Monday, December 1, 2008

Chemistry is Fun not Magic


Normally it is impossible for any substance to
burn under water, but while phosphorous with
some potassium chloride is kept is kept in large
trough of water and concentrated sulphuric acid
poured on it carefully using a funnel. The white
phosphorous burns violently under water. The
reactions are ;

KClO3 + H2SO4 ----- KHSO4 + HClO3
HClO3 ----- HClO4 + ClO2 + H2O
Chlorine dioxide combines with phosphorous to
make it burns.

Sunday, November 30, 2008

Most children with access to a chemistry set will have created a chemical crystal garden1.
Chemical gardens are the plant-like structures formed by dropping a seed crystal of a soluble metal salt into an aqueous solution of one of several anions2 such as aluminates, borates, carbonates, chromates, cyanoferrates, phosphates, or silicates. The most commonly-used (and studied) anion, however, is silicate, present in sodium silicate (water glass3) and hence the alternative names of silica or silicate gardens.
As the name indicates, the chemical 'horticulturalist' will observe a growth of fronds, similar in form to plants, taking place over a period of minutes to hours. This was of interest to 19th Century scientists who were interested in the origin of life.
These materials should be readily available from a good chemist, although they might have to be ordered.
Glass jar or large (600ml) beaker
100ml of water
150ml of sodium silicate solution
chromium (III) chloride hexahydrate4 crystals5 (green)
Iron (III) chloride crystals (orange)
Iron (II) sulphate crystals (green)
Copper (II) sulphate crystals (blue)
Nickel (II) sulphate crystals (green)
Aluminium potassium sulphate crystals (white)
Cobalt(II) chloride crystals (purple)
Rubber gloves
Eye protection
For aesthetic reasons an optional thin layer of sand may be placed on the bottom of the beaker or equivalent glass container.
If you live in a hard water area, boil the water first and allow it to cool before adding to the beaker or glass jar. Add the sodium silicate to the water, and stir it to dissolve.
The solution is now ready to receive the other chemicals. Using the tweezers, drop these carefully onto the surface, vertically above the place you would like them to grow, in a pattern as though you were sowing a real garden.
Do not shake or mix the solution. The 'plants' will begin to grow within a few minutes.
When the garden has ceased to grow (or if you wish to stop it growing), hold the beaker under a cold water tap and slowly allow fresh water to pour down one side of the beaker, thus flushing out the sodium silicate solution. This must be done very carefully to avoid damage to the crystal fronds.
When you are satisfied that all the solution has been washed away, and the beaker is filled with cold water, the top may be sealed with a lid or with a piece of plastic taped down. Under these conditions, with care, the chemical garden will survive for a long time.
Safety Precautions
Several of the chemicals involved, especially the chromium (III) chloride and nickel (II) sulphate, are skin irritants, and can cause contact dermatitis. Furthermore, iron (III) chloride is corrosive and stains the skin and many transition metal salts are toxic. Hence, the crystals should not be directly handled; use the tweezers!
So, What is Going On?
Certain metal salts, especially those of the transition metals, form precipitates when placed in the sodium silicate solution. As the metal salt dissolves, the resulting solution is less dense than the surrounding silicate solution and so rises up through the solution. As it reacts with the silicate anion, 'stalagmites' (like those found in caves) form from the bottom of the jar upwards - these are insoluble metal ion silicates. The surfaces of these insoluble silicates behave as a semipermeable colloidal membrane6, across which osmosis can occur.
Water from the sodium silicate solution travels across the semi-permeable membrane of the metal ion/sodium silicate precipitate, to the higher concentration of metal ions that are present on the inside. The water pressure inside the gel-like structures increases until the membrane bursts, thus allowing more of the metal ions to react with the silicate solution to create new membranes. This process repeats itself until the metal salt is fully dissolved, thus allowing the crystals to keep growing upward and sideways. As the metal salt solution is less dense than the sodium silicate solution, the precipitate tends to grow upwards.
Academic Interest in Crystal Gardens
So-called metallic trees were first observed by people such as the German chemist, Johann Rudolf Glauber (1604 – 1668) and first studied by another German chemist, Isidor Traube in the mid-19th Century. Traube showed that membranes could be produced artificially, which were permeable to water but not for certain dissolved substances. In this respect they were similar to those membranes surrounding plant and animal cells.
Among the semipermeable membranes prepared by Traube was one of copper (II) hexacyanoferrate (II), and such a membrane, formed in the walls of a porous pot, was used by the German botanist, Wilhelm Pfeffer, in 1877, for the quantitative measurement of osmotic pressure. These results showed that the osmotic pressure is proportional to the concentration of the solution, and also that it increases with rise of temperature. This research culminated in van't Hoff's law of osmotic pressure, formulated in 1887, in which he was able to apply the second law of thermodynamics. This pointed the way to a method for determining the relative molecular masses of substances in solution.
Indeed, osmotic pressure (for very dilute solutions) was found to obey the Ideal Gas Law, and
πV = (m/M)RT
π = osmotic pressure
v= volume (in dm3) containing a fixed mass of solute
m = mass of solute present
M = Relative Molecular Mass of solute
R = Universal Gas Constant
T = Absolute Temperature
and hence:
M = mRT/πv
Despite all this knowledge, the physical chemistry of the formation of chemical gardens is still imperfectly understood.
In 1984 Independent Television News (UK) organised a competition for all schools in Great Britain to suggest an experiment to be performed in space, aboard the Space Shuttle. This was won by a school from Kent, Ashford School, who suggested a chemical garden.
The students wished to find out what shape and direction the 'plants' in a chemical garden would grow under conditions of microgravity. They wrote a computer program to simulate what might happen to the garden in space and it predicted a range of possibilities from near-spherical shapes to a spherical bundle of long arms.
The shuttle Endeavour launched on 12 September, 1992 and the chemical garden experiment worked very successfully. The growths were in random directions and tended to be twisted. To the surprise of the students there were also a few perfect spiral forms. At the time of reporting they had no satisfactory theory of the origins of these spirals.
Crystal-growing under conditions of microgravity has a wider and deeper significance for the life sciences, where a major goal is to understand structure/function relationships of biological systems at the atomic level. For example, many important biological molecules such as proteins (which include enzymes) have yet to be adequately structurally analysed. Microgravity may provide an environment where perfect protein crystals could be produced, that are large and pure enough for more precise analysis, such as X-Ray Crystallography. This would have important applications in, for example, cancer research.
1 Chemical gardens are also referred to as crystals gardens, silica or silicate gardens or chemical crystal gardens.2 Negatively-charged ions ie, negatively-charged atoms or groups of atoms (eg, chloride ion, Cl- in sodium chloride or the sulphate ion, SO42- in copper sulphate).3 Water glass was used in the UK to preserve eggs during World War II. The eggs were soaked in water glass. The porous calcium carbonate of the eggshell reacted with the silicate to produce insoluble calcium silicate, which blocked the pores thus rendering the egg impermeable to air.4 This is the IUPAC recommended nomenclature, where Roman numerals are used to denote the oxidation number of the metal ion. Hence this compound is 'chromium three chloride, hexahydrate'.5 For the best results reasonably large crystals (up to pea-size) of each metal salt should be used.6 A membrane that allows the solvent (water) to pass but not the solute (such as dissolved sugar or large metal ions).

