Physics, Chemistry and Biology: Post your doubts here!

[Rahma Abdelrahman]

thanks for expl. I understood 36

28) doesn't the dry mass increase when number of cells increase?

37) if there are no limiting factors so why is there a death phase. The population should increase continuously.
And could you explain what are limiting factors and their effects.

Thanks again for your help

A limiting factor is a factor that limits the growth of a population when its supply becomes limited.
For example, a decrease in food supply, a decrease in available space due to population growth, the breakout of a particular disease or an increase in the number of predators in that habitat can all cause an increase in death rate of organisms thus achieving a stationary phase when death rate becomes equal to birth rate and finally enters a death phase when death rate exceeds birth rate.
Moreover, an increase in available food (like finding a new food source) and space can contribute to increasing birth rate thus delaying the effects mentioned above. <-- these are the biotic factors.
There are also abiotic factors that include:

1- Climate change​

2- A change in oxygen concentration in the atmosphere​

3- Pollution including water pollution and accumulation of toxic waste that is the metabolic products of the organism (can increase death rate) ​

4- Availability of a shelter from predators​

For plants, any factor that can affect rate of photosynthesis can become a limiting factor when it decreases in supply, for example, carbon dioxide availability, light intensity or temperature.
Hope that helps !

 

[Rahma Abdelrahman]

otherwise they dont?

They don't!

 

[Rahma Abdelrahman]

do v need 2 learn d reactivity series 4 metals? plz say no

I believe "YES", but it is easy, learn it that way: First metals are from G1 (K, Na) , then G2 (Ca, Mg) then G3 (Al) then non-typical transition metals(Zn) then typical transition metals (Fe, Sn, Pb) then Cu, Hg, Ag, Au, Pt. Don't forget to include Carbon after Al and Hydrogen before Cu. EASY!

 

[Rahma Abdelrahman]

y magnesium cloride does not react wid sulfuric acid?

MgCl2 is a neutral salt because it is formed by a reaction between a strong acid (HCl) and a strong base (MgO).

 

[Rahma Abdelrahman]

http://www.xtremepapers.com/papers/CIE/Cambridge IGCSE/Chemistry (0620)/0620_w08_qp_1.pdf
q29--how do u noe dat no of shells is 2 n not 6?

Firstly, it is not no. of shells, it's no. of electrons in the outer shell, which means 2 as a metal can never have 6 electrons in the outer shell!

 

[Maz]

Chemistry:
w10 - qp.12,
q. 16.
Why does the temperature increase ?

 

[AAnsarii]

Chemistry:
w10 - qp.12,
q. 16.
Why does the temperature increase ?

when u supply heat, it's absorbed n water is given off(by breakin bonds)
so when u add water, heat will be given out, thus the increase in temperature

 

[joulie]

Are you sure of the mark scheme answer for 37??!
I believe it should be B.
For 28, Yes, photosynthesis increases cell no. but when still under the soil, the seed is using the available food store in the cotyledon and is not getting extra nutrients from the soil so DRY mass remains constant!

it is D in 37 however in june 2007 Q 38 (that is the same question) the answer is B so i believe the true answer is B

 

[kainat husain]

PLEASE HELP
http://www.xtremepapers.com/papers/CIE/Cambridge IGCSE/Chemistry (0620)/0620_s08_qp_1.pdf
q13- how to do :/ the answer is D
q19 :/ :/ the answer is D
q22 the answer is D
q29 the answer is B
q30 the answer is D
q38 the answer is C Y NOT b :/
Q39 the answer is B

 

[AAnsarii]

PLEASE HELP
http://www.xtremepapers.com/papers/CIE/Cambridge IGCSE/Chemistry (0620)/0620_s08_qp_1.pdf
q13- how to do :/ the answer is D
q19 :/ :/ the answer is D
q22 the answer is D
q29 the answer is B
q30 the answer is D
q38 the answer is C Y NOT b :/
Q39 the answer is B

q13- X n Y react wid Br n O2, which will ALWAYS go to d anode, so u can make out dat x n y will go 2 d cathode

q 19- i think so:: u can't change d oxidation state, which is a chemical property, by adding xtra base. n potassium hydroxide is base coz it's "hydroxide", so pH increases. bases: CO3 ion(carbonates), OH(hydroxides), O(Oxides), hydrogencarbonates(dunno how dat appears)

q22-maybe coz they've 2 b added in xcess, so it'll b easier dat way. i think so, k, im not sure

q29- it 4ms a basic oxide, must b a metal. is below H in d reactivity series, so dat leaves only Cu. if u wanna learn d reactivity series dere's a good link http://www.bbc.co.uk/schools/ks3bit...al_behaviour/compounds_mixtures/revise5.shtml, if u haven't learnt it already, of course. H is btwn lead n copper{wikipedia}

q30- x is after H so will not displace d H 4m HCl. u noe dat Fe compound can b reduced by C, so even X will b reduced by C.

