20 May - Reflections of Term 2 Performance

It has finally come to the end of the second term and as usual, it is time to reflect about my science experience so far. Just like my Term 1 reflection, I would be reflecting about my test results and about the various topics covered thus far. However, I have decided to include one more component: comparing my performance this term with last term.

Term 2 test reflections

For this test, I got 37 out of 40 which was quite good a score. When converted into percentages it is 92.5%. I am somewhat satisfied with the results. In this test, I think I did better in the physics component rather than the chemistry component. I was quite surprised. I thought that I was better at chemistry than physics. I would be reflecting through the conceptual mistakes which I have made. The first mistake is as follows:

In a space shutter, the carbon dioxide build up inside the spacecraft was removed by the use of canisters packed with solid potassium hydroxide. Explain the chemistry in this reaction. Write a chemical equation for the above reaction.

At first I was quite shocked. I recalled that in the science lab, we used calcium hydroxide to absorb carbon dioxide. So, I thought that the chemical reaction for potassium hydroxide would be similar to that of calcium hydroxide. However, then the problem came. I did not know why calcium hydroxide reacted with carbon dioxide. But I did know that the product of the reaction between calcium hydroxide and carbon dioxide would be calcium carbonate. But the reaction for potassium hydroxide was slightly different because potassium is an alkali metal while calcium is an alkaline earth metal. In the end, I decided to just write out the chemical reaction to find out the truth. I wrote

KOH + CO₂ à KHCO₃

However, I was marked wrong. This was what the answer wrote:

Carbon dioxide is an acidic gas [1/2] will be neutralized [1/2] by the basic [1/2] potassium hydroxide to form salt and water [1/2].

The chemical equation is as follows:

CO₂ + 2KOH  à  K₂CO₃ + H₂O

I seemed quite easy to comprehend. On the contrary, it is not free of problems. One of the problems is that carbon dioxide is not acidic on its own. It needs water to show its acidic properties. The question stated that the potassium hydroxide was “solid potassium hydroxide” thus it can be inferred that no water was present. Thus, we might need to perform an actual experiment to find out if this reaction really works. From this question, I lost 2.5 marks which was 6.25% of the total marks. The other 0.5 marks were lost through carelessness, which accounts for 1.25% of the total marks.

Various topics covered in Term 2

Now, I shall go through the various topics individually. They are mainly: acids and bases, reflection of light and refraction of light. Acids and bases is a rather difficult topic because there are many different acids and bases available. Hence, the number of reactions is over a hundred. However, I believe that by understanding the general reactions such as acid and base react to produce salt and water, acids and bases can become a rather easy topic. However, some questions can be quite tricky, for example:

The pH of a lake polluted with acid rain can be raised by adding

A.      Calcium carbonate

B.      Hydrochloric acid

C.      Potassium hydroxide

D.     Sodium chloride

We obviously know that hydrochloric acid and sodium chloride do not help raise the pH. We are left with calcium carbonate and potassium hydroxide, both of which can neutralise acids. However, potassium hydroxide is an alkali and when added in access, might cause the pH to rise to high (above pH 7). Calcium carbonate would neutralise the acid and not cause the pH to rise over pH 7 because it does not release hydroxide ions.

Other questions require some background understanding which is not directly stated in the textbook:

Sulfuric acid reacts with substance X to produce a colourless solution. No other products are seen. What substance could be X?

A.      Magnesium

B.      Zinc carbonate

C.      Copper (II) oxide

D.     Sodium hydroxide

Sulfuric acid will react with magnesium to produce hydrogen gas. Sulfuric acid reacts with zinc carbonate to produce carbon dioxide. Both A and B will produce effervescence which can be seen. Sulfuric acid reacts with copper (II) oxide to produce copper (II) sulfate and water. Copper (II) sulfate solution is blue in colour and thus isn’t colourless. Therefore, we are left with option D (sodium hydroxide).

For the topics on reflection, I might have missed some important lessons due to competition, thus it was initially quite hard for me to keep up. However, after looking through the notes, I was soon able to get the rough idea. For reflection of light, I managed to grasp the idea of angle of incidence equals angle of reflection. Just by understanding this simple concept, I was able to draw ray diagrams for reflections rather easily. The formulas in physics are often easy to understand. However, it is the application of the formulas that is difficult and there are a wide range of applications.

I also found the idea of a virtual image and a real image quite confusing. At first, I thought that all images were virtual because we cannot touch them. However, I soon realise the difference. The notes said that images are virtual because they cannot be captured on a screen as the light rays do not pass through the image. I thought of another way of understanding the difference between real and virtual images. A virtual image is an image perceived by the brain – an illusion basically.

The next topic is about the refraction of light. I find this topic quite interesting because refraction causes many optical illusions such as the bending straw trick. Yet, I find refraction a little difficult to understand. Why is it that light bends? Why is it that when a light ray travels from an optically less dense medium to an optically denser medium, it is refracted towards the normal? Why not the other way round? Sometimes, I get confused with the other case whereby light travels from an optically denser medium into an optically less dense medium. I needed a distinct way to differentiate. Eventually, I thought of a rather ingenious way of distinguishing the two cases. I imagine a car. When a car travelling on a road (less friction to slow down) hits a sandy area (more friction to slow the car down), the car would swerve towards the sandy area.  By replacing the car with light rays, the road with the optically less dense medium and sandy area with the optically denser medium, I can understand how light refracts.

Comparison with Term 1

Finally, I shall compare my Term 2 performance with my Term 1 performance. In Term 1 test, I scored 37.5 marks; in Term 2 test, I scored 37 marks. Hence, it appears that I did not perform as well as in the first term, with a difference of 0.5 marks. However, it is a mere 1.3% difference and may not be significant. Last term, careless mistakes cost me 1.5 marks; this term, careless mistakes cost me only 0.5 marks. Thus, this test I have been more careful. However, it also means that I made more conceptual errors this test compared to last term. Thus, I believe that I need to do more revision for Science, particularly chemistry where I made more mistakes, so that I can brush up on my understanding. In addition, this term I had my National Schools competition and missed a few lessons, thus experienced a little difficulty coping. But eventually by looking through the notes again and again, I managed to keep up eventually. I believe that competition should not be used as an excuse as for not doing well academically. I believe that with time management and discipline, one can continue to excel academically. For example, at the competition site, I do not idle time away. Instead, I read through my notes so that I would not be lost when I return.