Acids, Bases, and pH

Objective: Distinguish common definitions and strengths.

In the Brønsted–Lowry model an acid donates a proton and a base accepts a proton. In familiar aqueous systems, acidic solutions have higher hydronium concentration than neutral water at the same temperature, while basic solutions have higher hydroxide concentration. The Arrhenius model describes acids as increasing H₃O⁺ and bases as increasing OH⁻ in water, with a narrower scope. Acid strength describes extent of ionization under defined conditions, whereas concentration describes amount per volume; a dilute strong acid and a concentrated weak acid are possible. Household labels do not by themselves establish pH or safety. Introductory examples include HCl as a strong acid in water and acetic acid as a weak acid in water.

Objective: Interpret the logarithmic pH scale.

For dilute aqueous solutions under an introductory approximation, pH = −log₁₀[H₃O⁺] when concentration is expressed in mol/L relative to standard state; rigorous definitions use activity. At 25 °C, neutral water has pH about 7; below 7 is acidic and above 7 basic in this conventional context. A one-unit pH decrease corresponds to roughly ten times greater hydronium activity, not one more hydrogen atom per molecule. The pH range can extend below 0 or above 14 in some systems. Indicators change color over limited pH ranges and require a reference chart. Report temperature and conditions for careful comparison.

Objective: Infer properties from pH and controls.

When comparing aqueous samples at the same temperature, a lower pH indicates greater hydronium activity. The pH difference is logarithmic: pH 2 versus pH 5 corresponds to approximately 10³ or one thousand times the hydronium activity in the basic classroom model. Do not infer total acid amount from pH alone, because volume and acid dissociation matter. pH meters need calibration and indicators have finite resolution. Experimental comparisons should use labeled samples, identical temperature, appropriate measuring tools, and controls. Neither a pH number nor an acid/base label alone determines the hazards of a substance.