q38- mixture of hydrocarbons, so it's not methane coz methane is only a SINGULAR hydrocarbon, not a MIXTURE, which is d case wid petroleum.

q39- ethene has only 2 C atoms, so it's got a low boiling pt. u noe dat methane is gas at rtp, so even ethene will b gas
then its not saturated coz its an "ene", 4m alkene, which are an unsaturated family.

 

[AAnsarii]

http://www.xtremepapers.com/papers/CIE/Cambridge IGCSE/Chemistry (0620)/0620_s08_qp_1.pdf q22, sby else also asked
thx

 

[thats_me]

PLEASE HELP
http://www.xtremepapers.com/papers/CIE/Cambridge IGCSE/Chemistry (0620)/0620_s08_qp_1.pdf
q13- how to do :/ the answer is D
q19 :/ :/ the answer is D
q22 the answer is D
q29 the answer is B
q30 the answer is D
q38 the answer is C Y NOT b :/
Q39 the answer is B

13 : Metals form at cathode as they form positively charged ions so they attract to the negative electrode so X & Y will be formed
19 : Adding in excess doesnot effect the oxidation state , but the pH increases because KOH is added in excess and it's basic so the pH becomes alkaline
22 : This is neutralization , we use the metal in powdered form i.e solid
29 : Forms an alloy can be C, Cu or Zn
Forms basic oxide can be Cu Or Zn
Below hydrogen is only copper (Reactivity series: K Na Ca Mg Al (C) Zn Fe Sn Pb (H) Cu Ag Au Pt)
30 : Refer the reactivity series Xis after Hydrogen in the series so it cannot displace hydrogen & it is below carbon so carbon CAN displace Xoxide
38 : Petroleum is a mixture of hydrocarbons , Methane is not a mixture , it is a hydrocarbon
39 : Ethene is a gas and has double bond (unsaturated) so B

 

[AAnsarii]

13 : Metals form at cathode as they form positively charged ions so they attract to the negative electrode so X & Y will be formed
19 : Adding in excess doesnot effect the oxidation state , but the pH increases because KOH is added in excess and it's basic so the pH becomes alkaline
22 : This is neutralization , we use the metal in powdered form i.e solid
29 : Forms an alloy can be C, Cu or Zn
Forms basic oxide can be Cu Or Zn
Below hydrogen is only copper (Reactivity series: K Na Ca Mg Al (C) Zn Fe Sn Pb (H) Cu Ag Au Pt)
30 : Refer the reactivity series Xis after Hydrogen in the series so it cannot displace hydrogen & it is below carbon so carbon CAN displace Xoxide
38 : Petroleum is a mixture of hydrocarbons , Methane is not a mixture , it is a hydrocarbon
39 : Ethene is a gas and has double bond (unsaturated) so B

22: shouldnt d hydroxide b added in solun?

 

[thats_me]

22: shouldnt d hydroxide b added in solun?

I think that is in titration when both the reactants are soluble(burette /indicator method)

 

[AAnsarii]

I think that is in titration when both the reactants are soluble(burette /indicator method)

aren't hydroxides soluble?

 

[AAnsarii]

w07_qp1 q33 whats a girder n how do u noe dat d ans is D?

 

[thats_me]

aren't hydroxides soluble?

As far as i knw Grp 1 hydroxides are soluble & grp2 hrdroxides are slightly slouble
but hydroxides of transition metals are insoluble...

 

[thats_me]

w07_qp1 q33 whats a girder n how do u noe dat d ans is D?

Sumthin related to a bridge Obv
It's D because u cannot plate the whole BRIDGE with zinc due to it's size and grease is also not a good opt
But painting the bridge can be done ..

 

[Rahma Abdelrahman]

w07_qp1 q33 whats a girder n how do u noe dat d ans is D?

It is D bec. you can EASILY paint a part of the bridge but greasing is unrealistic so it can't be used (imagine cars slipping off the road!
Plating with Zinc is not easy as it requires an electrolyte so it becomes too difficult and can't be easily done !

 

[Ahmed Khider]

Hey! Do u have the markscheme for chemistry paper 1 from 1994-2000?

Well i think i hve them but only as a hard copy..if u want u can tell me a question and i will give u its answer