Biology; Chemistry; Physics: Post your doubts here!

[zohra`xo]

http://papers.xtremepapers.com/CIE/Cambridge International O Level/Biology (5090)/5090_s11_qp_11.pdf

Can someone explain MCQ 18, please? Ty.

 

[TheStallion-Reborn]

http://papers.xtremepapers.com/CIE/Cambridge International O Level/Biology (5090)/5090_s11_qp_11.pdf

Can someone explain MCQ 18, please? Ty.

C is the answer. cx potasium hydroxide absorbs CO2 but water doesnt. wht does the mark scheme say?

 

[zohra`xo]

C is the answer. cx potasium hydroxide absorbs CO2 but water doesnt. wht does the mark scheme say?

I thought it was C too.. but MS says B. :\

 

[zohra`xo]

Farz karay it is C, I still don't understand how? Even if KOH absorbs CO2, how does that change the water level?

 

[TheStallion-Reborn]

I thought it was C too.. but MS says B. :\

Farz karay it is C, I still don't understand how? Even if KOH absorbs CO2, how does that change the water level?

ohkaaaay i get it...i was interpretting the question wrong...hang on,,ill xplain

 

[asma tareen]

as the amount of air in tube one has decreased water level is able to rise aand occupy space

 

[zohra`xo]

Ahaaa, samajh aa gaye lol. Tysm both!

 

[TheStallion-Reborn]

as the amount of air in tube one has decreased water level is able to rise aand occupy space

xactly...i was predominantly considering the water level in the testubes under question!

 

[asma tareen]

 

[asma tareen]

well m glad i could help

 

[simranlalchand]

Can anyone answer this question please
explain that genes may be copied and passed on to the next generation? and how can we tell from a genetic diagram that whether it is co dominance, complete dominance or incomplete dominance

 

[Saad Mughal]

1)THE COOLING EFFECT OF EVAPORATION; I know the phenomenon, but what exact wordings are we supposed to use here when answering a question in the exam. Is it that the energy is lost from breaking bonds of the liquid and turn it into a gas or is it that the molecules with the most energy evaporate, carrying (heat)energy away with them, leaving behind energy deficient molecules behind, which are cooler. If you could please tell me the exact wordings.
2) A question asked me why was oil a non-renewable resource and I answered that it is because once it is used, it cannot be reused again and is gone forever. The mark scheme however completely rejects this and declares this as correct; 'because oil is a finite source and will run out if we keep using it because it takes millions of years to form' confuses me :-/ any idea which supports the mark scheme so I can relate to it and remember it better?

Sorry for the terribly late reply. Chemistry I tell you.
If you want an explanation:
Evaporation is the process by which a liquid turns into vapor, it only occurs on the surface of the liquid. When a body is at high temperature, it has a large number of energetic molecules that move quickly, similarly, in evaporation, the more energetic molecules of the liquid break the surface tension of the liquid and turn into vapors leaving behind the relatively slower molecules, hence, the loss of the energetic molecules causes the average kinetic energy in the liquid to decrease which causes cooling.

If you want wording for examinations:
During evaporation, the liquid slowly turns to vapor, the process itself involves the energetic molecules of the liquid breaking free from the surface because of their greater kinetic energy, this leaves behind, in the liquid, less energetic molecules and hence, the average kinetic energy of the molecules in the liquid decreases which causes cooling.

For 2,
Well, as usama321 has already mentioned, you do not say that it can't be re-used since even some renewable resources can't be "re-used", therefore the proper manner to put it would be:
Oil is a non-renewable source of energy since its amount is limited/finite and will eventually run out; because oil comes from the fossil fuels, which are formed over millions of years, it is a non-renewable source of energy.

 

[Saad Mughal]

http://papers.xtremepapers.com/CIE/Cambridge International O Level/Physics (5054)/5054_s08_ms_2.pdf
strange that I don't know this by now, but yeah still......better now than delaying it any further; the marking scheme has 'A1' 'B1' 'C1' written infront of given statements what does it mean?

