[nilmot]

Sorry, just found out that it's not sulphur, it's phosphurous

Tom

[nilmot]

And the safe thing that makes it safe is potassium chlorate on the head of the match and phosphorous based chemical on the striking surface. They don't mix until match is stuck.

Tom

[NakedScientist (OP)]

QOTW needs resurrecting and that's partly my fault for taking ages to post the answer to this question and then publish the next one.

ANSWER TO "WHAT IS A SAFETY MATCH"

Matches were invented by the English apothecary John Walker in 1826-7 when he made a mixture of antimony sulphide and potassium chlorate for a client. He accidentally dropped some of the mixture which, upon impact, promptly ignited. Adding a stick and refining the recipe he produced the world's first friction matches, containing antimony sulfide, potassium chlorate, gum, and starch.

Walker was not an astute businessman and made no money from his invention. Indeed, it was another individual - Samuel Jones - who was already into matchmaking - who realised the huge commercial potential of a readily available source of fire and patented the invention under his own name. His brand of matches, based on the same recipe, were called Lucifers and were a runaway success.

But the problem with Walker and Jones's matches was that they were not terribly safe - they ignited in an explosive manner, produced a terrible smell when lit, and were poisonous ! They were, however, a runaway success and sales rocketed (pardon the pun).

Then, in 1832, Richard Bell established the first British match factory in London. He began producing a new phosphorus-containing match that had been invented by Frenchman Charles Sauria. The match head contained a mixture of sulphur, potassium chlorate, antimony sulphide, and white phosphorus. The addition of phosphorus made striking the match much easier to accomplish, but had the downside of poisoning people. The workers in the match factory developed phossy-jaw (an erosive disease of the lower jaw caused by long term exposure to white phosphorus), and children developed other bony abnormalities. A match box also contained enough white phosphorous to kill someone, and the matches kept setting fire to things they shouldn't - largely because all the chemicals needed for ignition were jammed together into the match head and all that was needed to kick-start the reaction was  some energy.

This problem was solved in 1844 when the Swedish chemist Gustav Pasch began separating the chemicals in a match head, placing some on the side of the matchbox, and the rest on the match head. The match could then only be struck against the side of the box - and that's the safety match. Another safety measure, which came later, was the subsitution of the less reactive red phosphorus for its more violent white counterpart.

The present day recipe for a match comprises a wooden or cardboard splint impregnated with ammonium phosphate (to stop it smouldering after being blown out), coated on the end with a mixture of gum, potassium chlorate, glass powder (to create the friction on striking), and sulphur.

The sulphur is the fuel which consumes the oxygen released by the potassium chlorate. The red phosphorus on the matchbox kickstarts the reaction rather than being used as a fuel (as in the early match recipes).

So there you have it, the history of matchmaking, and why a safety match is a safety match !

[NakedScientist (OP)]

HERE'S THE NEW QOTW :

WHY DO PLANETS SPIN, INCLUDING ORBITING THE SUN ?

[gsmollin]

NS, you should change the name to "Question of the Quarter", since the last question you posed was in September!

Anyway, planets spin and orbit the sun because the angular momentum posessed by the original gas and dust that collapsed to form the solar system is still conserved today.

In more detail, large clouds of gas and dust exist throughout the universe. They do not all collapse, because the clouds contain kinetic and thermal energy that resists gravitational collapse. However, if a volume of the cloud reaches a critical density, it will begin gravitational collapse. Frequently, the seed for the collapse is caused by a supernova explosion that compresses the cloud. Gravitational tidal forces from colliding galaxies can also begin collapse of gas clouds.

However it starts, the cloud must rid itself of its kinetic energy, and its gravitational binding energy, in order to collapse. It is mostly radiated away as thermal energy, but the cloud will eventually become opaque and convection will become important. Some protostars also radiate the energy as jets.

Another problem is the magnetic field of the collapsing cloud. As the size of the cloud shrinks, the strength of the magnetic field will grow, and may prevent further collapse. Ridding the cloud of the magnetic field is one of the many mysteries of stellar formation. It is possible that magnetic fields and rotational energy are eliminated together by the bipolar jets seen in nebula where protostars are presumably found. It is likely that the magnetic field energy has an important effect on the ultimate size and shape of the final solar system, just as the angular momentum, temperature, and kinetic energy of the collapsing cloud.

