Biology; Chemistry; Physics: Post your doubts here!

[TheStallion-Reborn]

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

ahh thankis! if u have a look at this marking scheme http://papers.xtremepapers.com/CIE/Cambridge International O Level/Physics (5054)/5054_s08_ms_2.pdf and here is the paper;
http://papers.xtremepapers.com/CIE/Cambridge International O Level/Physics (5054)/5054_s08_qp_2.pdf
in question 6 i just saw the graph and the resistance was increasing, which straight away hit me that the current has to go down vecause R=V/I; and then when the resistance becomes constant, so does the current. But the mark scheme has this 'M1' written infront of the statement 'Decreases' I knew it was decreasing just by looking at the graph at did not apply any method here, so how will I be awarded the marks? AAANNDD...another thing, if you look at the mark scheme, though it considers the term 'becomes constant' correct, it also says accept '2 Amp' ..how is someone supposed to figure out the value of the constant current??

 

[TheStallion-Reborn]

What is the effect of temperature on reversible reactions?

I NEVER get reversible equation...and then I stopped bothering because I didn't find any question on them. any important question regarding reversible reactions u have seen? and if u could please tell me what are the main points to be grasped in order to get hold of the topic :-/ and explain them a bit too like what is state of equilibrium n stuff and how will this knowledge be tested in the exam...

 

[usama321]

I NEVER get reversible equation...and then I stopped bothering because I didn't find any question on them. any important question regarding reversible reactions u have seen? and if u could please tell me what are the main points to be grasped in order to get hold of the topic :-/ and explain them a bit too like what is state of equilibrium n stuff and how will this knowledge be tested in the exam...

http://papers.xtremepapers.com/CIE/...l O Level/Chemistry (5070)/5070_s12_qp_22.pdf

Here it is. Check B9. You should get the hang of them. They are not that difficult, and you would have a choice if the other questions are too difficult.

 

[TheStallion-Reborn]

http://papers.xtremepapers.com/CIE/Cambridge International O Level/Chemistry (5070)/5070_s12_qp_22.pdf

Here it is. Check B9. You should get the hang of them. They are not that difficult, and you would have a choice if the other questions are too difficult.

thnku..pay attention to the latter part whats equilibrium aniways?

 

[usama321]

The idea is simple. In exothermic reactions, if the reactants are heated, the equilibrium shifts to the left, that is the rate of forward reaction decreases. This is so because exothermic reactions evolve heat and give off heat of their own, and giving them more heat slows down the reaction.
In endothermic reactions, providing heat shifts the equilibrium to the right. This is so beacuse endo reactions take in heat from the surroundings.

Now coming to the pressure factor. You have to see the number of moles of the reactants and the products to know the effect of pressure on the equilibrium. lets look at this equation
CO2 + 3H2 = CH3OH(g) + H2O(g)
There are 4 moles of the reactants, and two moles of the products. Now, if we increase the pressure, there would be more collissions in the reactants, as there are more moles of reactants then the products. As a result, the equilibrium would shift to the right, or the rate of reaction would increase. Decreasing the pressure would have the opposite effect
If the products had more reactants, increasing the pressure would have increased the rate of reverse reaction. Decreasing the pressure would have the opposite effect
Now coming to the B9 question, in the first question, he specifically asks about the rate of reaction, in terms of collisions between reacting particles. Now even if the reaction is exothermic, we have to tell the effect of increasing the temperature in terms of the collissions of particles. So, we will say that the kinetic energy of the particles would increase, faster collisions, harder and more frequent collissions and as a result the forward reaction increases.
Hope you understand it

 

[usama321]

thnku..pay attention to the latter part whats equilibrium aniways?

when a reversible reaction is in equilibrium, we say that the rate of forward and the backward reaction is equal. Shifting it to the right means, the forward reaction has increased. Shifting it to the left means the backward reaction increases

 

[usama321]

http://papers.xtremepapers.com/CIE/Cambridge International O Level/Physics (5054)/5054_s06_qp_4.pdf

In question 3 part a, do we include the components A and B in the diagram too? anyone please 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?

 

[TheStallion-Reborn]

The idea is simple. In exothermic reactions, if the reactants are heated, the equilibrium shifts to the left, that is the rate of forward reaction decreases. This is so because exothermic reactions evolve heat and give off heat of their own, and giving them more heat slows down the reaction.
In endothermic reactions, providing heat shifts the equilibrium to the right. This is so beacuse endo reactions take in heat from the surroundings.

Now coming to the pressure factor. You have to see the number of moles of the reactants and the products to know the effect of pressure on the equilibrium. lets look at this equation
CO2 + 3H2 = CH3OH(g) + H2O(g)
There are 4 moles of the reactants, and two moles of the products. Now, if we increase the pressure, there would be more collissions in the reactants, as there are more moles of reactants then the products. As a result, the equilibrium would shift to the right, or the rate of reaction would increase. Decreasing the pressure would have the opposite effect
If the products had more reactants, increasing the pressure would have increased the rate of reverse reaction. Decreasing the pressure would have the opposite effect
Now coming to the B9 question, in the first question, he specifically asks about the rate of reaction, in terms of collisions between reacting particles. Now even if the reaction is exothermic, we have to tell the effect of increasing the temperature in terms of the collissions of particles. So, we will say that the kinetic energy of the particles would increase, faster collisions, harder and more frequent collissions and as a result the forward reaction increases.
Hope you understand it

when a reversible reaction is in equilibrium, we say that the rate of forward and the backward reaction is equal. Shifting it to the right means, the forward reaction has increased. Shifting it to the left means the backward reaction increases

thnkeeew!!