B1 independent mark
C1 compensation mark; given automatically if the answer is correct, i.e. the working need not be
seen if the answer is correct; also given if the answer is wrong but the point is seen in the
working
M1 method mark: if not given subsequent A marks fall (up to next B, M or C mark)
A1 answer mark
cao correct answer only (including unit)
eeoo each error or omission
ecf error carried forward; it usually is even where not specifically indicated, i.e. subsequent
working including a previous error is credited, if otherwise correct

 

[TheLeagueofShadows]

Can anybody help me with q39 of June 2007 p1? I can't seem to get the hang of the deflection of alpha and beta particles in magnetic and electric fields.

 

[Saad Mughal]

Can anybody help me with q39 of June 2007 p1? I can't seem to get the hang of the deflection of alpha and beta particles in magnetic and electric fields.

Please try to post the link to the paper whenever you ask a question, it makes it easier for us to answer!
Alpha particles and beta particles are both deflected by magnetic and electric fields (beta are more strongly deflected since they are lighter). The beta particles are attracted by positive plate of the electric field whereas the alpha particles are deflected by the negative plate.
In magnetic fields, you can apply FLEMING'S LEFT HAND RULE to find out about the deflection of BETA particles, just take current to be opposite to convectional current (opposite to the direction in which the beta particle is travelling), in this case, the particle travels upwards towards the detector.
The answer is B, the beta particles are deflected upwards by the magnetic fields and alpha particles are not detected because:
(i) they have a small range (2-3 cm in air).
(ii) they are deflected opposite to beta particles.
Hope that helps.

 

[TheLeagueofShadows]

I still don't understand your answer. Firstly alpha particles can also travel from 5 to 10 cm in air so, they can't be discarded because of this reason in my opinion. Secondly, I know both Alpha and Particles deflect in electric and magnetic fields. The Electric field deflection is easy but I don't understand the Magnetic deflection.

In magnetic fields, you can apply FLEMING'S LEFT HAND RULE to find out about the deflection of BETA particles, just take current to be opposite to convectional current (opposite to the direction in which the beta particle is travelling), in this case, the particle travels upwards towards the detector.
Hope that helps.

The particle is travelling upwards to the detector but if you see from the source, it is travelling downwards! Can you please upload a diagram for your solution as i am not getting it :/

 

[Saad Mughal]

I still don't understand your answer. Firstly alpha particles can also travel from 5 to 10 cm in air so, they can't be discarded because of this reason in my opinion. Secondly, I know both Alpha and Particles deflect in electric and magnetic fields. The Electric field deflection is easy but I don't understand the Magnetic deflection.

The particle is travelling upwards to the detector but if you see from the source, it is travelling downwards! Can you please upload a diagram for your solution as i am not getting it :/

The diagram is misguiding. If you apply the Fleming's Left Hand Rule you can check the direction of beta particles. The direction of Alpha particles is opposite to that so only the beta particles will enter the detector. You are right, the range is not a contributing factor for this question (in real life it would be ) but the direction of the beta particles is the method of deducing the answer. If you don't get it then I'll upload a picture tomorrow morning.

 

[Khadija_1234]

What is the effect of temperature on reversible reactions?

 

[Kamihus]

What is the effect of temperature on reversible reactions?

In exothermic, increasing the temperature decreases the forward reaction
In endothermic, increasing the temperature increases the forward reaction

 

[TheStallion-Reborn]

Can anybody help me with q39 of June 2007 p1? I can't seem to get the hang of the deflection of alpha and beta particles in magnetic and electric fields.

it's B,( beta particles). Because when we apply the flemings left hand rule, taking the direction of the middle finger as the direction of the radiation and the direction of the index finger as the direction of the magnetic field, our thumb gives us the direction of deflection for ALPHA particles. Beta particles deflect less than alpha particles and in the opposite direction. So here, in this scenario, by taking the direction the magnetic field and the radiation, we find that alpha particles would deflect downwards, so the radiation deflecting upwards has to be beta! Hope that helps
PS (Direction of beta particles can also be found by using the right hand rule with the thumb giving the direction of the deflection of beta particles, better not use it and confuse urself because we don't have it in our syllabus.)