StudentQns' chemistry questions
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Originally posted by StudentQns:
Going back into my salt water scenario. I take the perspective of water that it increases in temperature. So does this mean that I take the perspective of the lower boiling point substance? Meaning if I put water and ethanol, then the boiling point range will be more than 78?
Then for melting point decreases which perspective do I take?
Thanks for the help
Another example of a question which shouldn't be asked at 'O' levels, because 'O' level students will not be able to comprehend the underlying chemistry of this.http://en.wikipedia.org/wiki/Boiling-point_elevation
http://en.wikipedia.org/wiki/Freezing-point_depression
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hi ultimaonline, is it something like when they say the boiling point of a substance increases when an impurity is placed inside of it. one of the two substance which has the power boiling point by itself will have an increased boiling point? Like water and salt so eventually the range if boiling will be 101-103 degrees. while for solids the melting point of the entire substance is taken in the perspective of the lowest melting point solid. like so when an alloy is formed it will be less than the melting point of both substance? Thanks for the help!
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Originally posted by UltimaOnline:
Another example of a question which shouldn't be asked at 'O' levels, because 'O' level students will not be able to comprehend the underlying chemistry of this.'A' level students should be able to work out, on a case-by-case basis, what happens in terms of inter-particle interactions, to explain why the boiling point of a species (eg. water) is elevated (in this case : ion-permanent dipole interactions 'holding back' water molecules from vapourizing) or why the freezing point of a species (eg. water) is depressed (in this case : ion-permanent dipole interactions disrupting the orderly lattice structure of the solid state) upon adding of a 2nd species (eg. an ionic salt).
Hi ultimaonline, I asked these questions as my textbook says that when an impurity urself inside a liquid, the boiling point increases. N in a solid melting point decrease. So I was pondering, if both liquids are placed together how would it affect the boiling point. So from examples I sort of deduced these assumptions. Thanks! -
Originally posted by UltimaOnline:
Another example of a question which shouldn't be asked at 'O' levels, because 'O' level students will not be able to comprehend the underlying chemistry of this.'A' level students should be able to work out, on a case-by-case basis, what happens in terms of inter-particle interactions, to explain why the boiling point of a species (eg. water) is elevated (in this case : ion-permanent dipole interactions 'holding back' water molecules from vapourizing) or why the freezing point of a species (eg. water) is depressed (in this case : ion-permanent dipole interactions disrupting the orderly lattice structure of the solid state) upon adding of a 2nd species (eg. an ionic salt).
Yea, I think I should just forget everything I just posted. In a question: methanoic acid has a melting point of 5 degrees. Boiling point of 101degrees. While ethanoic acid has a melting point of 17 degrees and a boiling point of 118 degrees. Substance X has a melting point of 15 degrees while a boiling point of 121 degrees. My answer: X is ethanoic acid with methanoic acid as an impurity. This is because impurities decrease the melting point as shown when the melting point decreases from 17 to 15 degrees. While an impurity increases the boiling point of substance as shown when rude boiling point increases from 118 to 121 degrees. Is this correct? This is just me using the O level stuff. But I think that it would be different in higher levels. Ultimaonline could u check this answer for me using O level standard. Thanks for the help! -
All your questions cannot be adequately addressed at O levels. Cambridge will not require you to predict, let alone explain, why or how boiling and melting points are modified by impurities, only that they are.
So in a nutshell, don't try to figure it out at 'O' levels, because it simply can't be done using limited 'O' level knowledge.
Of course, anyone who feels otherwise, can feel free to advise StudentQns on his queries.
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Originally posted by StudentQns:
Going back into my salt water scenario. I take the perspective of water that it increases in temperature. So does this mean that I take the perspective of the lower boiling point substance? Meaning if I put water and ethanol, then the boiling point range will be more than 78?
Then for melting point decreases which perspective do I take?
Thanks for the help
the main reason (at O level) why ethanol and water put together, bp more than 78 degrees (using primary school knowledge) is there is added impurities
To utimaonline: For A levels, is it the hydrogen bonds in water alone is weaker than the hydrogen bonds formed between water and ethanol (as the donor atom, hydrogen bond and the electronegative atom of the acceptor molecule is more linear than what is in water alone).
Plus, organic cation is RNH3+ .
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Originally posted by StudentQns:
Yea, I think I should just forget everything I just posted. In a question: methanoic acid has a melting point of 5 degrees. Boiling point of 101degrees. While ethanoic acid has a melting point of 17 degrees and a boiling point of 118 degrees. Substance X has a melting point of 15 degrees while a boiling point of 121 degrees. My answer: X is ethanoic acid with methanoic acid as an impurity. This is because impurities decrease the melting point as shown when the melting point decreases from 17 to 15 degrees. While an impurity increases the boiling point of substance as shown when rude boiling point increases from 118 to 121 degrees. Is this correct? This is just me using the O level stuff. But I think that it would be different in higher levels. Ultimaonline could u check this answer for me using O level standard. Thanks for the help!I don't even learn this in A levels. Don't worry too much.
