Biology; Chemistry; Physics: Post your doubts here!

[Dashing Cancerian]

phy by charles chew
chem by prescott
bio by lam peng or d.g. mackean

 

[scouserlfc]

Ok... Thanks... I need your help again.. I am posting a pic of a question.. The ans is D in the marking scheme but i cant understand whats the logic behind it.. Pls hav a look and help me if you know...

Dont get it really a very good question, and i dont think this is in O levels !!!!!!!!!!!!!
but never mind have a look at this web page
http://malaysia.answers.yahoo.com/question/index?qid=20120608080553AAJgHwN
this is really confusing i know,i cudnt figure out ill try to to work it out alone when not on computer probably ill figure it out but for now i think if u dont get what its said on the page,then u already know that chlorine is going to bleach so it makes the indicator go colourless so at positive electrode its going to be colourless and as no other option has this u go with D only

 

[KyleoAhsan]

Well I am not so sure about the answer but I just wanted to try, hope this is correct if I am wrong please correct me ^_^!
Now when concentrated aqueous Sodium Chloride is Electrolysed the ions get separated, i.e. H+ OH- and Na+ Cl- now for the negative electrode two things will go that is Na+ and H+ but H+ will be discharged since its lower in the electrochemical series leaving Na+ behind and at the other side Cl- will get discharged as it is lower in the electrochemical series hence, Na+ OH- are left in the solution but at the positive electrode chlorine is formed which bleaches the indicator solution hence at positive electrode it is colourless but at negative electrode there is Sodium Hydroxide which is Alkaline or atleast OH- ions are there which is responsible for the alkaline characteristic so around the negative electrode it is blue/purple indicating that it is slightly alkaline!

 

[Adeelkhan17]

Well I am not so sure about the answer but I just wanted to try, hope this is correct if I am wrong please correct me ^_^!
Now when concentrated aqueous Sodium Chloride is Electrolysed the ions get separated, i.e. H+ OH- and Na+ Cl- now for the negative electrode two things will go that is Na+ and H+ but H+ will be discharged since its lower in the electrochemical series leaving Na+ behind and at the other side Cl- will get discharged as it is lower in the electrochemical series hence, Na+ OH- are left in the solution but at the positive electrode chlorine is formed which bleaches the indicator solution hence at positive electrode it is colourless but at negative electrode there is Sodium Hydroxide which is Alkaline or atleast OH- ions are there which is responsible for the alkaline characteristic so around the negative electrode it is blue/purple indicating that it is slightly alkaline!

are u sure chlorine is lower in reactivity series ??? isnot it hydroxide ions ??
WHEN EVER two ions are present , like two positive and two negative , or some thing like that , preferential discharge takes place , which depends on
position in Electrochemical series and concentration and type of electrode
since they are inert electrode so ...hydrogen gas given at Cathode because it have more tendency to gain electron than sodium while
chlorine is MOST likely to be discharged due to it's concentration

it is kind of tricky question ..is it from O level paper ? and if yes , which paper and question ?

 

[Yashil Burrut]

Hey...i really need help in conduction(chemistry)...first time posting a question...
When do compounds/elements conduct electricity by ions or electrons???

 

[Yashil Burrut]

Well I am not so sure about the answer but I just wanted to try, hope this is correct if I am wrong please correct me ^_^!

Now when concentrated aqueous Sodium Chloride is Electrolysed the ions get separated, i.e. H+ OH- and Na+ Cl- now for the negative electrode two things will go that is Na+ and H+ but H+ will be discharged since its lower in the electrochemical series leaving Na+ behind and at the other side Cl- will get discharged as it is lower in the electrochemical series hence, Na+ OH- are left in the solution but at the positive electrode chlorine is formed which bleaches the indicator solution hence at positive electrode it is colourless but at negative electrode there is Sodium Hydroxide which is Alkaline or atleast OH- ions are there which is responsible for the alkaline characteristic so around the negative electrode it is blue/purple indicating that it is slightly alkaline!

