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# The Solid Phase Help (page 2)

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By — McGraw-Hill Professional
Updated on Sep 5, 2011

## Hardness Of Solids

Some solids are literally “more solid” than others. A quantitative means of expressing hardness, known as the Mohs scale , classifies solids from 1 to 10. The lower numbers represent softer solids, and the higher numbers represent harder ones. The standard substances used in the Mohs scale, along with their hardness numbers, are shown in Table 10-1. The test of hardness is simple and twofold: (1) a substance always scratches something less hard than itself, and (2) a substance never scratches anything harder than itself.

Table 10-1 The Mohs Scale of Hardness (Higher numbers represent harder substances. Relative hardness is determined by attempting to scratch one substance with another.)

An example of a soft solid is talc, which can be crumbled in the hand. Chalk is another soft solid. Wood is somewhat harder than either of these. Limestone is harder still. Then, in increasing order of hardness, there are glass, quartz, and diamond. The hardness of a solid always can be determined according to which samples scratch other samples.

Many substances have hardness numbers that change with temperature. In general, colder temperatures harden these materials. Ice is a good example. It is a fairly soft solid on a skating rink, but on the surface of Charon, the bitterly cold moon of the planet Pluto, water ice is as hard as granite.

Hardness is measured by maintaining laboratory samples of each of the 10 substances noted in Table 10-1. A scratch must be a permanent mark, not just a set of particles transferred from one substance to the other. Substances commonly have hardness values that fall between two whole numbers on the scale. The Mohs hardness scale is not especially precise, and many scientists prefer more elaborate methods of defining and measuring hardness.

## Density Of Solids

The density of a solid is measured in terms of the number of kilograms contained in a cubic meter. That is, density is equal to mass divided by volume. The kilogram per meter cubed (kg/m 3 or kg · m −3 ) is the measure of density in the International System (SI). It’s a rather awkward unit in most real-life situations. Imagine trying to determine the density of sandstone by taking a cubical chunk of the stuff measuring 1 m on an edge and placing it on a laboratory scale! You’d need a construction crane to lift the boulder, and it would smash the scale.

Because of the impracticality of measuring density directly in standard international units, the centimeter-gram-second (cgs) unit is sometimes used instead. This is the number of grams contained in 1 cubic centimeter (cm 3 ) of the material in question. Technically, it is called the gram per centimeter cubed (g/cm 3 or g · cm −3 ). To convert from grams per centimeter cubed to kilograms per meter cubed, multiply by 1,000. Conversely, multiply by 0.001.

You certainly can think of solids that are extremely dense, such as lead. Iron is quite dense too. Aluminum is not so dense. Rocks are less dense than most common metals. Glass is about the same density as silicate rock, from which it is made. Wood and most plastics are not very dense.

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