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.Related Articles:

• Chemistry Phenomenon – Sublimation Process
• O Level Chemistry – Physical & Chemical Properties of Alkalis
• O Levels Chemistry 2008 Exam Question: Organic Chemistry in Paper 1 (MCQs)
• O Level Chemistry: Assigning State Symbols in Chemical Equations (Exercise)
• O Level Chemistry SPA: Color of Copper Hydroxide Vs Copper Carbonate Vs Soluble Copper Salts

Chemical Bondings comprise of the following sub-topics:

• Ionic Bonding
• Covalent Bonding
• Metallic Bonding (not in Combine Science syllabus)

and examiners will be looking out for specific keywords from students.

There is this tendency that students used the wrong keywords associated with one type of bonding for another type, or miss out on keywords to score the grades they truly deserve in this basic Chemistry topic.

Let’s take a look at one of the exercises that i have recently gave to my Sec 4 GCE-O Level Chemistry Class.

Example 1: Student A

Remarks:

For Part (a)

1. Student do not know what to write in order to obtain the 2 marks in Part (a) of the question. He actually wrote a few lines of redundant information in which the examiners will not give him any marks for.

2. He will get 1 mark for mentioning that ions are held together by strong electrostatic forces of attraction which requires a large amount of energy to overcome.

3. However, he missed out on the other set of keywords that examiners are looking out for, which is: “Crystal Lattice Structure”. Other accepted keywords are “Giant Ionic Structure” or “Giant Lattice Structure”.

For Part (c):

1. We do not usually mention bonds in Ionic Compound. Instead, we should always give the answer with keywords such as “electrostatic forces of attraction between the ions”. This is what examiners are looking out for.

NOTE:

Chemistry students

- you need to know how your examiners / teachers mark your answers. You need to know what they want!

They have hundreds, if not tens of thousands of scripts to mark. As such, do not expect them to read your answer slowly and carefully, and award you marks for anything you write that may sound true.

In fact, they have an “Answer Script” that they follow very very very closely. For the above question of Part (a),  their “Answer Script” may just shows the following answers:

1. “Ions are held by strong electrostatic forces of attraction” (1 mark)
2. “Held in Crystal Lattice Structure” (1 mark)

As such, when they screen through your answer script and see the two sets of keywords, they will generally award you the full 2 marks – of course your answer must not be phrased until it’s out of point (or with redundancy).

Know What Examiners Want In Your Answer:

Do a search on archive blogposts that i have shared in last 3 years on Common Errors made by Chemistry Students.

If you have been studying very hard but just can’t seemed to score full marks for each question, you can check out Up Your Chemistry Grades Now – a Chemistry Critical Guidebook that i have written 2 years ago that address:

1. What Examiners Want In Your Answer
2. Common Mistakes Made by Students Year-after-Year

PS: Grab a f-r-e-e introductory copy of Up Your Chemistry Grades Now by Clicking HERERelated Articles:

• O Level Chemistry: Question on Chemical Bonding
• O Level Chemistry Essential Concepts Revision Workshop – Post Analysis Series – Part 1
• Chemistry Examination Tips – Paper 2
• O Level Chemistry Questions – Qualitative Analysis (QA) / SPA Tips
• O Level Chemistry: Question on Chemical Bonding

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.

Hope you find the information beneficial to scoring your A’s for upcoming GCE O Levels Chemistry Examinations (or other Chemistry-related examinations).

PS: Leave me a comment. I would love to hear from you. =)Related Articles:

• Dec Holidays ‘O’ Levels Chemistry Headstart & Revision Mastery Workshops – Successful Completion
• Successful Completion: O Level Organic Chemistry Mastery Workshop 2009
• How to Revise Effectively for GCE O Levels Chemistry Exams?
• O Levels Organic Chemistry: Common Conceptual Mistakes by Students
• O Level Chemistry Students: Thank You & All The Best!

Just a quick note on 2 common errors (mentioned in Up Your Chemistry Grades Now) that you would need to be aware of, so that you can avoid them at all cost.

Hope you will find this information useful to you.

