What Is the pH Scale?
Measurement of Acidity & Alkalinity 

CLASS NOTES from Science Prof Online

Hydrochloric acid (in beaker) reacting with ammonia fumes to produce ammonium chloride (white smoke).

What Is an Acid?
An acid is any ionic compound that releases hydrogen ions (H+) in solution. Ionic compounds are molecules held together by ionic chemical bonds that result from the attraction of oppositely charged atoms, called ions.

 Weak acids release fewer hydrogen ions and have a sour taste. Strong acids release more hydrogen ions and are highly corrosive. The more hydrogen ions in the solution, the more acidic it is.

What Is a Base? 
Bases, also called alkaline substances, are compounds that release hydroxyl ions (OH-) in solution. Weak bases have a bitter taste (opposed to sour taste of acids and sweetness of aldehydes and ketones) and are slimy to the touch. The more hydroxyl ions in a solution, the more alkaline it is.

Measurements of pH
Acidity or alkalinity of a solution is measured by concentration of hydrogen ions (H+) versus hydroxyl ions (OH-) and is expressed as pH level, an exponential scale that ranges from 0 (very acidic) to 14 (very basic). 

Article Summary: The pH scale is a measurement of the acidity or alkalinity of a solution. To understand the scale, it is important to first understand what acids and bases are.

Acids, Bases, Buffers, and the pH Scale
pH Scale Graphic by Edward Stevens
Because the scale is logarithmic, a change of just one unit represents a tenfold change in H+ concentration. When the concentration of hydrogen ions (H+) equals the concentration of hydroxyl ions (OH-) a solution is neutral.

​Salts and pH Buffering
Salts are inorganic molecules can act as buffers to reduce fluctuations in pH by releasing cations and anions other than H+ and OH- in solution. An example of a salt buffer is a product like Tums the stomach antacid, which is made of the ionic compound calcium carbonate (CaCO3). Carbonate reacts with a free hydrogen ion (H+), forming bicarbonate. Then bicarbonate reacts with a free hydrogen ions to create carbonic acid (H2CO3), which then can dissociate (break down further) into water (H2O) and carbon dioxide (CO2). During this process, free hydrogen ions are locked up, preventing the pH from lowering. 

​Additional Resources On pH Scale
​The pH Scale animated lesson from John Kyrk
Acids Bases and Metals super fun interactive BBC tutorial with a British accent :) + quiz
pH Scale interactive lesson and quiz from scool.co.uk
Think of pH as Water Balance animated tutoral, Springfield Technical Community College

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Portions of this article originally appeared on Suite101 online magazine.​​

Page last updated 5/2013
Interactive pH Scale Simulation
FREE from
Test the pH of things like coffee, spit, and soap to determine whether each is acidic, basic, or neutral. Visualize the relative number of hydroxide ions and hydronium ions in solution. Switch between logarithmic and linear scales. Investigate whether changing the volume or diluting with water affects the pH. Or you can design your own liquid!
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