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Biology; Chemistry; Physics: Post your doubts here!

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Snowysangel

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In one of the past papers, they asked us the use of acetylene :/ are there other hydrocarbons outside our syllabus whose uses we should know?
 
Saad Mughal

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Snowysangel said:
In one of the past papers, they asked us the use of acetylene :/ are there other hydrocarbons outside our syllabus whose uses we should know?
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You should just know that acetylene, like all alkynes, has a triple bond. Also, it's not necessary that everything will be from the syllabus, some questions are such that they are closely related to the syllabus. That is why it is RECOMMENDED that you do at least 2003-2012 past papers to get an idea of how questions are asked.
The use of acetylene, if you already didn't know, is welding. It is used as oxyacetylene gas welding since the temperatures produced on burning can be quite high.
 
Talha Irfan

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Hskk said:
a) Taking the average always provides a more accurate result as it decreases the reaction time error.

b i) The m.s. says 'distance (travelled by wave) / length of tray varies' Basically, since the ends are sloping outwards, the tray is not a proper rectangle so the distance of one wave can not be measured accurately.
ii) The best understandable answer I think would be to describe a way of avoiding parallax error eg. view the ruler vertically etc. That's what what the ms accepts too.

c i) This one's pretty easy, you see there is a space between the start of the ruler and 0 so the reading would not be accurate with this ruler.
ii) One ruler is used to measure the length of the 'dead space', the other one used to measure the depth. Both readings are added.

d i) That's just your normal graph.
ii)The speed of the waves becomes so high that the time cannot be measured by the student.
iii) Since the graph has a decreasing gradient, you can see that the speed will eventually become constant.

Best of luck (y)
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ur replies are impressive :p
 
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Snowysangel

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Saad Mughal said:
You should just know that acetylene, like all alkynes, has a triple bond. Also, it's not necessary that everything will be from the syllabus, some questions are such that they are closely related to the syllabus. That is why it is RECOMMENDED that you do at least 2003-2012 past papers to get an idea of how questions are asked.
The use of acetylene, if you already didn't know, is welding. It is used as oxyacetylene gas welding since the temperatures produced on burning can be quite high.
Click to expand...
How did u know that? The same goes for magnesium oxide...the questions don't give away the answer :/
Also, when we have to find the concentration of something, can we give it in terms of g/dm3? Or is always mol/dm3? Like in q6 in the December 2003 exam
 
Saad Mughal

Saad Mughal

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Snowysangel said:
How did u know that? The same goes for magnesium oxide...the questions don't give away the answer :/
Also, when we have to find the concentration of something, can we give it in terms of g/dm3? Or is always mol/dm3? Like in q6 in the December 2003 exam
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Well, I've come across the use of acetylene quite frequently. I first read about it in 8th class Science. What about Magnesium Oxide?
The unit is already stated by the blank space for the answer, mol/dm3.
 
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Saad Mughal said:
Well, I've come across the use of acetylene quite frequently. I first read about it in 8th class Science. What about Magnesium Oxide?
The unit is already stated by the blank space for the answer, mol/dm3.
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No they didn't give any unit. They gave us the mass of something in 2 dm3 of a solvent and asked us to find its concentration :S
Magnesium is used in furnaces as a refractory material, and then there's silicon
 
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Snowysangel said:
No they didn't give any unit. They gave us the mass of something in 2 dm3 of a solvent and asked us to find its concentration :S
Magnesium is used in furnaces as a refractory material, and then there's silicon
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I checked the paper from xtremepapers, the unit was there, mol/dm3.
Yeah, magnesium oxide is used in furnaces, because of the high melting point and its ability to absorb heat.
In which paper did this come? I haven't come across it as of yet.
Silicon is a semi-conductor, it is used in computer chips.
 
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Snowysangel

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Saad Mughal said:
I checked the paper from xtremepapers, the unit was there, mol/dm3.
Yeah, magnesium oxide is used in furnaces, because of the high melting point and its ability to absorb heat.
In which paper did this come?I haven't come across it as of yet.
Silicon is a semi-conductor, it is used in computer chips.
Click to expand...
Oh it's not given in red-spot. How do u know all this btw? Do u go to merchant??
 
Saad Mughal

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Snowysangel said:
Are the uses of other substances given as well?
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Other substances such as? The use of magnesium oxide isn't in the book but its bonding and structure tell us about its use.
In general, most uses are given in the metals and non metals chapter.
 
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Snowysangel

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Saad Mughal said:
Other substances such as? The use of magnesium oxide isn't in the book but its bonding and structure tell us about its use.
In general, most uses are given in the metals and non metals chapter.
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But according to one of the examiners report, we have to be very specific in our answers. We can't write answers such as for cooking or something, when its used specially in furnaces :S so how would we know that? I mean I know magnesium oxide is an ionic compound and thus has a high melting point, but how do u know that it absorbs heat?
 
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Snowysangel said:
But according to one of the examiners report, we have to be very specific in our answers. We can't write answers such as for cooking or something, when its used specially in furnaces :S so how would we know that? I mean I know magnesium oxide is an ionic compound and thus has a high melting point, but how do u know that it absorbs heat?
Click to expand...
Obviously, when a substance has a high melting point, it keeps absorbing heat until it reaches the temperature at which it melts. And all the more strong compounds such as silicon oxide (silica), magnesium oxide, calcium oxide, aluminium oxide (I guess) are used as refractory materials because they have very high melting points. The group 1 and lower group 2 and group 3 metal compounds are not used because they're melting points are relatively lower. Hope that helps. :)
 
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