Q&A - PSLE Science

[emerald]

Thanks to atutor2001 but 1 more question pertaining to ans 2. Imagine an inverted beaker in a basin of water. A candle is burning inside the beaker. After the candle had finished burning, some water from the basin entered the beaker.


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.

Could you please explain? Thanks.

[Full.010894Cream]

emerald:

Thanks to atutor2001 but 1 more question pertaining to ans 2. Imagine an inverted beaker in a basin of water. A candle is burning inside the beaker. After the candle had finished burning, some water from the basin entered the beaker.


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.

Could you please explain? Thanks.

perhaps carbon dioxide is more soluble than oxygen in water

[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.

[atutor2001]

Full.Cream:

emerald:

Thanks to atutor2001 but 1 more question pertaining to ans 2. Imagine an inverted beaker in a basin of water. A candle is burning inside the beaker. After the candle had finished burning, some water from the basin entered the beaker.


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.

Could you please explain? Thanks.

perhaps carbon dioxide is more soluble than oxygen in water

Great point, in fact this is the main contributing factor for the water to enter because the change in molecular size is negligible.

[Full.010894Cream]

atutor2001:

Great point, in fact this is the main contributing factor for the water to enter because the change in molecular size is negligible.

think both are probably outside PSLE scope.

I was just thinking that since the candle is really a hydrcarbon and not carbon, then both water and CO2 (and a whole lot of other oxides) will be formed. Therefore the CO2 formed is definitely lesser than the O2 used. But think this is also beyond PSLE.

[atutor2001]

Full.Cream:

atutor2001:

Great point, in fact this is the main contributing factor for the water to enter because the change in molecular size is negligible.

think both are probably outside PSLE scope.

I was just thinking that since the candle is really a hydrcarbon and not carbon, then both water and CO2 (and a whole lot of other oxides) will be formed. Therefore the CO2 formed is definitely lesser than the O2 used. But think this is also beyond PSLE.

Thanks, have not thought of that. Fully agree with you. Furthermore most of the gaseous oxides are more soluble. Wish you are from MOE

[tianzhu]

Hi


I think chemical reactions of combustion are beyond PSLE’s scope.

When you first place the inverted cup over the candle, the trapped air gains heat and expands and some air may escape through the mouth of the glass submerged in the water. After some time, the candle flame die off due to insufficient oxygen. The air then cools and contracts, and surrounding air forces the water up into the glass.

[Dharma]

emerald:

Thanks to atutor2001 but 1 more question pertaining to ans 2. Imagine an inverted beaker in a basin of water. A candle is burning inside the beaker. After the candle had finished burning, some water from the basin entered the beaker.


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.

Could you please explain? Thanks.

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 %)

[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.