Chemical substance

Steam and liquid water are two different forms of the same chemical substance, water.

In chemistry, a chemical substance is a form of matter that has constant chemical composition and characteristic properties. It cannot be separated into components by physical separation methods, i.e. without breaking chemical bonds. They can be solids, liquids or gases.

Chemical substances are often called 'pure' to set them apart from mixtures. A common example of a chemical substance is pure water; it has the same properties and the same ratio of hydrogen to oxygen whether it is isolated from a river or made in a laboratory. Other chemical substances commonly encountered in pure form are diamond (carbon), gold, salt (sodium chloride) and refined sugar (sucrose). However, simple or seemingly pure substances found in nature can in fact be mixtures of chemical substances. For example, tap water may contain small amounts of dissolved sodium chloride and compounds containing iron, calcium and many other chemical substances.

Chemical substances exist as solids, liquids, gases, or plasma and may change between these phases of matter with changes in temperature or pressure. Chemical reactions convert one chemical substance into another.

In geology, substances of uniform composition are called minerals, while physical mixtures (aggregates) of several minerals (different substances) are defined as rocks. Many minerals, however, mutually dissolve into solid solutions. An element is a chemical substance that is made up of a particular kind of atoms and hence cannot be broken down or transformed by a chemical reaction into a different element, though it can be transmutated into another element through a nuclear reaction. This is so, because 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. However, the number of chemical substances that are elements can be more than 120, because some elements can occur as more than a single chemical substance (allotropes). For instance, oxygen exists as both diatomic oxygen (O₂) and ozone (O₃). 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. Around a dozen elements, 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. Certain elements such as silicon sometimes resemble metals and sometimes resemble non-metals, and are known as metalloids.

Man’s Greatest Inventions

When people think of mankind’s history of inventions, many will mention things like the wheel, or fire. However, it can be argued that these were discoveries rather than inventions. For real inventions, you have to look to the later historical period. While there are a number of inventions that might be considered the " best, " there are six that particularly stand out.

Compass. One of the most important early inventions was the compass. A compass allows a sailor to determine the direction of magnetic north, which made navigating easier. It was the invention of the compass that allowed Chinese navigators to take their ships on voyages as far as the coast of Africa, and possibly even to the west coasts of North and South America.

Steam Engine. While the ancient Greeks toyed with getting power from steam, the most they were able to achieve were simple toys like spinning spheres. It wasn’t until James Watt of Scotland invented the real stream engine in 1775 that this power could finally be harnessed for practicable purposes, from steamships to trains. As the motive force for electrical generation, steam transformed society in the 19th and 20th centuries, and helped spur technological improvement at an ever-increasing rate.

Light Bulb. Edison’s invention of the light bulb had far-reaching implications beyond the simple fact of being able to brighten a dark room with electricity. It led to changes in many of humanity’s fundamental habits. The 24-hour daily cycle became optional, and many people now took employment in businesses and factories running night shifts. Night life on the street became a bit less menacing, since cities could now make them almost as bright as day, which was a vast improvement over the weak gas lamps of the past.

Computer. Although inventors like Englishman Charles Babbage and his associate Ada Lovelace had worked on the concepts that underlay modern-day computers back in the 19th century, the technology did not yet exist to build the devices they envisioned. It was not until the invention of vacuum tubes that the power of the computer was realized. Modern computers came into being over a period of decades, and were the result breakthroughs made by many nations and individuals, such as the groundbreaking work of Alan Turing of England and American Jack Kilby’s invention of the integrated circuit. Modern computers contain far more calculating power than the early machines, and have affected almost every aspect of our live. One of the most obvious and pervasive effects that the computer has had can be seen in the rise of the The Internet as a primary means of communication and information dissemination.