At cathode:
both Na+ and H+ ions migrate to the cathode. However H+ ions will be discharged in preference to Na+ ions since it is lower in the electrochemical series...
2H+ +2e--------> H2

At anode(+)
Both Cl- and OH- migrate to the anode. However Cl- will be discharged in preference as IT IS PRESENT IN GREATER CONCENTRATION.
2Cl- ------> Cl2 +2e.

hopes this helps u becoz ive written these on my own!!!

(let me tell u ... OH- is lower in the reactivity series..)
SO4 2- No3- Cl- Br- I- OH-

 

[KyleoAhsan]

At cathode:
both Na+ and H+ ions migrate to the cathode. However H+ ions will be discharged in preference to Na+ ions since it is lower in the electrochemical series...
2H+ +2e--------> H2

At anode(+)
Both Cl- and OH- migrate to the anode. However Cl- will be discharged in preference as IT IS PRESENT IN GREATER CONCENTRATION.
2Cl- ------> Cl2 +2e.

hopes this helps u becoz ive written these on my own!!!

(let me tell u ... OH- is lower in the reactivity series..)
SO4 2- No3- Cl- Br- I- OH-

Thanks for correcting my mistake, I did not refer to the electrochemical series! Yeah, If I knew that OH- was lower than Cl- then my explanation would be same as yours! sorry for my silly mistake !

and the answer for your question is when a substance is molten or aqueous its because of the ions that electricity is conducted. where as a metal conducts electricity due to its normal structure, the electrons are delocalised and surrounds the metal positive ions and held together by the strong forces between the negative electrons and the positive metal cations. These delocalised electrons are free to move throughout the structure so it can carry an electric current!

 

[Yashil Burrut]

Thanks for correcting my mistake, I did not refer to the electrochemical series! Yeah, If I knew that OH- was lower than Cl- then my explanation would be same as yours! sorry for my silly mistake !

and the answer for your question is when a substance is molten or aqueous its because of the ions that electricity is conducted. where as a metal conducts electricity due to its normal structure, the electrons are delocalised and surrounds the metal positive ions and held together by the strong forces between the negative electrons and the positive metal cations. These delocalised electrons are free to move throughout the structure so it can carry an electric current!

Thanks 4 helping me....so in metals the sea of delocalised electrons conduct electricity, while in compounds in aq or molten state its due to ions!!!...Yeah..now ive understood it..thxs

 

[smart_boy]

Dont get it really a very good question, and i dont think this is in O levels !!!!!!!!!!!!!
but never mind have a look at this web page
http://malaysia.answers.yahoo.com/question/index?qid=20120608080553AAJgHwN
this is really confusing i know,i cudnt figure out ill try to to work it out alone when not on computer probably ill figure it out but for now i think if u dont get what its said on the page,then u already know that chlorine is going to bleach so it makes the indicator go colourless so at positive electrode its going to be colourless and as no other option has this u go with D only

Thanks.. That site explained well.. Next i need to ask what is the difference between 'dendrons' and 'dendrites' in the human nervous system..??? And out of sensory and motor neurones, which has which and why..??? Pls help me out...

 

[Ram97]

can any1 suggest me the best buks available for physics chemistry and bio? im using matters for physics and chemistry and the oxford biology for o levels by marry jones...nd i find all three of them useless...

For Physics:
Complete Physics by Stephen Pople
Physics A course for O'Level Second Edition by Charles Chew

For Chemistry:
Chemistry by Richard Harwood
Chemistry A Course for O'level Third Edition by C.N.Prescott

For Biology:
GCSE Biology Third Edition by D.G.Mackean

 

[MeowMeow]

I need help in biology5090 syllabus,couldnt get some answers

4b) investigate the necessity for chlorophyll, light and carbon dioxide for photosynthesis, using appropriate controls.

