GED Science Practice Test: pH, Acids, and Bases

Substances can be described in terms of their acidity or basicity.  In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. The pH scale ranges from 0 to 14, with 0 being the most acidic and 14 being the most basic. The scale represents a measurement of the ratio of hydrogen ions (H+) to hydroxide ions (OH) (see the chemistry lesson on chemical bonding). Hydrogen ions are sometimes called hydronium ions. Solutions with a pH less than 7 are said to be acidic, and are called acids. Solutions with a pH greater than 7 are basic or alkaline, and are called bases. Solutions with a pH of 7 are said to be neutral. Pure water, which has a pH very close to 7, has equal amounts of H+ and OH- ions. It is important to understand that there are many substances that have a particular pH.  Thus, pH is an example of an extrinsic property.  The following diagram shows the pH scale and some common substances at each pH.


pH measurements are important in medicine, biology, chemistry, agriculture, forestry, and food science.  For example, a slight change in the pH of water in streams can drastically affect other conditions of the stream (such as dissolved oxygen) and the health of fish populations.  Additionally, increasing the acidity of food products helps to preserve the foods.  This is one of the ideas behind pickling, which involves putting vegetables in acidic vinegar mixtures.

The pH of a substance can be changed by mixing it with a second substance of a different pH.  For example.  If you had lemon juice at a pH of 2, you could raise its pH by mixing it with water at a pH of 7.  When you bring acids or bases closer to neutral by combining them with another substance, this is called neutralization.


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