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You probably know water’s chemical description is H2O. A water molecule consists of one atom of oxygen bound to two atoms of hydrogen. The hydrogen atoms are “attached” to one side of the oxygen atom, resulting in a water molecule having a positive charge on the side where the hydrogen atoms are and a negative charge on the other side, where the oxygen atom is. Since opposite electrical charges attract, water molecules tend to attract each other, making water kind of “sticky.” The side with the hydrogen atoms (positive charge) attracts the oxygen side (negative charge) of a different water molecule.
All these water molecules attracting each other mean they tend to clump together. This is why water drops are, in fact, drops! If is wasn’t for some of Earth’s forces, such as gravity, a drop of water would be ball shaped — a perfect sphere. Even if it doesn’t form a perfect sphere on Earth, we should be happy water is sticky.
Water is called the “universal solvent” because it dissolves more substances than any other liquid. This means that wherever water goes, either through the ground or through our bodies, it takes along valuable chemicals, minerals, and nutrients.
Pure water has a neutral pH. Pure water has a pH, of about 7, which is neither acidic nor basic.
Physical and Chemical Changes:
A physical change is one in which there is no change in the molecules which make up a given substance. Turning water into ice or vapor does not constitute a chemical change because the same molecules make up the liquid, solid and vapor states of water. The only difference between ice, steam and water is this: molecules in ice essentially have no freedom. The only vibrate within the crystal. The molecules in water are free to move within the limits of the container, as limited by gravity. The molecules in stream are completely free to move within the container, if any. They are essentially unaffected by gravity.
A chemical change occurs when new molecules are formed as a result of the change.
When water turns to steam at 212°F, a physical change occurs. On the other hand, when propane gas is ignited, it turns to carbon dioxide and water vapor in a chemical change. And change, of course, continues all the time. Let’s briefly consider the types of change by examine the compounds and mixtures.
Compounds and Mixtures:
How can one distinguish between compounds and mixtures? A compound has a definite and unvarying composition.
Water is a typical compound. It is composed of two elements hydrogen and oxygen in definite proportions. Regardless of where one ands water, it always consists of these two elements and always in the same proportion. Salt is another common compound. Whether it comes from a salt mine or is produced in a laboratory, salt is a compound of the two elements sodium and chlorine in an unvarying ratio.
Water, as a typical compound, also suggests another characteristic of the compound, namely a unique “personality” of its own. Although made up of hydrogen and oxygen, water is quite different from these two elements both physically and chemically. And so we should add to our definitions: a compound has well defined characteristics of its own, usually entirely different from those of its component elements.
Further, water freezes at 32°F and boils at 212°F. This indicates another characteristic of the compound: a pure compound has a definite freezing and a definite boiling point.
And finally, water, as a typical compound, is a uniform substance no matter whether one is considering a drop, a glassful or a lake of it. Thus, a compound is homogeneous.
In sharp contrast, a mixture will vary in the amount of the ingredients it contains. A mixture of sand and salt, for example, may have a bit of salt and a large amount of sand. Or it may be a blend of a large amount of salt and sand. No exact ratios of substances are necessary to constitute a mixture. At the same the ingredients in a mixture continue to maintain their essential properties. The salt still tastes salty; the sand continues to be gritty. The properties of the mixture are simply the total of the separate properties of the salt and sand. In this salt sand mixture the original ingredients could be recovered through some type of mechanical process. And finally, a mixture may have varying proportions of its ingredients in different parts of the sample. There may be more salt than sand at the bottom and less at the top of a mixture. In a word, mixtures are usually heterogeneous.
To a chemist, the term “pure” has meaning only in the context of a particular application or process. The distilled or de-ionized water we use in the laboratory contains dissolved atmospheric gases and occasionally some silica, but their small amounts and relative inertness make these impurities insignificant for most purposes. When water of the highest obtainable purity is required for certain types of exacting measurements, it is commonly filtered, de-ionized, and triple-vacuum distilled. But even this “chemically pure” water is a mixture of isotopic species: there are two stable isotopes of both hydrogen (H1 and H2, the latter often denoted by D) and oxygen (O16 and O18) which give rise to combinations such as H2O18, HDO16, etc., all of which are readily identifiable in the infrared spectra of water vapor. And to top this off, the two hydrogen atoms in water contain protons whose magnetic moments can be parallel or antiparallel, giving rise to ortho- and para-water, respectively. The two forms are normally present in a o/p ratio of 3:1.
The amount of the rare isotopes of oxygen and hydrogen in water varies enough from place to place that it is now possible to determine the age and source of a particular water sample with some precision. These differences are reflected in the H and O isotopic profiles of organisms. Thus the isotopic analysis of human hair can be a useful tool for crime investigations and anthropology research.
Current views of water structure
The present thinking, influenced greatly by molecular modeling simulations beginning in the 1980s, is that on a very short time scale (less than a picosecond), water is more like a “gel” consisting of a single, huge hydrogen-bonded cluster. On a 10-12-10-9 sec time scale, rotations and other thermal motions cause individual hydrogen bonds to break and re-form in new configurations, inducing ever-changing local discontinuities whose extent and influence depends on the temperature and pressure.
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