4d)investigate and state the effect of varying light intensity, carbon dioxide concentration and temperature on the rate of photosynthesis (e.g. in submerged aquatic plants)

4i)identify and label the cuticle, cellular and tissue structure of a dicotyledonous leaf, as seen in crosssection under the microscope, and describe the significance of these features in terms of function, i.e.
• distribution of chloroplasts – photosynthesis;
• stomata and mesophyll cells – gas exchange;
• vascular bundles – transport;

5f)explain why diet, especially energy intake, should be related to age, sex and activity of an individual

5i)identify the main regions of the alimentary canal and the associated organs: mouth (buccal) cavity, salivary glands, oesophagus, stomach, duodenum, pancreas, gall bladder, liver, ileum, colon, rectum and anus

5o)describe the digestion in the alimentary canal

6d)investigate, using a suitable stain, the pathway of water in a cut stem

6f)) identify the positions of xylem and phloem tissues as seen in transverse sections of unthickened, herbaceous, dicotyledonous roots, stems and leaves

14e)describe the use of fermenters for large-scale production of antibiotics and single cell protein

16t) describe the menstrual cycle, with reference to the alternation of menstruation and ovulation, the natural variation in its length and the fertile and infertile phases of the cycle

17d)explain that genes may be copied and passed on to the next generation

17i)predict the results of simple crosses with expected ratios of 3:1 and 1:1, using the terms homozygous,
heterozygous, F1 generation and F2 generation

17(j) explain why observed ratios often differ from expected ratios, especially when there are small numbers
of progeny

Please do help with the ones u know cause i really need help Sorry for the long post i dont mean to.A big thank you to anyone who is willing to help​

 

[Suchal Riaz]

Well I am not so sure about the answer but I just wanted to try, hope this is correct if I am wrong please correct me ^_^!
Now when concentrated aqueous Sodium Chloride is Electrolysed the ions get separated, i.e. H+ OH- and Na+ Cl- now for the negative electrode two things will go that is Na+ and H+ but H+ will be discharged since its lower in the electrochemical series leaving Na+ behind and at the other side Cl- will get discharged as it is lower in the electrochemical series hence, Na+ OH- are left in the solution but at the positive electrode chlorine is formed which bleaches the indicator solution hence at positive electrode it is colourless but at negative electrode there is Sodium Hydroxide which is Alkaline or atleast OH- ions are there which is responsible for the alkaline characteristic so around the negative electrode it is blue/purple indicating that it is slightly alkaline!

yes u r right, it would make up sodium hydroxide i guess which is an alkali.

 

[Suchal Riaz]

thnkeew

we don't even need to read the book. our sir of both Physics and Chemistry give lecture of about 2 periods and then we complete the MCQs from classified and ask queries next day. Then we do structured question in one day. Then after discussing difficult questions we move on. That is all. Understanding the chapter and doing practice of last 10-12 years of papers is enough.

 

[smart_boy]

I need help in biology5090 syllabus,couldnt get some answers​

​

4b) investigate the necessity for chlorophyll, light and carbon dioxide for photosynthesis, using appropriate controls.​

​

4d)investigate and state the effect of varying light intensity, carbon dioxide concentration and temperature on the rate of photosynthesis (e.g. in submerged aquatic plants)​

​

4i)identify and label the cuticle, cellular and tissue structure of a dicotyledonous leaf, as seen in crosssection under the microscope, and describe the significance of these features in terms of function, i.e.​

• distribution of chloroplasts – photosynthesis;​

• stomata and mesophyll cells – gas exchange;​

• vascular bundles – transport;​

​

5f)explain why diet, especially energy intake, should be related to age, sex and activity of an individual​

​

5i)identify the main regions of the alimentary canal and the associated organs: mouth (buccal) cavity, salivary glands, oesophagus, stomach, duodenum, pancreas, gall bladder, liver, ileum, colon, rectum and anus​

​

5o)describe the digestion in the alimentary canal​

​

6d)investigate, using a suitable stain, the pathway of water in a cut stem​

​

6f)) identify the positions of xylem and phloem tissues as seen in transverse sections of unthickened, herbaceous, dicotyledonous roots, stems and leaves​

​

14e)describe the use of fermenters for large-scale production of antibiotics and single cell protein​

​

16t) describe the menstrual cycle, with reference to the alternation of menstruation and ovulation, the natural variation in its length and the fertile and infertile phases of the cycle​

