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The pH of a solution is given as the negative log of the hydronium ion concentration in a solution. The hydronium ion is formed when H+ combines with H2O to form H3O+
When an acid is added to water the molecule constituting the acid HA is split into H+ and A- ions. The H+ ions produced this way combine with the H2O to form H3O+ or hydronium ions.
As pH is given as the negative log of the H3O+ ions it will decrease when the acid is added to water.
Therefore the acid reduces the pH of the water and increases the H+ or H3O+ concentration.
pH is a measure of acidity or basisity (alkalinity) of a solution. It is approximately equivalent to the negative log of the concentration of hydronium (H3O+) ions. Because of the negative log relationship, increased hydronium ions translates to decreased pH.
When added to water, an acid releases hydrogen ions. Hydrogen ions in water combine with the water molecules to form hydronium ions, thereby decreasing the pH and increasing the acidity of water.
Water exposed to air is mildly acidic because it absorbs small quantities of CO2 from the atmosphere. The CO2 breaks down to carbonic acid that then dissociates to release hydrogen ions into the water, decreasing the pH.
To answer your question, acid added to water increases the hydrogen ion concentration (H+), decreases the pH, and increases the acidity.
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