PS: Do leave me a comment. I would love to hear from you.Related Articles:

• O Level Chemistry Questions: Acids, Bases & Salts
• O Level Chemistry Essential Concepts Revision Workshop – Successful Completion
• O Level Chemistry Essential Concepts Revision Workshop – Post Analysis Series – Part 1
• How to Revise Effectively for GCE O Levels Chemistry Exams?
• O Level Chemistry: Mole Calculation Question & Common Mistakes Made

(Picture adapted from David Scarlett)

With your school’s Preliminary Examinations (2 weeks – 1 month time) coming, i understand that most of the O Level students are frantically trying everything to revise through what your school has gone rush through for the past months. Many are feeling lost because for the last months, you have been spending most of your time in Co-Curricular Activities (CCA) and Singapore Youth Festivals (SYF) 2009. I am glad you have stepped down on all of these activities. Many have also struggled understanding on the topics of Ammonia & Air / Atmosphere (most school skipped them totally) as well as Organic Chemistry (rushes through to finish syllabus in school). GCE O levels Examinations is just 3 months away! Let’s FOCUS!!!

For the next 3 months, i will be sharing with you on specific tips and strategies on how to Revise for your O Levels Chemistry Exam and Score your As!

Let us see what you need to know =)

The GCE ‘O’ Level Chemistry Examination not only tests the number of chemical facts and definitions you have remembered, but also if you know how to apply the chemistry concepts and solve application problems.

About 20-30% of the marks are given for recall of facts and definitions, the rest are for understanding and applications of chemistry concepts. This is contrary to the many beliefs by parents (and students) that by plain memorising at the eleventh hours, you can score well in Chemistry exams by rote-learning and memorising your way to victory. This is a definite NO-NO!

Following are Skills required to DO WELL in GCE ‘O’ Level Chemistry Exam:

• Knowledge of DEFINITIONS, CHEMICAL FORMULAE, UNITS & SYMBOLS
• Knowledge of CHEMISTRY CONCEPTS such as Reactivity Series of Metals & Its Applications
• Knowledge of FACTS such as Uses of Petroleum Fractions
• Knowledge of OBSERVATIONS that occurs in many experiments
• Ability to use FLOW-CHARTS to solve questions related to Qualitative Analysis
• Ability to SKETCH, PLOT and INTERPRET graphs and diagrams
• Knowledge of the SOCIAL, ECONOMIC and ENVIRONMENTAL ASPECTS of Chemistry such as effects of pollutions
• Ability to use CHEMICAL CALCULATIONS to solve problems related to Mole Concepts and Chemical Reactions

As you can see, a large part of the marks will come from Chemistry Definitions and essential Chemistry Concepts. You will DO WELL if you know all the above points.. Sounds like a “tall-order”?? You can do it if you put in the effort! Check with you teachers to clear any misconceptions that you have. That is what my current group of Chemistry students has been doing – asking me questions before / after class, sending me questions via email / facebook, etc..

But if you are looking for a QUICK FIX to get you the As that you truly deserve, then you would need to know exactly what the examiners are asking you so that you know how to answer accordingly to score FULL MARKS.

In October 2008, i have launched a Chemistry guidebook “Up Your Chemistry Grades Now!” that took me more than 2 years to compile. At present, hundreds of copies have been sold to over 11 13 countries (London, South Africa Saudi Arabia & Brunei among the latest addition).

Q) How do you like the idea of scoring the marks you truly deserve by knowing what examiners are looking out for?  Yes! It is exactly what this book is for.

Main highlights of this book include:

- Glossary of terms & KEY DEFINITIONS you need to know
- Chemistry Tips & Exam Strategies
- Common Errors Made by Most Students & How to Avoid them
- What Examiners are really looking out for in your answers?

You can take a “peep” and download the Introductory Section by CLICKING HERE ! I am certain you will love this!

NOW! Like i always tell my chemistry students, don’t take a word i say until you are certain about it.

>> Hear what students / parents say about the book by CLICKING HERE!

Until the next blogposts, Take Care and Study Hard!

Best Regards,

Sean Chua

Master Trainer for Chemistry & Author

WINNERS Education GroupRelated Articles:

• How to Revise for O Level Chemistry Effectively & Quickly
• Mid-Year Chemistry Exams (O Levels) – How to Revise Quickly & Effectively ?
• How To Revise for O-Level Chemistry & Improve Your Grades
• Chemistry Tips: How to Revise for your GCE ‘O’ Level Chemistry Exams
• Up Your Chemistry Grades Now! – Truth & Myth