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I love it when teachers ask questions! I alway enjoy a good science question because they make me really think about the subject to be sure I give the right answer, but when a teacher asks a question, I feel obligated to give an answer (if I can) that will let them explain the subject to their students. Recently, I have been talking with a teacher friend, about chemical and physical changes, and the more we talked, the more I thought the rest of you might find this useful.
The word rock means different things to different people. As with many words, its scientific meaning is different from many of the meanings used in everyday language, so lets start with a short video on the scientific definition of a rock.
To perform the streak test, you rub a specimen on a piece of unglazed porcelain tile. Go to your local hardware store, and look at the backs of their floor tiles. You want a tile that is made of porcelain, and the unglazed back should have a white color.
These tiles are very hard (hardness 6.5), and they have a rough surface. As you rub the mineral across that surface, it acts like a file, powdering up some of the mineral. The color of that powdered mineral is its streak color.
With nearly 3000 different known minerals, you might think that identifying mineral specimens would be difficult. Actually, it is usually fairly easy. Many of those 3000 minerals are very rare, only found in very limited areas. If you can learn to identify twenty or thirty of the most common minerals, you will be able to identify most of the specimens that you are likely to find.