To utimaonline: I think only can solve by physics definition of 1 degrees celsius plus the enthalpy changes of the bonds formation and breaking
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Originally posted by StudentQns:
Going back into my salt water scenario. I take the perspective of water that it increases in temperature. So does this mean that I take the perspective of the lower boiling point substance? Meaning if I put water and ethanol, then the boiling point range will be more than 78?
Then for melting point decreases which perspective do I take?
Thanks for the help
Mixtures of solutions are quite beyond the scope of secondary level. You have chosen the example of ethanol and water which can form azeotrope based on the % of each component.
The azeotrope in this case boils at a lower temperature than both pure ethanol and pure water.
For cases of solute and solvents (e.g. salt + water), I believe the rule of thumb would be to take it from the perspective of the solvent.
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Originally posted by atomos:
Mixtures of solutions are quite beyond the scope of secondary level. You have chosen the example of ethanol and water which can form azeotrope based on the % of each component.
The azeotrope in this case boils at a lower temperature than both pure ethanol and pure water.
For cases of solute and solvents (e.g. salt + water), I believe the rule of thumb would be to take it from the perspective of the solvent.
Oh ok thanks :) cos this impurity thing was slightly covered. They say that when impurity added, the melting point increases and boiling point decreases. And could u let me know if my methanoic acid and ethanoic acid question is correct? It was a practise question from the textbook. As for ultimaonline, get your facts right. this was slightly TAUGHT in SCHOOL. won't it be natural to want to get an answer? Thanks atomos and sbs! -
Originally posted by StudentQns:
Yea, I think I should just forget everything I just posted. In a question: methanoic acid has a melting point of 5 degrees. Boiling point of 101degrees. While ethanoic acid has a melting point of 17 degrees and a boiling point of 118 degrees. Substance X has a melting point of 15 degrees while a boiling point of 121 degrees. My answer: X is ethanoic acid with methanoic acid as an impurity. This is because impurities decrease the melting point as shown when the melting point decreases from 17 to 15 degrees. While an impurity increases the boiling point of substance as shown when rude boiling point increases from 118 to 121 degrees. Is this correct? This is just me using the O level stuff. But I think that it would be different in higher levels. Ultimaonline could u check this answer for me using O level standard. Thanks for the help!Did the question state that substance X must be a mixture of methanoic acid and ethanoic acid?
How about the possibility that substance X is a mixture of ethanoic acid (solvent) and a (unnamed) solute impurity?
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Originally posted by StudentQns:
As for ultimaonline, get your facts right. this was slightly TAUGHT in SCHOOL. won't it be natural to want to get an answer?
What's taught in schools differ from school to school, and even differs amongst teachers in the same school. Cambridge doesn't care what your school taught you.And I wasn't directing at you when I said, "this qn shouldn't be asked at 'O' levels", I was directing at Cambridge, that they shouldn't, and wouldn't, ask students this question at 'O' levels. Get your facts right.
For the record, I won't be replying to any more of StudentQns posts (whether 'O' level qns now, or 'A' level qns in future). Ever.
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Yes ultimaonline U already said that u already said u weren't going to answer. so I was approaching other chem helpers on the forum such as atomos, sbs, frekiwang. Thanks for all the help so far. bye
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Originally posted by atomos:
Did the question state that substance X must be a mixture of methanoic acid and ethanoic acid?
How about the possibility that substance X is a mixture of ethanoic acid (solvent) and a (unnamed) solute impurity?
oh yes! That makes a lot more sense than the combination of the two acids. Something like thr raoult's law. Thanks for the help! :) -
Originally posted by StudentQns:
Yes ultimaonline U already said that u already said u weren’t going to answer. so I was approaching other chem helpers on the forum such as atomos, sbs, frekiwang. Thanks for all the help so far. bye
I will still continue to help you, but as a neutral in the issue, I really also seconded utimaonline view that this is really out of syllabus, maybe your teacher is too over ambtious .
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Originally posted by StudentQns:
oh yes! That makes a lot more sense than the combination of the two acids. Something like thr raoult's law. Thanks for the help! :)are you really in O level? Cause using my JC Chem and (partial univeristy 2) biology knowledge, I really don't heard of raoult law
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Originally posted by SBS n SMRT:
are you really in O level? Cause using my JC Chem and (partial univeristy 2) biology knowledge, I really don't heard of raoult law
"Raoult's Law" is usually taught within the topic of "colligative properties of solutions" in University level Chemistry : -
Originally posted by SBS n SMRT:
I will still continue to help you, but as a neutral in the issue, I really also seconded utimaonline view that this is really out of syllabus, maybe your teacher is too over ambtious .