Wednesday, November 26, 2008

Chemistry is Fun not Magic


This is an interesting chemical experiment which beautifully

express the phenomenon of osmosis through semipermeable

membrane of silica gel. It looks a magic where colourful silica

grow in solution appear as colourful flowring garden The magic

solution in which garden grow is a solution of sodium silicate in

water. It is prepared by diluting water glass (concentrated

solution of sodium silicate available in market) five times with

distilled water. Salts used to make magic rocks which readily

available are:

Purple- Manganese chloride
Blue- Copper sulphate
Red- Cobalt chloride
Pink- Manganese chloride
Orange- iron chloride
Yellow- Iron chloride
Green - Nickel nitrate
White- Lead nitrate

The silica garden is prepared by placing crystals of various

coloured salts ( Apr. 0.4 mm size) in a magic solution of sodium

silicate prepared as above taken in a clean glass container .

In a few hours hollow tubes of metallic silicate gets shoot
up from these crystals which look like trees. If you add too

many crystals the solution will turn cloudy and immediate

precipitation will occur. A slower precipitation rate will give

you a nice garden. Once the garden grown , you can replace

the sodium silicate solution carefully with pure water. Initially

a colloidal and semipermeable shell of silicate is formed around

the crystal. Inside this is a strong solution of the salt and out side

is a weak solution of sodium silicate. Hence water permeates

into the shell and pressure rise until the shell bursts. At this

stage the salt solution escapes but immediately comes in the

contact with the sodium silicate solution and react with it to

form again semipermeable shell of the metallic silicate. Thus

the original condition is reproduced over and overagain and a

projection of silicate continusly grow.
The optimal concentration of magic solutio lie between

1.56 M and 0.625 m with respect to si9lica in experiment

where growth of silica tubes is vigorous. More concentated

solutionproduce meagre growth and thre is vigorous growth

from intermediate concentration while the more diluted

solution produces mearly a gelatinous mass. In more

concentrated solution the semipermeable membraneof

sodium silicate surrounding the seed crystal is broken

only with difficulty to produce the growth of tube. On the

other hand in dilute solution the membrane that is formed

acquire a more plassticcharacter and is not easily rupture

rather distords without breaking.