The angular mometum possessed by the original cloud is conserved: It cannot be radiated like the energy can. As the size of the cloud shrinks under gravitational collapse, the rotational rates begin to speed up. The cloud may break up into two to more clouds depending upon the amount and distribution of its angular momentum. The cloud forms a flat disc along a common rotational axis. If a star is to form at the center, it must rid itself of most of its angular momentum. This is accomplished by the protostar shedding mass through a solar wind. Much of that material remains in the disc, and adds to the coalesceing material in the disc. If the angular momentum is large enough, a binary star system may form. If there is less angular momentum, then planets form around the star. The planets all orbit in the same direction as the original cloud rotated, in a disc. They also spin in the same direction, at least in the beginning. As planetary formation continues, large planetoids may strike a planet and tip its axis of rotation. There can also be gravitational resonances, and tidal effects in planets near the central star, or stars, that will affect rotational rates and axis directions.

A related question is "Why do the planets all orbit at low inclinations to the ecliptic, i.e. why aren't there planets orbiting over the north and south poles of the sun?"

[NakedScientist (OP)]

There's not much I can add to gsmollin's excellent synopsis regarding planetary spin.

So here's this week's QOTW :

IF THE RETINA NEEDS LIGHT TO 'SEE', HOW DOES IT SEE THINGS THAT ARE 'BLACK' ?

Fire away...

TNS

[neilep]

Last time I looked I discovered that I'm not an Optician or eye doctor, but would the lack of light in itself be a way for the brain to construct the object that is not reflecting the light ?, if light is being received from everything but the object which is black then the the 'gap' itself is the construct ....even items which are black do reflect something, enough for the brain to construct outlines, edges, impressions, contours etc etc

'Men are the same as women...just inside out !'

[DrN]

I agree with Neil. Black absorbs light, rather than reflecting it, so it must be an absence of light. hence 'darkness' at night being perceived by our eyes as black.

[NakedScientist (OP)]

ANSWER TO LAST WEEK'S QOTW :

"IF THE RETINA NEEDS LIGHT TO 'SEE', HOW DOES IT SEE THINGS THAT ARE 'BLACK' ?"

The answer to this question lies within the retina itself. The retina consists of a sheet of cells, several layers thick, at the back of the eye. The top layer contains the photoreceptors or 'rods and cones', which contain light-sensitive pigment called rhodopsin comprising a protein - opsin - fused to retinal (the light sensitive part of the molecle) which is derived from vitamin A.

There are about 110 million rods in each eye (which see in black and white) and about 6.5 million cones (which see in colour - 62% are red cones, 32% are green, and 2% are blue). These photoreceptors connect to 'bipolar cells' which then link up with retinal ganglion cells. The retinal ganglion cells are responsible for transmitting the image to the brain. Here's a retinal schematic :



Intuitively one would think that when light shines on a photoreceptor it activates it, but in fact the opposite is true.  Light shining on a photoreceptor actually switches it OFF. When a photon of light hits a rod or cone it causes the rhodopsin to dissociate into its retinal and opsin components. This leads to the production of a chemical messenger which turns off the flow of sodium ions into the cell, hyperpolarising it, and hence making it less active.

The increased activity seen in the absence of light is referred to as the 'dark current' and, paradoxically, the retina is far more active in the dark than it is in the light.

So you do actually actively 'see' black, because the lack of light hitting photoreceptors makes them much more active.

Here's an overview of retinal physiology :
http://sky.bsd.uchicago.edu/lcy_ref/synap/retina.html

TNS

[NakedScientist (OP)]

HERE'S THIS WEEK'S QOTW

"HOW DOES WINDSCREEN DE-ICER SPRAY WORK ?"

TNS

[DrPhil]

I have tried several different brands and found that they don't work very well.

[chris]

It worked okay for me the other day !

Sorry to hear your's works less well, but how does it work in theory ?

