Biology; Chemistry; Physics: Post your doubts here!

[Saad Mughal]

however they have drawn a straight line in the marking scheme for the section you have shown in red............a straight line with a negative gradient, not even slightly curved...

Which marking scheme?
The marking scheme says "decrease to 5 m/s at 25 s".

 

[papajohn]

how a low density cloud of hydrogen in space becomes a star??

Stars are formed in clouds of gas consisting mainly of hydrogen called nebulae. In space gravity pulls the gas and dust into a blob. Small blobs form around it(planets and moons). The central blob gets larger and larger. Gravitational Potential Energy is converted into thermal (heat) energy causing the temperature to rise. After some time, the the pressure and temperature become so high that the process of fusion begins. The blob lights up and a star is formed. Hydrogen atoms fuse together to form helium. As a result of the fusion the star expands.
Extra Info:
After several billion years, the main source of hydrogen has nearly run out. Heavier elements (iron) are formed. The star expans to an enormous size (Red Giant). Finally when the pressure is too high, and the mass exceeds a critical limit, the star explodes. This explosion is called a supernova. However remember that this information would not be asked in the exam but it is good to know it as applied questions regarding it may be asked.

 

[fatima0000]

Stars are formed in clouds of gas consisting mainly of hydrogen called nebulae. In space gravity pulls the gas and dust into a blob. Small blobs form around it(planets and moons). The central blob gets larger and larger. Gravitational Potential Energy is converted into thermal (heat) energy causing the temperature to rise. After some time, the the pressure and temperature become so high that the process of fusion begins. The blob lights up and a star is formed. Hydrogen atoms fuse together to form helium. As a result of the fusion the star expands.
Extra Info:
After several billion years, the main source of hydrogen has nearly run out. Heavier elements (iron) are formed. The star expans to an enormous size (Red Giant). Finally when the pressure is too high, and the mass exceeds a critical limit, the star explodes. This explosion is called a supernova. However remember that this information would not be asked in the exam but it is good to know it as applied questions regarding it may be asked.

thanks, this one is really helpful

 

[Saad Mughal]

any body please explain me cathode ray oscilloscope ??
im worried nd cant get through it

A cathode ray oscilloscope consists of a cathode, a grid (optional), an anode, y-plates, x-plates and a florescent screen.
The cathode is heated (like a filament) and it emits electrons (thermionic emission).
The anode is connected to a large positive potential (hence it is positively charged), it attracts the electrons and accelerates and concentrates them onto a straight beam and travels to the screen.
The y-plates are connected to the voltage (that is applied) (they are responsible for the vertical movements on the CRO) and deflects/moves the electrons accordingly.
The x-plates are connected to the time base (they are responsible for the horizontal movements), they move the electrons left or right.
The florescent screen is a screen made of florescent (it is chosen because it glows and produces a bright spot where the electrons strike them).
The grid, is connected next to the cathode, it has a negative potential connected across it, it deflects the electrons coming from the cathode (and hence, controls the number of electrons hitting the screen). The grid is the brightness control of the CRO.
The whole area inside the CRO is a vacuum (no air), since air molecules would bombard the electrons.
A diagram is attached to help you understand. If you have any queries, just ask.

 

[fatima0000]

human activities which led to an increase in background radiation??

 

[Saad Mughal]

human activities which led to an increase in background radiation??

Construction and mining (exposing radioactive sources underground).
Nuclear waste and Nuclear power plants give off background radiation.
Global warming causes destruction of ozone which leads to more cosmic radioactive rays from the sun.

 

[usama321]

4
An aqueous solution of a sulphate is made from a solid hydroxide, of a metal M, by the reaction:M(OH)2(s)+H2SO4(aq)→MSO4(aq) + 2H2O(l)
For which hydroxide would the method
not
work?
A
barium hydroxide
B
copper(
II
) hydroxide
C
iron(
II
) hydroxide
D
magnesium hydroxide

 

[Saad Mughal]

4
An aqueous solution of a sulphate is made from a solid hydroxide, of a metal M, by the reaction:M(OH)2(s)+H2SO4(aq)→MSO4(aq) + 2H2O(l)
For which hydroxide would the method
not
work?
A
barium hydroxide
B
copper(
II
) hydroxide
C
iron(
II
) hydroxide
D
magnesium hydroxide

B should be the answer.

 

[usama321]

B should be the answer.

that is what i chose, and the mark scheme says it is A

 

[usama321]

i think they have got it wrong, cuz barium is even more reactive than Zn and Mg

 

[Saad Mughal]

that is what i chose, and the mark scheme says it is A

Which paper?

 

[Saad Mughal]

i think they have got it wrong, cuz barium is even more reactive than Zn and Mg

Yeah, barium comes after potassium and sodium.

 

[usama321]

winter 2002, mcq 4

 

[Saad Mughal]

winter 2002, mcq 4

They're correct. We've been dumbfounded by the basics.
Kill me now. Barium sulphate is not even soluble! How could it be in (aq.) form after reacting.

 

[usama321]

They're correct. We've been dumbfounded by the basics.
Kill me now. Barium sulphate is not even soluble! How could it be in (aq.) form after reacting.

did you mean copper sulphate is not soluble?

 

[papajohn]

They're correct. We've been dumbfounded by the basics.
Kill me now. Barium sulphate is not even soluble! How could it be in (aq.) form after reacting.

Oh! i see now..

 

[usama321]

i did not understand?

 

[Saad Mughal]

did you mean copper sulphate is not soluble?

Barium Sulphate (BaSO4 is not soluble). We learned this in the solubility table, didn't we? So how can it be used to make aqueous solution?

 

[usama321]

Barium Sulphate (BaSO4 is not soluble). We learned this in the solubility table, didn't we? So how can it be used to make aqueous solution?

then copper should be the answer?

 

[Saad Mughal]

then copper should be the answer?

No, A should be the answer since BaSO4 is not soluble and hence, the method would NOT work.