Urea is an important historical compound.  In 1773 Hilaire Rouelle discovered that human urine contained this substance.  At the time it was strongly believed that the animate world bore no relation in chemical terms to the inanimate world, (a theory called ‘vitalism’).  In simple terms, molecules found in living organisms were somehow ‘special’ and could not be prepared from the everyday compounds that stocked a chemistry lab of the period.  A little more than fifty years after its discovery, in 1828, urea was the first organic substance to be artificially synthesized in the lab.  The chemist, Friedrich Woehler, prepared urea from two ‘ordinary’ compounds, potassium cyanate and ammonium sulphate, and thus showed that vitalism was not a viable theory.  The modern science of organic chemistry was born out of the interest in molecules that play a part in the processes that occur in living organisms.

Urea is used extensively in the manufacture of urea-formaldehyde resins, which in turn are used to form plastics.  The compound is also involved in the production of fertilizers which release nitrogen to plants to facilitate their growth.
 
The structural formula and molecular shape of urea are shown below.  Compared with most biological organic compounds, urea is relatively simple.  (Both illustrations were produced using HyperChem.)

Since the white crystals have a relatively low melting temperature, (about 133 degrees Celsius), urea makes an excellent melt specimen.  Note however that the compound is harmful if swallowed or inhaled, and causes irritation to the skin, eyes, and respiratory tract.

The images in the article were photographed using a Nikon Coolpix 4500 camera attached to a Leitz SM-Pol polarizing microscope. Crossed polars were used in all polarized light images.  Compensators, ( lambda and lambda/4 plates ), were utilized to alter the appearance in some cases.  A 2.5x, 6.3x, 16x or 25x flat-field objective formed the original image and a 10x Periplan eyepiece projected the image to the camera lens.

Provided that the thickness of the crystal layer is suitable, urea can produce strikingly colourful images. In the first image (above), lambda/4 and lambda compensator plates were used to decrease the contrast.  In the two images below, only crossed polars were used.  If the rotating stage of the microscope was moved to a different angle, the dark areas would be brighter and the bright areas darker.  This particular orientation was chosen to reveal the yellow-orange ‘arms’.

The following images show a higher magnification view of the same area.

Sometimes the small details are more interesting than the overall structure.  The image to the right is a magnified view of the small shape at the upper centre of the left image.

Consider the gray areas in the image below.

Under higher magnification, and with the aid of two lambda/4 compensators, roughly triangular detail appears.

The first image in the article shows what looks like blue-red quills on a feather.  If these quills are magnified, and both lambda and lambda/4 compensators are used, one can see unusual details in the bands.  (The left image is my favourite urea photomicrograph.  There’s no accounting for taste!)

Here is a challenge.  Can you find the structure shown in the right image within the left image?

Small ‘blobs’ such as the one shown below occurred occasionally in the darker areas.  Compensators reveal the complex structure in the area outside the ‘blob’.

Under higher magnification, detail is resolved in the outside area.  The same magnification also shows the subtle detail in a different ‘blob’.

The two photomicrographs that follow display the change caused by rotating one of the two lambda/4 compensators.  Notice that each orientation highlights the detail of different areas.

Consider the top right corner of the left image below.  Under much higher magnification,  this corner has a completely different appearance when dark-ground illumination is used to reveal the detail of the crystal edges.

Urea it seems to me, is an ideal substance to study under the polarizing microscope.  It is not extremely dangerous, and once the melt specimen has been prepared, the sample can be re-melted many times to provide fascinating crystal fields.  Experiment by using a pencil eraser to push gently on the centre of the coverglass while the compound is still molten.  This will provide a crystal thickness gradient which will result in more interesting fields.

Equipment

The images in the article were photographed using a Nikon Coolpix 4500 camera attached to a Leitz SM-Pol polarizing microscope. Crossed polars were used in all polarized light images.  Compensators, ( lambda and lambda/4 plates ), were utilized to alter the appearance in some cases.  A 2.5x, 6.3x, 16x or 25x flat-field objective formed the original image and a 10x Periplan eyepiece projected the image to the camera lens.