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Acids and Bases at a Glance
pH scale, indicators, and neutralization
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Acids and bases are two major classes of substances that explain many everyday reactions, from lemon juice and vinegar to soap and antacid tablets. The pH scale from 0 to 14 is a fast way to compare how acidic or basic a solution is. A pH of 7 is neutral, values below 7 are acidic, and values above 7 are basic. Understanding acids and bases helps students connect observable properties, particle behavior, and chemical reactions in the NGSS HS-PS1 framework.
At the particle level, acids increase the concentration of hydrogen ions, H+, or hydronium ions, H3O+, in water, while bases increase the concentration of hydroxide ions, OH-, or accept H+ from acids. The pH scale is logarithmic, so a change of 1 pH unit means a tenfold change in H3O+ concentration. Neutralization reactions occur when acids and bases react to form water and an ionic compound called a salt. Strong and weak acids or bases differ by how completely they ionize in water, not by how concentrated the solution is.
Key Facts
- pH = -log[H3O+]
- pOH = -log[OH-]
- At 25°C, pH + pOH = 14.00
- For water at 25°C, Kw = [H3O+][OH-] = 1.0 × 10^-14
- Acid plus base neutralization often follows H+ + OH- = H2O
- A 1-unit decrease in pH means 10 times greater [H3O+]
Vocabulary
- Acid
- An acid is a substance that donates H+ ions or increases H3O+ concentration in water.
- Base
- A base is a substance that accepts H+ ions or increases OH- concentration in water.
- pH
- pH is a logarithmic measure of hydronium ion concentration that indicates how acidic or basic a solution is.
- Indicator
- An indicator is a dye or substance that changes color over a certain pH range.
- Neutralization
- Neutralization is a reaction between an acid and a base that produces water and a salt.
Common Mistakes to Avoid
- Calling every low-pH solution a strong acid, which is wrong because strength means percent ionization, while pH also depends on concentration.
- Treating the pH scale as linear, which is wrong because each pH unit represents a tenfold change in [H3O+].
- Assuming neutralization always gives pH 7, which is wrong because weak acids, weak bases, and unequal amounts can produce acidic or basic final solutions.
- Forgetting units and significant figures in pH calculations, which is wrong because concentration is measured in mol/L and the decimal places in pH match the significant figures in concentration.
Practice Questions
- 1 A solution has [H3O+] = 1.0 × 10^-3 M. Calculate its pH and state whether it is acidic, neutral, or basic.
- 2 At 25°C, a solution has pH = 10.50. Calculate the pOH and [OH-].
- 3 A student says that 0.10 M acetic acid must be just as strong as 0.10 M hydrochloric acid because their concentrations are equal. Explain why this reasoning is incorrect.