According to the UK Chemguide, Kw in chemistry represents the ionic product for water, and it helps chemists predict how the acidity of water changes as it warms or colds. The abbreviation includes an upper-case K and a lower-case w that drops below the capitol K line-position. Chemguide explains that Kw only holds true for pure water, and it does not apply to solutions which include any other substance.
Chemguide states that all water molecules act as either a very strong acid or an extremely strong base. The water molecules ionize into these forms at any time. Acting as a base, one water molecule accepts a hydrogen atom from another molecule to form the positively charged hydronium ion, known as H+, and the negatively charged hydroxide ion, known as OH-. Chemguide notes that this process happens in extremely small quantities and in all types of water.
According to Chemguide, the Kw of a sample is simply the concentration of hydronium ions multiplied by the concentration of hydroxide ions. In most samples tested at room temperature, the resulting value for Kw is 1.00 x 10-14 mol^2 dm^-6. This means that the pH of even pure water changes with its temperature. Chemguide states that water only retains a neutral pH of 7 at a certain temperature.