​

17d)explain that genes may be copied and passed on to the next generation​

​

17i)predict the results of simple crosses with expected ratios of 3:1 and 1:1, using the terms homozygous, ​

heterozygous, F1 generation and F2 generation​

​

17(j) explain why observed ratios often differ from expected ratios, especially when there are small numbers ​

of progeny​

​

Please do help with the ones u know cause i really need help Sorry for the long post i dont mean to.A big thank you to anyone who is willing to help​

Dont you think you have a really LONG list of Doubts..! Still I ll try my best to help you as far as I can...
4b) investigate the necessity for chlorophyll, light and carbon dioxide for photosynthesis, using appropriate controls.
This section is all about practical work involved in photosynthesis. u need to describe experiments and conclude from them that various substances are needed for photosynthesis...
First... to prove that chlorophyll is necessary for photosynthesis, you need to take a plant with variegated leaves, destarch it by keepin in the dark for a day and then put it in sunlight. When you test that leaf for starch after that.. you will find that the green part has starch and white doesnt.. This is because only the chlorophyll containing cells carried out photosynthesis and the white part didnt.. so the glucose produced got stored as starch in only that part... This proves that chlorophyll is necessary for photosynthesis
Next, to prove that light is needed, you can keep a plant with destarched leaves, in light, after covering one leaf with aluminium foil... After some hours the leaves with no foil will show positive result for starch and the leaf covered with foil will show no starch.. this is because the foil prevented light from reaching leaf cells and photosynthesis couldnt be carried out...
To prove that CO2 is needed, take two identical potted plants with destarched leaves and cover both with a transparent plastic bag, making them airtight. inside one bag keep a container of soda lime and in the other, a container of sodium hydrogencarbonate solution... Keep them in light for some hours and then test leaves of both for starch.. As soda lime absorbs CO2 from the air the plant which had soda lime will show no starch in leaves whereas the other one will.. because sodium hydrogencarbonate releases CO2 in the air around the plant...
Finally, control means that you set up a similar apparatus in each experiment which shows that it is the condition you are investigating that is responsible for the change observed.

4d)investigate and state the effect of varying light intensity, carbon dioxide concentration and temperature on the rate of photosynthesis (e.g. in submerged aquatic plants)
Light intensity: Keep an aquatic plant in water with a funnel and test tube inverted - placed above the plant to collect any gas released. keep a strong desk lamp switched on, close to it and record the number of bubbles per minute or the vol of gas collected per minute(using a measuring cylinder). keep the desklamp at distances of 10,20, 30 cm and so on.. Record for each distance.. you ll find that most gas is released wen light is closest as more light intensity means more photosynthesis..

I ll answer the rest later if i can... sorry...

 

[MeowMeow]

Dont you think you have a really LONG list of Doubts..! Still I ll try my best to help you as far as I can...
4b) investigate the necessity for chlorophyll, light and carbon dioxide for photosynthesis, using appropriate controls.
This section is all about practical work involved in photosynthesis. u need to describe experiments and conclude from them that various substances are needed for photosynthesis...
First... to prove that chlorophyll is necessary for photosynthesis, you need to take a plant with variegated leaves, destarch it by keepin in the dark for a day and then put it in sunlight. When you test that leaf for starch after that.. you will find that the green part has starch and white doesnt.. This is because only the chlorophyll containing cells carried out photosynthesis and the white part didnt.. so the glucose produced got stored as starch in only that part... This proves that chlorophyll is necessary for photosynthesis
Next, to prove that light is needed, you can keep a plant with destarched leaves, in light, after covering one leaf with aluminium foil... After some hours the leaves with no foil will show positive result for starch and the leaf covered with foil will show no starch.. this is because the foil prevented light from reaching leaf cells and photosynthesis couldnt be carried out...
To prove that CO2 is needed, take two identical potted plants with destarched leaves and cover both with a transparent plastic bag, making them airtight. inside one bag keep a container of soda lime and in the other, a container of sodium hydrogencarbonate solution... Keep them in light for some hours and then test leaves of both for starch.. As soda lime absorbs CO2 from the air the plant which had soda lime will show no starch in leaves whereas the other one will.. because sodium hydrogencarbonate releases CO2 in the air around the plant...
Finally, control means that you set up a similar apparatus in each experiment which shows that it is the condition you are investigating that is responsible for the change observed.

