How Does Water Behave

Let us first explore the basic nature of water. Take a look at Fig. 1.1 below. This represents the chemistry of water in a nutshell (multilayered, though). 

How are these three atoms combined together This is one of the fundamental issues in chemistry structure of molecule. We omit the issue of how we determine the structures of molecules. The result of many, many experimental works is summarized in the figure. That is, two separate small hydrogen atoms attach to a 

As you see, the proton H+ that splits from a water molecule is actually bound with another water molecule, its oxygen atom to be exact, and exists as a hydronium ion (H3OQ in water. This (chemical) reaction is called autodissodation equilibrium. 

As indicated above, OH is much stronger base than water itself, and hence, there are a lot more H O than H3O and Off- in pure water, as these two, i.e., H+ (of H3O ) and OH , tend to bind to form H O back. The extent of autodissociation is governed by the so-called mass action law [H30][OH ]=K, and is called an eqnihbrium constant and known to be a very small number, 10 at room temperature. As you recall, [H3O ] represents the molar concentration (mol/L) of the chemical species H3O. More rigorously speaking, [A] in an equilibrium constant expression represents activity which is a number and its magnitude related to the molar concentration. This relationship says that the product of the concentrations of hydronium ion and hydroxide ion in water is constant. So, if you add an acid (which gives off H ) to water, you have increased the hydronium ion concentration, and accordingly the hydroxide ion in the solution will be reduced. 

Now we have listed all the important basic chemical properties of water molecules. Water as we know is a snbstance and is liquid at ambient temperatures. It turns into solid (ice) at lower temperatures below 0°C, or it boils at 100°C and changes into gas (vapor, steam in this case) under the standard condition. The standard condition here means that the atmospheric pressure is 1.0 atm (or 1.013 hPa or bar). Actually, the freezing point (temperature) and the boiling point of pure water under this condition are defined to be 0°C and 100°C, respectively. [Today, scientists try to redefine the standard condition to be 1.000 hPa or 1.000 bar (1,000 mbar) instead of 1 atm. For nonrigorons treatments, the old standard condition is still valid and is what is employed in this book]. 

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