Coloured bands on soap bubbles and oil layers are caused by interference of light waves with themselves. Sunlight is a composite of seven colours (violet, indigo, blue, green, yellow, orange and red, in that order) each of which lies in a specific wavelength range. For example, violet light has a wavelength of 380 nanometers (one nanometer is a billionth of a metre) and red, above 600 nm.
Light waves falling on any thin film are partially reflected from the top and bottom surfaces of the film. A wave reflected from the bottom surface has to travel a longer distance than the one reflected from the top. This difference in distance travelled by the waves, called path difference, leads to a difference in the phases of the light waves. (The phase describes the electric and magnetic fields associated with light waves.) This phase difference depends on the thickness of the film at the point of reflection and angle of viewing. If two waves are ‘out of phase’, then they cancel each other (destructive interference) and if they are ‘in phase’ they add up (constructive interference.) Thus the regions of the spectrum (colours) which interfere constructively can be seen whereas those which interfere destructively are lost. Even if there is a slight variation in the thickness of the film, it will be coloured differently.
Carbon changes into diamond and graphite in the depths of the earth under high pressure and temperature. The process might have taken millions of years. A French scientist, Moissan, verified this theory by experiment. He heated carbon (as charcoal) and iron together in an electric furnace at 3,500 degrees Celsius. Carbon dissolved in the molten iron and the mass was cooled suddenly by dipping in water. Then it was put in acid. The iron dissolved and the residue was found to contain a few very small diamonds (less than one millimeter in size) and some graphite. These results were confirmed by others too. Today diamonds are mass-produced by this method.
Grade denotes the minimum strength obtainable in a cement sample. This is determined after making a mould of it, tom, under specified conditions on the 28th day of its moulding. The strength is expressed in megapascals (MPa).
A phenomenon called polarity prevents oil and water from mixing. All molecules carry electrical charge which is distributed uniformly or non-uniformly over the length of the molecule. In polar compounds, the positive and negative charges are concentrated at the two ends of the molecule. When such substances are mixed together, the positive and negative regions of their molecules attract each other and as a result a clear solution is obtained. Water is a polar substance and mixes freely with other polar substances. Oil molecules, on the other hand, are non-polar. When polar and non-polar substances are mixed together, the mutual attraction of polar molecules separates out the non-polar molecules and the two substances do not mix. 
The solubility of a substance depends on the nature of bonding in both the solute and the solvent. Salt is an ionic substance; that is, its atoms are held together by opposite electrical charges. Water molecule also has electrical charges – the hydrogen atoms are positively charged and the oxygen atom negatively charged. When salt is put in water the charges on the water molecule pull the oppositely charged atoms in salt apart, thus dissolving it. Oil molecules, on the other hand, do not carry any charge and so does not have any effect on the ionic bonds in salt when put in it. As a result, salt does not dissolve in oil.
Bubbles are balls of thin soap films. They show rainbow colours because the light falling on them undergoes what is known as interference. Light travels in the form of waves which have troughs and peaks at regular intervals. For instance, one wavelength of a light-wave has one trough and one peak. When light falls on a soap bubble it is reflected from both the upper and lower surfaces of the film. Thus, there are two waves of reflected light emerging out of surfaces of the soap bubble. If the distance between the two surfaces of the film is less than one wavelength of light, the two reflected waves are so close that interference occurs. The peaks of the two waves may add up and become a bigger peak. Or the trough of one wave may fall with the peak of the other thus cancelling its effect. As the seven component colours of white light have different wavelengths, the adding or cancelling effect of interference is not the same on all of them. In other words, some colours become brighter and some duller. Thus a viewer sees a mixture of rainbow colours instead of the white light emerge from the soap bubble.
Foam or lather is nothing but a large collection of small soap bubbles. A soap bubble is, in turn, a very thin film of soap solution enclosing some in air. Because of the low surface tension of soap solution, the film can stretch and spread and form innumerable bubbles with a very large total surface area. Because of this, whatever slight tint present in the thin film of the coloured soap solution gets subdued. Although a soap film is more or less transparent, the lather or foam looks white because the light striking this large collection of bubbles gets scattered. That is why all kinds of foam look white.
One of the laws of fusion is that the melting point of substances which expand on freezing is lowered by the increase of pressure, while it is raised in the case of those which contract on solidification. Ice belongs to the first category of substances, that is, it expands on freezing. Ice has an open structure which collapses when subjected to pressure, producing water which occupies lesser volume. That is why ice melts when subjected to pressure.