It's not really my teacher but its me.I'm way too curious and when my textbook says something like impurities increases bp decreases mp without any explanation that the impurity has to be a non volatile solute, then I want to find my answer. Although its too high level knowing about these facts allows me to understand why these cases happens although I don't quite understand vapor pressure. But it really aids me. Thanks do much for the help you've been giving me! :) -
Hi, I have a nets question to ask. What is the meaning of soft/hard and strong/weak? What does saying pure metals are soft while alloys are hard and stronger? Thanks for the help!
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Pure Metals are soft.
Depend on what type of metals, some can be as some as plasticine. They are soft because they only have single type of metal atoms in them. The atoms arrangement of pure metals are like this B.
Soft because they atoms layer can slide over one another easily.
For alloy, alloy means metal made with two or more elements. The atom structure looked like this (A) and (D). They are hard because atoms cannot slide over one another easily.
For (C), it's alloy, but the second element seems to filled up the spaces between the primary element. This is very weak alloy which will not come out for exams.
In Exam, you will need to know how to draw (A), (B) and (D).
Strong/weak is use to describe it's strength and weakness.
If you remember your D&T, you learn about high speed steel and other type of drill bits. High speed steel (black colour) is consider very strong but brittle. Strong because it's able to penetrate metals, brittle is because once it's drop on the floor, chances are the drill bit is rendered useless due to cracks.High speed steel is actually an alloy of steel and carbon. Carbon gives it's black signature black colour. The more carbon you put, the stronger the metal gets (visual the two different atoms in an alloy) and it gets brittle (means break easily) when you add more carbons.
Strong can also be use to describe Aluminium. Aluminium by itself is a very strong metal.
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Oh so soft means like easy to scratch, bend like plastacine while strong is the same thing?
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Originally posted by StudentQns:
It's not really my teacher but its me.I'm way too curious and when my textbook says something like impurities increases bp decreases mp without any explanation that the impurity has to be a non volatile solute, then I want to find my answer. Although its too high level knowing about these facts allows me to understand why these cases happens although I don't quite understand vapor pressure. But it really aids me. Thanks do much for the help you've been giving me! :)
Actually knowing too much high order stuffs can be helpful, but may cause even more confusion as you will not have the basis of these theories and hence , you cannot apperciate them.E.g. In O levels, when we say enzymes lower activation energy, rate increases
(You can also use k = Ae^(-Ea/RT) for the explanation, Ea decereases, k increases, that's ahrennius (sic) equation which is technically out of A level, but what is the point for knowing such a covoluted example in O level, unless you are in Olympiad, as it will not improve any understanding but just a neater form of explanation)
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Originally posted by StudentQns:
Oh so soft means like easy to scratch, bend like plastacine while strong is the same thing?
Soft = weak bonds (i.e. weak forces of attraction between the atoms / ions/ cations)Hardness refer to the bond strength between 2 atoms
Btw, use these terms carefully
malleable = can be bended without breaking
Ductile: Can be drawn into wires
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Oh ok thanks! Also how do I tell if an ionic compound has a higher melting point than another ionic compound? Like NaCl and MgO I know that MgO has a higher melting point due Mg bring 2+ and O is -2 while Na is +1 and Cl is -1 so the ionic bonds between mg and o is stronger than Na and Cl. As they are more oppositely charged. But then I'm not sure if difficult ones are given like CuCO3 and Na2CO3... Thanks for the help!
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Originally posted by StudentQns:
Oh ok thanks! Also how do I tell if an ionic compound has a higher melting point than another ionic compound? Like NaCl and MgO I know that MgO has a higher melting point due Mg bring 2+ and O is -2 while Na is +1 and Cl is -1 so the ionic bonds between mg and o is stronger than Na and Cl. As they are more oppositely charged. But then I’m not sure if difficult ones are given like CuCO3 and Na2CO3…
Thanks for the help!
True for your NaCl and MgO explanation, (but actually ionic radii matters but that's A level).
As utimaonline will say for CuCO3 and Na2CO3, it is A level knowledge to tell which has stronger ionic bonds, and in O level, your understanding will simply yield a partial explanation, and hence, that should not be an O level question
But, I will answer as you are such an inquiring student (you should really take H3 in JC), Ionic bond strength (or lattice energy, i.e. the enthalphy change when 1 mole of an ionic solid is formed from its isolated gaseous ion) is proportionate to ( q+ q- ) / (r+ + r -), hence, Cu is double charge and Na is single charge, LE for CuCO3 is larger than Na2CO3, hence, mp should be higher. BUT WAIT , the anionic cloud of CO3 of CuCO3 may be polarised more due to higher polarising power due to higher charge density of Cu2+ than Na+, hence, causing it to have lower thermal stability, hence, the real answer is debatable even at A levels, but at university, there may be more understanding.
Morale of story: Such a question will never be asked even in A levels. However, if they do ask, they will give a context and answer according to it.
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Oh ok :) this was a question gave by my school teacher to challenge all of us. Was pretty scared cos I thought it would come out for o levels. Thanks so much for the help sbs smrt really appreciate your help! :)