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Substances to refer to any particular variety of matter that always has the same properties an composition, regardless of how and where a specimen is obtained.
For example, water is a substance.
Under given conditions, any sample of water has the same properties and composition.
The properties and composition can be used to identify water.
On the other, hand wood is not a substance, it properties and composition can vary widely.
Wood is actually a mixture of many substances.
A sample of any substance is homogenous; that is the properties and composition are the same through out the sample.
There are two major kinds of substances:
Mixtures - Like compounds, the components of a mixture are always composed of two or more elements, but mixtures differ from compounds in that:
Elements - A substance that cannot be decomposed into two or more other substances by means of a chemical change is called an element.
An element consists entirely of atoms with the same atomic number.
Over one hundred and tentative different elements are known to exist.
Most are metals such as mercury, iron, lead, copper, silver and gold.
Others are nonmetals such as oxygen, sulfur, iodine and neon.
Still others are semi-metals (metalloids) such as silicon beryllium and boron.
Compounds - A substance that can be decomposed into other substances by a chemical reaction is a compound.
Every compound consists of two or more elements chemically combined in definite atomic proportions.
For example, carbon dioxide is always two parts oxygen and one part carbon -- a ratio of 1:2 of carbon to oxygen.
Please note that the properties of compounds are usually VERY different then those of the elements they come from.
For example when hydrogen and oxygen, two gaseous elements, are combined in a 2:1 ratio, they form water.
- The components of a mixture can either be elements or compounds.
- The atomic ratio of compounds in a mixture is not fixed.
- The properties of a mixture are always intermediate between those of its components.
- Some mixtures, such as solutions of salt water or mixtures of gases are homogeneous, but others such as concrete are heterogeneous
- Substances in mixtures are still independent and can be removed from each other without chemical reaction.