1

General Science / Re: Magnets

« on: 28/02/2006 11:41:03 »

Essentially, there are three types of magetism:

1. Ferromagnetism eg permanent magnets. This is where all the magnetic moments align in the same direction (this is how you get north pole and south pole). The strongest magnet known is a neodymium-iron-boron compound (Nd2Fe14B), which is used in earphones and high-performance speakers.

2. Anti-ferromagnetism - magnetic moments align in opposite directions and cancel leaving no net moments.

3. Ferrimagnetism - moments align in opposite directions leaving net residual moment. There is a family of compound called rare earth garnets (Ln3Fe5O12), a complex oxide. Its ferrimagnetic properties have unsual temperature dependence.

Let's say the lanthanide has bigger moments and is pointing up; and the iron has smaller moments pointing in opposite direction. This results in a net magnetic moment pointing up. However, as you heat the compound, the lanthanide magnetic moment is destroyed leaving only the iron's magnetic moment which is pointing in a different direction. So the overall effect is a change of direction of magnetic moment on heating the compound.

Personally, I find this quite fascinating.

Angel

2

General Science / Re: Why do leaves change colour?

« on: 28/02/2006 11:17:31 »

thank you Broca

Angel

3

General Science / Why do leaves change colour?

« on: 27/02/2006 19:24:41 »

Why do leaves of some plants change colour in autumn? I know it's somthing to do with singlet oxygen affecting the chlorophyll in leaves but I can't remember exactly. If someone can help me out it will be great.

Angel

4

Chemistry / Re: In chemical bonding, what are sigma and pi bonds?

« on: 27/02/2006 19:14:49 »

As you learn more about bonding models, you will find the above is not entirely correct. At the moment Molecular Orbital Theory is the best model for explaining bonding in molecules.

Angel

5

Chemistry / Re: In chemical bonding, what are sigma and pi bonds?

« on: 27/02/2006 19:10:57 »

The answer to your question all depends on how deep you want to go into this subject. Are you in University?

Think of sp3, sp2 and sp as hybrid of s and p orbitals.
For example in methane (CH4) the orbitals of the carbon atom is sp3 hybridised (one s + three p), this forms 4 equivalent orbital for hydrogen to bond to. and these orbitals get away as far as possible from each other to form a tetrahedral shape.

Can think about the other two hybridisation in the same way.
sp2 (one s + two p) - trigonal planar in shape. eg formaldehyde HCHO.
sp (one s + one p) - linear in shape. eg acetylene (=ethyne)

sigma bond = single bond (originate from overlap of s orbitals(H-H) or end-on overlap of p orbitals (C-C)
pi bond = double bond (originate from side-way overlap of p orbitals)

I hope the above is not too text-book like. It will be helpful to look at diagrams on books.

Angel

6

Chemistry / Re: Oxidation query

« on: 22/02/2006 18:24:40 »

I think MnO2 Manganese(II) oxide is black and I think this is the product you get if you heat Mn in air.

Angel

7

Chemistry / Re: Oxidation query

« on: 22/02/2006 18:21:34 »

quote:Originally posted by ROBERT

I think the "black" in black & white photographs is Silver.

The substance used in photography is a silver(I) halide complex. The halide X- can be oxidised to X by shining light on it and releases an electron. This electron reduces Ag+ to silver metal (black).

But silver metal itself is quite stable and not easily oxidised by O2 in air.


Angel

8

Chemistry / Re: Rocket Engine Fuels

« on: 22/02/2006 17:53:33 »

The rocket engine fuel I've come across is hydrazine, N2H4. But then I don't think this can be easily made with household materials.

Angel

9

Chemistry / Nano-Kids

« on: 19/11/2005 09:12:57 »

I saw this on the web the other day. It's amazing what a bit of chemistry can do!

http://cohesion.rice.edu/naturalsciences/nanokids/

ref. Vol. 80 No. 4 April 2003 • Journal of Chemical Education

Angel

10

That CAN'T be true! / Barometer

« on: 09/12/2003 08:30:49 »

This is a story about a physic student.

He was doing an exam at the end of his physics degree and one of the question was "How do you measure the height of a skyscraper using a barometer?" This student wrote, "Tie a string onto the barometer and lower it from the top of the skyscraper and the height of the skyscraper is the length of the string plus the length of the barometer." And (not surprisingly) this student failed his physics exam!

But this student claimed there's nothing wrong with his answer so he wrote to the examiners who later reply, "The answer showed absolutely no understanding of physics at all. However we would like to meet you and ask for an oral response to the question."

