub-atomic particles present in the nucleus. i.e. protons + neutrons.

Since when atom becomes ion, only electrons are transferred, the mass will stay the same.

Acids, Bases & Salts are very important chapter in GCE O Levels Chemistry (IB Chemistry and IP Chemistry).

In fact, there are considered heavy-weightage topics (in terms of marks as well as content) that are related to many other chemistry topics. Many students lost their interest in Chemistry as early as Sec 3 / Year 3 after their school gone through these topics – they complained that they don’t understand these topics, and no matter how well they memorise (YES! Chemistry is NOT about memorising, contrary to many beliefs), they just can’t seemed to score in it.

Starting today, i am going to share with you several questions from Up Your Chemistry Grades Now showing some of the misconceptions made by many students, year-after-year. By knowing such common errors, it allows you to be aware and avoid such mistakes at all cost. In fact, this is the one of the strategy that we used in our weekly coaching classes and topical mastery workshops.

Do you think the student’s answers are correct? Think about it before you click to see the full post that contains the Examiner’s Comments below.