Chris

"I never forget a face, but in your case I'll make an exception"
 - Groucho Marx

[chris]

It worked okay for me the other day !

Sorry to hear your's works less well, but how does it work in theory ?

Chris

"I never forget a face, but in your case I'll make an exception"
 - Groucho Marx

[DrPhil]

Works well in theory but not in practice.

[DrN]

is it like adding salt to water reduces the temperature at which it freezes, by interfering with the hydrogen bonding? so de-icer would presumably do the same. I remember that the salt experiment reduced the freezing point by 10 C (probably added to saturation), so based on this theory, whether it worked or not would depend on how cold it was! using salt, it would mean the temp would have to be below -10 C before it didn't work. presumably the solvents in de-icer take the temp threshold further.

[DrPhil]

My guess??... Depression of the freezing point is due to a lowering of the concentration of water molecules. As the deicing chemical dissolves in a little water, the particles are randomly distributed amongst the water molecules.  The particles simply get in the way of the water molecules when they attempt to form the highly ordered pattern of the solid phase.  Hence a lower temperature is required to get the solution to freeze.

[DrN]

yes, thats a more coherent way of putting it! I looked at my can of de-icer and its essentially propan-2-ol, and it works up to -15 C. the only thing is - how does the propanol dilute frozen, solid, water? it must break the bonds somehow. chemistry was never my strong point.

[DrPhil]

I know that salts have an exothermic heat of solution (or is it heat of hydration; I'm not a chemist either) that assists in melting the ice. But that doesn't explain how the non salt-based products work.

I also know that aircraft deicers are usually applied hot. The hot liquid melts the ice and then the freezing point depression properties of the solution prevent re-freezing. But that doesn't explain how the deicers that we may use on our cars might work. If you're like me you store the stuff in the trunk of your car and it probably starts out at the same temperature as the ice it's trying to melt.

Then there are the glycols which have a couple of “-OH” (hydroxyl) groups that can break up the hydrogen bond in water. I suppose that this could break apart the ice crystals.

[NakedScientist (OP)]

ANSWER TO LAST WEEK'S QOTW :

"HOW DOES WINDSCREEN DE-ICER SPRAY WORK ?"

You're almost there. De-icer is indeed iso-propanol, an alcohol not greatly dissimilar to the alcohol in a bottle of gin or whisky.

Alcohols dissolve because they have a hydroxyl (OH) group attached to the molecule. This chemical group is very polar (pardon the pun in the context of de-icer). The oxygen atom loves electrons and pulls them towards itself very strongly, including the electron from the hydrogen that is bonded to it.

This makes the hydrogen a little bit positive, and the oxygen a little bit negative and enables it to take part in a process called hydrogen bonding. This is what makes water 'sticky' and why you can bend a stream of water with the static electricity on a comb, because all of the molecules cling together by the positive hydrogens attracting the negative oxygens on adjacent molecules.

So the alcohol can dissolve in water because it can take part in the same hydrogen bonding process. But because the alcohol molecule is much bigger than a water molecule, and a funny shape, it prevents the water molecules lining up so easily to get close enough together to form a solid crystal i.e. freeze. To do that you need to make the conditions much much colder. This is how antifreeze works. A big ethylene glycol molecule links up with lots of water molecules (and dissolves quite happily), but because it is a funny shape it prevents big regular ice crystals from forming, so even if you car radiator does begin to freeze the best it can do is form slush which won't burst your pipes.

So how does the de-icer melt the ice to start with? Well, the alcohol in the tin is concentrated. When you add it to the ice on the window, the diluting effect of the ice and concentrated alcohol mixing produces a little bit of heat which speeds up the melting process. Then the alcohol and water mix thoroughly, the alcohol dissoves in the water, lowering its melting point and preventing re-freezing.

Occasionally you can make the ice re-freeze but this is usually on an exceptionally cold day with a particularly thick layer of ice, so the concentration of alcohol in the water remains too low.

So that's how de-icer works (in theory !)

TNS

[NakedScientist (OP)]

Here's this week's QOTW

"WHAT MAKES YOUR JOINTS 'CRACK' FROM TIME TO TIME ?"

TNS