4d)investigate and state the effect of varying light intensity, carbon dioxide concentration and temperature on the rate of photosynthesis (e.g. in submerged aquatic plants)
Light intensity: Keep an aquatic plant in water with a funnel and test tube inverted - placed above the plant to collect any gas released. keep a strong desk lamp switched on, close to it and record the number of bubbles per minute or the vol of gas collected per minute(using a measuring cylinder). keep the desklamp at distances of 10,20, 30 cm and so on.. Record for each distance.. you ll find that most gas is released wen light is closest as more light intensity means more photosynthesis..

I ll answer the rest later if i can... sorry...

Thank you so much for your help,im really sorry that there is a lot of question.I really really appreciate your help ur a nice person

 

[arsalaan]

Can some one tell me that which book is best for A Level Biology(9700)

 

[Enferno]

I want to know that, if we are given a salt and said to prepare it , like potassium sulphate , how do you know that which method we have to prepare it from, like metal and acid, metal oxide and acid , metalhydroxide and acid or metal carbonate and acid. Please reply urgently, tomorrow is my chemistry paper.

 

[smart_boy]

I want to know that, if we are given a salt and said to prepare it , like potassium sulphate , how do you know that which method we have to prepare it from, like metal and acid, metal oxide and acid , metalhydroxide and acid or metal carbonate and acid. Please reply urgently, tomorrow is my chemistry paper.

Firstly it depends upon the salt..
NaCl, K2CO3, Cu(NO3)2 these are all soluble salts whereas BaSO4, AgCl, PbBr - all these are insoluble.
For insoluble salts, a double displacement reaction needs to occur between two salts or an acid and salt... For example
NaCl + AgNO3 gives AgCl and NaNO3
Silver chloride is insoluble whereas sodium nitrate is soluble. So it can filtered, washed with distilled water, and then dried
For soluble salts, you need to see if any reactants are given..
Method 1: Acid-base titration
If both reactants and the salt needed are soluble, then u take the acid in a burette, alkali and indicator in a conical flask, add acid to flask till neutralisation, repeat experiment without indicator, crystallise the salt from the neutralised solution by heating it.
eg. nitric acid + Sodium hydroxide = sodium nitrate + water
Method 2: Mix excess insoluble base with acid
If one base given to u is insoluble, then u can add excess of it to acid, so that all acid gets neutralised
then u can filter any solid left in the solution and heat the filtrate till salt crystals appear. Then let it cool to get the salt
eg. Calcium carbonate + Hydrochloric acid = Calcium chloride + Carbon dioxide + Water

 

[Enferno]

Firstly it depends upon the salt..
NaCl, K2CO3, Cu(NO3)2 these are all soluble salts whereas BaSO4, AgCl, PbBr - all these are insoluble.
For insoluble salts, a double displacement reaction needs to occur between two salts or an acid and salt... For example
NaCl + AgNO3 gives AgCl and NaNO3
Silver chloride is insoluble whereas sodium nitrate is soluble. So it can filtered, washed with distilled water, and then dried
For soluble salts, you need to see if any reactants are given..
Method 1: Acid-base titration
If both reactants and the salt needed are soluble, then u take the acid in a burette, alkali and indicator in a conical flask, add acid to flask till neutralisation, repeat experiment without indicator, crystallise the salt from the neutralised solution by heating it.
eg. nitric acid + Sodium hydroxide = sodium nitrate + water
Method 2: Mix excess insoluble base with acid
If one base given to u is insoluble, then u can add excess of it to acid, so that all acid gets neutralised
then u can filter any solid left in the solution and heat the filtrate till salt crystals appear. Then let it cool to get the salt
eg. Calcium carbonate + Hydrochloric acid = Calcium chloride + Carbon dioxide + Water

still didnt got my answer

 

[smart_boy]

still didnt got my answer

Potassium sulphate is water-soluble so u can use acid-base titration for making it... ur reactants will be potassium hydroxide and sulphuric acid...
The other method cant be used as most potassium compounds are soluble in water... I hope u get it now...