Category : Tips

Physical Chemistry Calculations – Be Aware Where Errors Occur

Mr Khemistry

Physical Chemistry Calculations

Physical Chemistry – Recently i was revising acid-base equilibrium with one of my students and i realised she has trouble getting the right answer even though the procedure is correct. We went through the process and every step was conceptually correct. Then i got her to show me how she calculated on her GC, i saw the workings and it was right. But when she pressed “=”, the answer came out wrong!

Boy, was i puzzled. We went through the steps carefully again and still we arrived at the same outcome. Finally, after i queried her on one of the steps, we both realised what’s wrong. She did not use brackets to sequence the operations, thus the calculator did not process the workings in the sequence we wanted.

A few more questions with calculations we encountered different problems, like missing out a zero or misreading a digit. I pointed out to her none of the questions we did had any mistakes conceptually, only the numerical answers were wrong. I asked her would she be discouraged and come to the conclusion physical chemistry is difficult? She said yes. And no wonder, after thinking so hard and so many calculations, the final answer always turned out to be wrong.

I taught her to carefully check through her workings when doing calculations and look out for common errors. Most importantly, not to be discouraged and come to the wrong conclusion physical chemistry is hard! Most of the calculations in physical chemistry is NOT hard, we just need to practise and look out for common errors.

Do you find physical chemistry daunting? You can do well in it!

Come join us at our weekly group tuition to see how you can confidently approach calculations too!

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Solving Organic Deductive Questions

Mr Khemistry

Solving Organic Deductive Questions

Compound J, C8H6O4, forms a molecule K, C8H10O3, when reacted with LiAlH4 in dry ether. However, when J was reacted with NaBH4 instead, a product of molecular formula C8H8O4 was formed.

J gives an orange precipitate with 2,4-dinitrophenylhydrazine but does not give any precipitate when warmed with Fehling’s reagent. K reacts with bromine water to form L, C8H7O3Br3.

Suggest the structures of J, K and L. 


First information is the molecular formula, it has almost the same C to H ratio and has more than six carbons => likely to have benzene ring C6H6

(Note that there is still two carbon atoms not accounted for)

When it undergoes reduction with LiAlH4, 4 hydrogen atoms were added but 1 oxygen atom was removed. => likely to have one COOH group, accounts for 2 oxygen atoms

But when it undergoes reduction with NaBH4, only 2 hydrogen atoms were added. 
=> confirmed one of the reduction by LiAlH4 was on COOH

J undergoes condensation with 2,4-DNPH but does not undergo oxidation with Fehling’s reagent. => carbonyl but not aliphatic aldehyde. Possible to be aromatic aldehyde or ketone.

K  undergoes electrophilic substitution (tri-substitution) with bromine water => phenol present and also 2,4,6 positions does not have any substituent groups. This means that the two remaining carbon atoms must be at 3 and 5 positions. Additionally, this means that ketone is no longer possible as we would need two carbon on the same substituent group.

Summary: From the above deductions, J has phenol, aromatic aldehyde and carboxylic acid on positions 1,3,5.

K has phenol and two primary alcohols.

L has tri-substituted Br on positions 2,4,6, phenol, aromatic aldehyde and carboxylic acid on position 1,3,5.

deductive question

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Distinguishing Organic Compounds

Mr Khemistry

Distinguishing Organic Compounds

Distinguishing alkenes





Suggest one simple chemical test to distinguish them from each other. State the reagents and conditions needed and give the expected observations for each compound.



This pair of compounds are both dienes, meaning they both contain two C=C bonds. 

We will not be able to distinguish them by using chemical tests for alkenes. This is true for any pair of compounds with the same functional group(s). For example, esters, secondary alcohols, amides etc.

The tip is to distinguish them base on their structures. Use structure-specific tests such as iodofoam test and KMnO4.

For the pair of compounds above, note that the double bonds are closer for cyclohexa-1,3-diene. Using hot acidified KMnO4 will yield carboxylic acid for both compounds but one of the product for cyclohexa-1,3-diene is is ethandioic acid which will further oxidize to carbon dioxide and water.


Add KMnO4(aq), H2SO4(aq) and heat to both compounds.

Observation: Both solutions will change from purple to colourless but the solution of cyclohexa-1,3-diene will produce effervescence of CO2 that form white ppt with limewater

Extra practise:

distinguishing esters



Distinguishing organic compounds is one of the easier types of questions in H2 Chemistry, be sure to score here!

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Revision for A level Chemistry

Mr Khemistry

Revision for A level Chemistry

Students and parents (as well as teachers) are all getting nervous about the impending Prelims. Some parents are asking me why aren’t their children making progress in their studies even though they spent long hours revising and attending tuition.

One of the main reason is that students have to reflect on how they are studying, not just trying to remember the content.

