Q&A - PSLE Science

[James Ang]

atutor2001:

emerald:

My girl's teacher said that water entered the beaker to take up the space of the oxygen that was used up by the burning candle. He also said that little carbon dioxide was produced when the candle was burning.

This another fine example of the inadequacy in our teaching.

In Sec 3, students will be thought that 1 mole of O2 combines with 1 mole of C to give 1 mole of CO2.

Therefore, it is not true that the number of molecules of CO2 is less than the number of molecules of O2, causing a decrease in the volume that allows water to enter the beaker.

However, the size of 1 molecule of O2 is not the same as 1 molecule of CO2.

The size of one O2 molecule is slightly bigger than that of one molecule of CO2. (one O2 molecule is about 3.5 angstroms while one CO2 molecule is about 3.3 angstoms - 1 angstroms = 1/10,000,000,000 metre)

Therefore, when O2 is converted to CO2, the volume of the gas decreases allowing the water to enter the beaker.

I was equally troubled by the explanation given by my kids teacher. Anyway, I remembered my teacher telling me the same thing and it just goes on being passed down till present day. What to do? This is how science is thought here.

The volume taken up by 1 mole of any gas is the same at 24dm3 (rtp) so the space taken by 1 molecule of O2 is the same as that of CO2. The standard answer for the rise in water level is 1/5 due to oxygen taking up 20% of air. The amount of CO2 produced is the same amount due to the balanced chemical equation so the CO2 is assumed to have dissolved readily in water to form carbonic acid, a weak acid.

[atutor2001]

Dharma:

The candle burns in the inverted beaker because there is oxygen in the air inside the beaker. Once the oxygen is used up during burning, the flame goes off and water rises in the beaker to take up the space occupied by oxygen previously. The water level in the inverted beaker will rise by 1/5 of the height of the air inside the inverted beaker at the start of the experiment. CO2 is produced when things burn …so the burning of the candle flame produces CO2(small %)

Thanks for the explanation. However, the issue that was bothering us was that although \"oxygen in the beaker was used up\", the oxygen did not disappear but still remained inside the beaker except that it was in a different form and should still occupy space. Therefore, water should not have been able to enter the beaker because the oxygen was still inside.

I think Full.cream has given the best explanation that the oxygen now exist as dioxide which can dissolve in water. Therefore, the oxygen merged into the water leaving more space in the beaker for the water to occupy.

[James Ang]

Dharma:

The candle burns in the inverted beaker because there is oxygen in the air inside the beaker. Once the oxygen is used up during burning, the flame goes off and water rises in the beaker to take up the space occupied by oxygen previously. The water level in the inverted beaker will rise by 1/5 of the height of the air inside the inverted beaker at the start of the experiment. CO2 is produced when things burn …so the burning of the candle flame produces CO2(small %)

based on the balanced equation;

O2 (g) + C(s) -> CO2(g)

so 1 mole of O2 -> 1 mole of CO2 and since molar volume of gas is the same at 24dm3 (rtp), therefore 1 cm3 of O2 -> 1 cm3 of CO2, so equal amount of CO2 is produced as the amount of O2 used up. The standard answer usually assumed that the CO2 is dissolved in the water, i.e.

O2 (g) + C(s) -> CO2(aq)


perhaps someone ought to investigate this \"problem\" empirically and provide an answer based on realistic experimental setting.

[tianzhu]

Hi


This link should put to rest any doubts or misconceptions that we have over the question on inverted cup over candle.

http://www.science.edu.sg/ssc/detailed.jsp?artid=2430&type=6&root=5&parent=5&cat=53

[Full.010894Cream]

tianzhu:

Hi


This link should put to rest any doubts or misconceptions that we have over the question on inverted cup over candle.

http://www.science.edu.sg/ssc/detailed.jsp?artid=2430&type=6&root=5&parent=5&cat=53

COOL! :celebrate:

Why do we adults love to complicate things!!! :oops:

[tianzhu]

I’ve been asked this question in another blog. Please refer to this link.

http://farm3.static.flickr.com/2628/3949882075_7fe15e9fde.jpg

For this question, Figures A, B and C are likely possibilities.
It's not in the options. Figure D is out as from the plan view, it can be seen that the object is not triangular.

What’s your take?

[James Ang]

Question No. 109 :

In the experiment to determine the percentage of oxygen in air, after the burning of the candle in an inverted jar, water rises up to about one-fifth of the gas jar. Does the carbon dioxide given off, which is sparingly soluble in water, not occupy space?

This common demonstration which involves the burning of a candle has occasionally been interpreted wrongly. As the candle flame consumes the oxygen in the inverted jar and eventually is extinguished, the water level inside the jar will rise dramatically, perhaps even flooding the candle. This effect has sometimes been erroneously attributed to the loss of oxygen from the trapped air as the candle burns. That cannot be the reason, because the burning itself releases gases and vapours, notably carbon dioxide (CO2) and water(H2O). The rise in the water level is actually due to the cooling of the air inside the inverted jar as the candle dims and dies out. When you first place the jar over the candle, the trapped air is heated; it expands, and some of it may even bubble out through the submerged mouth of the jar. As the flame dims, the air cools and contracts, and then the atmospheric pressure outside the jar forces the water up into the jar.


The above explanation makes it seems that the answer is wrong if we attribute the 1/5 rise due to the oxygen being used up while burning. I do not think that is the original intention of the primary level question is to test students if they understand what is expansion and contraction, otherwise why mention 1/5 rise?

[atutor2001]

tianzhu:

Hi


This link should put to rest any doubts or misconceptions that we have over the question on inverted cup over candle.

http://www.science.edu.sg/ssc/detailed.jsp?artid=2430&type=6&root=5&parent=5&cat=53

Thank you. Out of curious, I actually carried out the experiment. Unfortunately no bubbles came out of the glass container that I used and the candle flame when off quite quickly. So it means to say that the air surrounding the candle flame at the moment when I covered it with the inverted container has already expanded \"alot\". Very interesting. Thanks again.

PS

Ya! the explanation on expansion is correct because I repeated the experiment and noticed that the water level only started to rise after the flame has gone off.

Furthermore, more water enters the container (same size) when I used a bigger candle.

So sad, I have learnt the wrong thing for so many donkey years.

[Dharma]

tianzhu:

I’ve been asked this question in another blog. Please refer to this link.

http://farm3.static.flickr.com/2628/3949882075_7fe15e9fde.jpg

For this question, Figures A, B and C are likely possibilities.
It's not in the options. Figure D is out as from the plan view, it can be seen that the object is not triangular.

What’s your take?

Hi Tianzhu,

One important information is in the 1st sentence of the Qn .. “Ming Ming rested 2 objects on a table”

Option A – shows that the smaller object is either floating or is glued to the bigger object. It does not rest on the table.

Option B & C – No issue. Possible

Option D – Smaller object – No problem as it rests on the table. The bigger object is a triangular prism. Plan view of the prism in this case will give a square. Just like a pitched roof of a house, when we have a aerial view, we normally see a square or a rectangle.

I feel that the answer should be B,C and D.

[tianzhu]

Hi Dharma


Good Morning

I agree with you that option A is out. The key point is that the two objects are resting on a table.

But for figure D, the plan view of a triangular prism is not a complete square or rectangle but should be a square or rectangle with a dividing line in the centre.

So my revised answer is option (3).