During this student's oral exam, he gave another answer to the barometer question. He said, "You could stand on top of the skyscraper, drop the barometer and measure the time taken to reach the ground. Using the mass of the barometer, acceleration due to gravity and time taken to reach the ground, the distance travelled can then be calculated." [] The examiners were shocked. He went on, "Or better still, you could go to the janitor of the skyscraper and say he can have a new barometer if he can just tell me the height of this skyscraper" []

There are rumours saying that the physics student in this story was Neils Bohr who later got a Nobel Prize for Physic [^]

Angel

"Anyone who has never made a mistake has never tried anything new." -Albert Einstein

11

General Science / Re: Pencil lead

« on: 06/11/2004 13:58:46 »

thx moses lawn

Angel

12

General Science / Pencil lead

« on: 04/11/2004 20:26:45 »

Hello~ I haven't been around for ages...the amount of work to do has got on top of me.

Anyone knows how pencil lead of different hardness (eg. 2B, B, HB....etc) are produced? I say hardness of pencil lead because if it's softer then more graphite will be left on the paper. Is this due to the % compositions of graphite in pencil lead? What other substances are also in pencil lead?

Angel

13

Physics, Astronomy & Cosmology / Re: Why does light refract?

« on: 01/08/2004 20:53:17 »

There are no delocalised electrons in glass (SiO2) since all atoms are covalently bonded, so electrons cannot absorb the energy and become excited.

Angel

14

Physiology & Medicine / Re: Smooth muscle-tropomyosin

« on: 10/06/2004 11:55:35 »

Is tropomyosin the molecule that is around actin filament?

I seem to remember that during muscle contraction, Ca2+ ions attaches to the tropomyosin exposing the myosin binding site on the actin molecule. This enable myosin to bind with actin filament and allow cross bridge cycle (rachet mechanism) to take place.

I've done this in school, but I remember it only vaguely so I could be wrong.

Angel

15

Physics, Astronomy & Cosmology / Re: What is the largest atom?

« on: 03/06/2004 12:38:56 »

quote:Originally posted by Dan B

[] yeah, a neutron star is more dense than the atomic nucleus.

I don't think it's 'more' dense than the atomic nucleus. A neutron star is 'as' dense as the atomic nucleus.

Angel

16

Physics, Astronomy & Cosmology / Re: What is the largest atom?

« on: 02/06/2004 09:00:26 »

Most atoms have more neutrons than protons, this is to increase the nuclear force without increasing the electrostatics repulsion (produced between protons) and hence making the nuclei more stable.

Very large nuclei are unstable because strong nuclear force that holds the nucleons together has a very short range so cannot reach every nucleon. The following shows how nuclei become more stable:


Also have a look at the graph for binding energy per nucleon:


Fe-56 is indeed the most stable nuclei.

Source: http://europa.eu.int/comm/research/energy/fi/fi_bs_np_en.html

Angel

17

Geology, Palaeontology & Archaeology / Re: Where does sand come from ?

« on: 28/05/2004 08:18:27 »

Bear in mind that the fine dust particles will eventually turn into rocks once again but that over millions of years.

Angel

18

Physiology & Medicine / Re: Upper GI scope

« on: 27/05/2004 11:46:54 »

Jay, what happened exactly? I'm going to uni to do chemistry in September, any advice for me? It seems that the lab reactions you are doing are pretty hazardous.

Angel

19

Physiology & Medicine / Re: Where air comes from when puffing out your cheeks?

« on: 27/05/2004 11:42:27 »

quote:Originally posted by cannabinoid

Yeah, even if you breath out as much as you possibly can, there is still some air in your lungs, esophagus, and mouth.  Puffing your cheeks is a musclature movement of the mouth that forces the air that remains in your mouth and throat into your cheeks.

You find the weirdest things to do with the body, QC.

This message brought to you by The Council of People Who Are Sick of Seeing More People

Jay, I would've said that puffing your cheeks decrease the pressure inside the mouth so the air in lung and esophagus is drawn out by diffusion. This is exactly what breathing is, moving your rib cage outwards and your diaphragm downwards decreases the pressure in the lungs so the air from the outside is drawn in.

Angel

20

Geology, Palaeontology & Archaeology / Re: Where does sand come from ?

« on: 27/05/2004 11:35:56 »

Yep, I would've also guessed that sand comes from the weathering of rocks especially sandstones.
As to your second question, maybe few metres of sand then followed by layers of small rocks? Anyway, Richard is the person to ask.

Angel