There are many students who will just keep doing ten-year series until the A levels, that is not good revision! After marking, they should ask themselves what was the mistakes made and why.

Practise do not make perfect, only practise with reflection brings improvement. One suggestion i make to students is to write down why they did not get the correct answer and not just look at the model answer thinking that they understood it. Asking themselves why they made the mistake and writing down the misconception will help them not to repeat the same mistakes again.

Another suggestion is to go over the tuitions/revision packages/tys etc and write down the common question types for each topic. See how the questions are being structured and what are the standard questions asked. There is no reason why students can’t do common standard questions.

These 3 months are crucial for students, hope they will manage their time well.

Wishing all J2 students the best for their forthcoming Prelims!


Lesson Structure

Mr Khemistry

Recently i implemented a new lesson structure for the J2 classes, inspired by my Harvard teaching and learning course.

It’s easily remembered by the acronyms ABCDE, inspired by my son’s fascination with alphabets.

A for Aim – What’s the learning objective we’re trying to master today?

B for Big Idea – What’s the underlying concept behind this learning objective?

C for Culling – Which are the important information in the question stem?

D for Doing – In the prescribed time limit for the marks allocated.

E for Evaluation – Reflecting on the gaps in understanding, or careless mistakes.

In having this structure, students knew what is expected of them every single lesson, to go through the lesson knowing there is one main concept we need to learn. Some people might be worried that such structure will crimp creativity, actually i think the reverse is true. Having a well-defined lesson structure allows students the mental space and boundaries to explore, either with their discussions or with their questions and answers.

In the last lesson when i taught energy cycles, i was amazed at the variety of answers i got when they presented on the whiteboard after the D stage.

At the E stage, the different ways which they drew the energy cycle was really unexpected. Even the incorrect answers led us to identify new misconceptions.

I used to think lesson plans were unnecessary, but now i’m starting to see the value in having a broad structure for the lesson 🙂

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Posture for Learning

Mr Khemistry

Did you know that there’s a posture for learning? I didn’t until quite someone pointed out the link between posture and our physiological state of mind. 

A couple of examples:

You get angry and your nostrils flare, face turns red and your fists clench. 

Watching a scary movie and your heart beats faster, palms get sweaty and your breathing becomes shallow.

These are our bodies’ physical response to our state of mind, it works the other way too i.e. adjusting your body’s posture will alter your state of mind.

E.g. Before you step up the stage to give a presentation, you breathe deeply to calm your nerves.

Breathing deeply also works to cool down anger.

So what IS the posture for learning?

As teachers, we know that our enthusiasm and passion for what we are teaching will rub off on the students we are teaching. We learn how to engage students and keep their attention. 

How about students? What should you do to ensure that you are in the best frame of mind for learning? First, take note of what NOT to do.

  1. Passive listening 
  2. Folding of arms
  3. Leaning back on your chair or slouching/sleeping on your desk
  4. Staring into space
  5. Constantly checking your handphone

What kind of posture encourages learning?

  1. Relevant notes on your desk with pen and highlighter ready
  2. Sitting up and leaning slightly forward
  3. Engaging with what the teacher is saying, ask questions when in doubt

Happy Learning! 🙂



Thinking about the “M” word

Mr Khemistry

No, not money…Memory! 🙂

Many have argued that our system of education is antiquated because a good grade is usually dependent on memorizing large chunks of information. 

Why do we need to remember so much information when we have unprecedented access to huge amounts of data to literally anything at our fingertips?

I would say that without knowing the basic knowledge, there’s no way we can critically think about higher order questions. For example, in organic chemistry, we cannot begin to think about synthesis and deductive questions if we do not have basic knowledge of reagent and conditions involved in various reactions. 

So how do we improve our memory?

Firstly, it’s important to recognize our memory works in three steps, Encoding, Storage and Retrieval. 

Encoding – In the context of school/tuition, it is mostly via visual and sound encoding. Semantic (what something means) encoding is the most effective as the information will likely be stored under our Long Term memory.

Storage – Information is either stored in the Short Term Memory (STM) or Long Term Memory (LTM). Research have shown that students who simply read through their notes and tutorials tend to store the information in their STM. Constant reviewing, practicing and reflecting upon their mistakes helps move the information into the LTM.

Retrieval – Some methods of improving retrieval of information: State dependent learning, being in the environment you are in when you first learn helps you recall better. Schemas/acronyms, mental shortcuts that help you organize and understand new information. Chunking/mindmaps – grouping similar information together allows easier recall. Frequent short tests/quiz helps retention as well. 

Lastly, remember our brain is a muscle and it gets stronger when it is used more. Conversely, if you don’t like to think and enjoy being a passive consumer….it atrophies. Our brains can be filled with junk or it can be filled with all kinds of valuable knowledge. It can also get malnutrition from lack of “brain food” and burdened by negative emotions/stress hormones. Treat it well.

