Mohs Hardness scale is used to determine the hardness of an unknown substance by using materials of known hardness. The idea is to scratch the surface of the unknown substance by a material of known hardness. If a scratch can be made, then the sample has lesser hardness than the material of known hardness. The process can be repeated with a different material of known hardness to get an idea of the hardness of the unknown sample.
Here, the student has a plastic piece, copper coin, glass plate and steel file. The hardness of these substances on Mohs hardness scale are 2, 3, 5.5 and 6.5, respectively. To know the hardness of the unknown sample, the student can try to scratch the sample with the steel file. If a scratch is made, the sample has lesser hardness than steel, or 6.5. Try scratching it with glass plate next. If a scratch cannot be made, the sample has a hardness between those of steel and glass, between 5.5 and 6.5. If a scratch can be made by glass, use copper next and so on. This way, we can figure out a range of hardness values for the unknown mineral sample.
Hope this helps.
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