 

[TheStallion-Reborn]

http://papers.xtremepapers.com/CIE/Cambridge International O Level/Physics (5054)/5054_s06_qp_4.pdf

In question 3 part a, do we include the components A and B in the diagram too? anyone please help

they are terminals, (if by component u didn't mean so) yes, hw would u complete the circuit widout these two cx u have to find out what is in between them

 

[TheStallion-Reborn]

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?

the last thing first, im not sure but i think incomplete dominance and co-dominance is the same thing. Well, if that is the case, you will find two capital alleles in the diagram like AB or XY.
And your first question, I didn't really it. But I'll try to explain it. In both male and female organisms, gametes contain half the genetic information. It is copied from the organism itself but the chromosomes of gametes are 23 in number, instead of 46. So the two gametes from male and female organisms, each containing half of the genetic information fuse together, completing the required genetic material for the new organisms.

 

[TheStallion-Reborn]

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 :/

it is travelling downwards because it is emmited in that direction. however the part under consideration is it's direction as it leaves the magnetic field. I answered ur question earlier too. check it out it might help

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.)

 

[TheStallion-Reborn]

ahh...rite....dumb question! In parallel circuits, current into the circuit divisions, while the voltage remains the same throughout, while resistance is 1/R=1/R1+1/R2. In series circuit the curent is uniform throughout the circuit and the voltage is divided among the components whereas the resistance is R=R1+R2. aRE MY CONCEPTS RIGHT?? and what affect does an increased current have on a bulb and what affect does increased voltage have on a bulb. please help,,it's always so confusing

 

[Suchal Riaz]

ahh...rite....dumb question! In parallel circuits, current into the circuit divisions, while the voltage remains the same throughout, while resistance is 1/R=1/R1+1/R2. In series circuit the curent is uniform throughout the circuit and the voltage is divided among the components whereas the resistance is R=R1+R2. aRE MY CONCEPTS RIGHT?? and what affect does an increased current have on a bulb and what affect does increased voltage have on a bulb. please help,,it's always so confusing

in parallel voltage in every connection has full voltage(battery voltage) thus maximum current than in series. although less current flows through that connection where the resistance is high.
if u attach 5 bulbs to 5 volts battery in series they will be very dim as each will get just 1 volt and as combined resistance is high the current in all circuit will be very small. if u attach them in parallel each will get full 5 volts and the current will be high as voltage is high and resistance is of just a single bulb not 5 bulbs in one connection. hope it helps.

 

[nidz]

Ok, in simple words
The reaction b/w HCl and CuCO3 is,
CuCO3 + HCl ---> CuCl2 + H2O + CO2
The hydrogen from the hyrdochloric acid displaces the copper in the cooper (II) carbonate since hydrogen is more reactive than copper.
This will form carbonic acid and copper (II) chloride.
The carbonic acid (being soluble) H2CO3 will break up into its ions and react to form water and carbon dioxide,
H2CO3 ----> CO3 2- + 2H+ ----> H2O + CO2.
Hope you get it now.

Yeah got it thanks Didn't know a simple acid carbonate reaction could be linked to the reactivity series like that

 

[TheStallion-Reborn]

in parallel voltage in every connection has full voltage(battery voltage) thus maximum current than in series. although less current flows through that connection where the resistance is high.
if u attach 5 bulbs to 5 volts battery in series they will be very dim as each will get just 1 volt and as combined resistance is high the current in all circuit will be very small. if u attach them in parallel each will get full 5 volts and the current will be high as voltage is high and resistance is of just a single bulb not 5 bulbs in one connection. hope it helps.

thankyou but my question remains the same; what affect does an increased current have on a bulb and what affect does increased voltage have on a bulb (AS IN BRIGHTNESS ETC)

 

[nidz]

What is the correct test for Hydrogen?
All examiner reports say "pops in a flame"
Does it mean we are not suppose to mention lighted splints? :/

 

[nidz]

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?

Found it in Physics Nov 07 P4

Marking Scheme Code
B1 Independent mark
C1 Compensation mark:
– is always followed by an A mark
– is given automatically if the answer is correct
– is given if the answer is wrong but the point is seen in the working
A1 Answer mark
ecf error carried forward; correct working using an error in previous working is credited

 

[TheStallion-Reborn]

What is the correct test for Hydrogen?
All examiner reports say "pops in a flame"
Does it mean we are not suppose to mention lighted splints? :/

simple. bring a lighted splint near the source, the splint extinguishes with a pop sound. y risk any of your precious marks?

 

[fatima0000]

i want help with this question. can anyone please explain this question to me?
Q no 4: part (b) nov 12 ppr
http://papers.xtremepapers.com/CIE/Cambridge International O Level/Biology (5090)/5090_w12_qp_22.pdf
thanks in advance