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Reading and learning

Mr Khemistry

Reading and Learning

Recently i had a revelation on why students struggled with paper 3 i.e. free response questions, as compared with structured questions in paper 2.

It has to do with the ability to read and glean useful information from words. This problem is especially stark for science students, because they tend to fall back on formulas and calculations, that when they are asked to READ, they flounder.

In fact, collectively, teachers notice a decreasing trend in the students’ capabilities to read critically. Their language ability seemed to have deteriorated quite significantly through the years. Certainly we’ve postulated several theories on why this has happened but since it does us no good to spend too much time speculating on the cause of this decline in reading ability, we shall instead turn our focus to how we can guide our students to read effectively.

How to read and glean important information from questions

  1. Get a teacher or someone who’s an expert on the subject matter to listen to how you interpret free response questions. Read aloud and write down any information you can extract from the question context. Proceed to write down your answer in response to the question.
  2. Likewise, ask the teacher to pen down his/her response to the question.
  3. Compare what is the difference and what is missing from your answer. Critically think about why these interpretations are missing from your answer and how you can implement this “disciplinary reading culture” into your future question analysis.
  4. It is important to recognize that every discipline has their own “reading culture” which allows for the experienced practitioner to derive richer contextual meaning (compared to a lay-person) from the materials given.

Try out the above exercise and let me know if it’s useful to you 🙂


Meta Learning Part 2

Mr Khemistry

Meta Learning Part 2

This post is a follow-up to Meta Learning post.

Recently one of my students did not complete her homework, citing “no stamina”. I told her most likely it is due to mental fatigue, as she is using a lot of her mental energy to do the organic questions. This is commonly why some students do very well at the start of the paper but flounder towards the end. To maximize mental energy, i advised her to delegate her daily schedule to a predictable routine. 

Why is it important to plan out your schedule during holidays? The main reason is because your daily routines are disrupted and precious mental energy is used for mundane decisions. Like what to study/revise, what to eat, what to wear etc. If you planned your holiday schedule and keep to a daily routine, you would save mental energy for the time that you are actually revising and clarifying concepts.

Next is to optimize the time that you spent studying. Have you observed whether you learn faster very early in the morning or late in the evening? When you learn is just as important as how you learn. 

The two most important commodities in studying is time and mental energy. Spending lots of time staring at your notes isn’t going to help if you are not focused. If you are not mentally sharp while revising, what you learnt today will be easily forgotten tomorrow. “Strike while the iron is hot” can be translated to “Study while you are alert”. 

Take a step back and study your own physiology and psychology. It might be the secret to a much improved performance 🙂



Meta Learning

Mr Khemistry

Meta Learning

I’ve recently completed my reading challenge for 2018, twelve book, in four months and a week.
Realised that i’ve set too low a bar for myself. Walked right over it. Will be doubling the number of books to read for the rest of the year.

Reading and learning are intricately linked, but reading doesn’t automatically mean that you are learning. This shocked a new student when i meet him earlier this month.

I asked him, “How do you learn?” He replied, “I look through my notes, but after that i cannot remember the reactions”


“You read and you forget. You write and you remember. You teach and you understand.”

Shared with him the analogy of learning how to ride a bicycle. “When you want to learn how to ride a bicycle, how do you do it?”

“I get on the bicycle and try to ride it…”

*Raised eyebrows*

“Oh….so i do, then i will remember”

“Yes, practise makes progress. Then when you have practised, try teaching it to someone who might be struggling with it. Then you hear it for the second time and learn better.”


This brings me to a point about what i’ve learned from a webinar conducted  by Jim Kwik on meta learning, science of learning HOW to learn.

Schools mostly teach us what to learn, some may teach why we learn and the history of the theory. But not many actually teaches us HOW to learn, which arguably is THE skill to have in this day and age. Most of the information we are learning can simply be obtained from google online, so it makes little sense to memorise such big chunks of information…in so short a time…which stresses out many students, parents and teachers!

The way we teach the next generation is the same way that we learn from the previous, unless WE change as educators and parents. We need to teach them HOW to learn. Because formal education can make them a living but continuing self education will fulfill their highest potential. 

It is easy to do well in school because everything is tried and tested. You just have to observe the past papers and do them well, chances are you will do pretty well too. However, in the real world, things are changing rapidly. How do we best prepare our students to thrive in this rapidly evolving world?

The solution lies in teaching them how to learn and then letting them do the learning. But the bottleneck is time. Either we teach them how to learn, then they go home to do their learning/studying OR we do the learning/studying on their behalf and THEN tell them what to learn. No prizes for guessing which route we are taking.


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