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Chemicke latky, produkty

A chemical substance is any material with a definite chemical composition.[1] For example, a sample of water has the same properties and the same ratio of hydrogen to oxygen whether the sample is isolated from a river or made in a laboratory. A pure substance cannot be separated into other substances by any mechanical process.[2] Typical chemical substances found in the home are water, salt (sodium chloride) and sugar (sucrose). Generally, substances exist as solid, liquid, or gas, and may change between these phases of matter with changes in temperature or pressure.
The concept of a chemical substance became firmly established in the late eighteenth century after work by the chemist Joseph Proust on the composition of some pure chemical compounds such as basic copper carbonate.[3] He deduced that: "All samples of a compound have the same composition; that is, all samples have the same proportions, by mass, of the elements present in the compound". This became known as the law of constant composition, and it is one of the foundations of modern chemistry.
A chemical substance that cannot be broken down or transformed by ordinary chemical processes into a different substance is called a chemical element (often referred to simply as an element). An element consists of particles called atoms, which consist of negatively charged electrons centered about a nucleus of positively charged protons and uncharged neutrons. All of the atoms in a sample of an element have the same number of protons, though they may be different isotopes, with differing numbers of neutrons.
There are about 120 known elements, about 80 of which are stable, that is, they do not change by radioactive decay into other elements. The majority of elements are classified as metals. These are elements with a characteristic lustre such as iron, copper, and gold. Metals typically conduct electricity and heat well, and they are malleable and ductile.[4] Around a dozen elements[5] such as carbon, nitrogen, and oxygen are classified as non-metals. Non-metals lack the metallic properties described above, they also have a high electronegativity and a tendency to form negative ions called anions. Certain elements such as silicon sometimes resemble metals and sometimes resemble non-metals, and are known as metalloids.

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