Science · Solutions & reactions
Acids, Bases, and pH
Estimated time includes reading and one quiz. Take the time you need.
Start here: key ideas
- Acids increase hydrogen ion concentration in water.
- Bases decrease hydrogen ion concentration or increase hydroxide ions.
- A lower pH is more acidic; a higher pH is more basic.
- Each pH unit represents a tenfold change in hydrogen ion concentration.
- Neutralization combines acid and base effects.
- Buffers resist sudden pH change.
What you’ll be able to do
- Recognize acids and bases.
- Interpret the direction and tenfold steps of pH.
- Explain neutralization.
- Explain how a buffer limits pH change.
Compare acids and bases, read the pH scale, and explain neutralization and buffers.
Optional review
If you’d like to review the basics, visit Solutions, Concentration, and Solubility. You can start this lesson without completing those first.
Recognizing acids and bases
An acid increases hydrogen ion, H+, concentration in water. A base accepts H+ or increases hydroxide ion, OH−, concentration.
In water, H+ is carried as hydronium, H3O+. An acid can donate H+ and a base can accept H+. H+ is also called a proton; use the transfer of H+, rather than the name of a definition, to identify the roles.
Indicators change color over certain pH ranges. Strong and weak describe how fully an acid or base reacts in water. Concentrated and dilute describe amount, so these word pairs are not the same.
Acids often taste sour and react with some metals, while bases often feel slippery, but never identify lab chemicals by touching or tasting them. Use indicators or measurements. A strong acid may be dilute, and a weak acid may be concentrated.
Direction on a logarithmic scale
pH describes H+ concentration. At room temperature, pH 7 is neutral, values below 7 are acidic, and values above 7 are basic.
The scale is logarithmic. A solution at pH 3 has ten times more H+ than one at pH 4 and one hundred times more than one at pH 5. Lower pH means more acidic.
Worked example. Compare pH 2 and pH 5. The difference is three pH units. Each step is tenfold, so pH 2 has 10 × 10 × 10 = 1,000 times more H+ than pH 5.
Supporting calculation with a supplied relationship
For concentrations written as [H+] = 1 × 10−n mol/L, pH = n. For example, [H+] = 1 × 10−4 mol/L gives pH 4. Use the stated relationship for these simple powers of ten; general logarithm calculations are not a required skill here.
The pH scale
At about 25°C, below 7 is acidic, 7 is neutral, and above 7 is basic. One step down in pH means about ten times as many hydrogen ions per volume in the model used here. Learn the pattern, not the pictured products’ exact values. Their pH depends on what they contain and how concentrated they are.
The pH scale — OpenStax College, Chemistry (CC BY archive). CC BY 3.0. Original source image reproduced unchanged; explanatory caption added.
Neutralization and resistance to change
Neutralization is an acid-base reaction. H+ and OH− can form water, while the remaining ions form a salt. The final pH is not always exactly 7 because amounts and strengths matter.
A buffer can take up added H+ or release H+ when needed. This limits sudden pH changes, but a buffer has a limited capacity.
When equal effective amounts of strong HCl and strong NaOH react, H+ and OH− form water and the other ions form salt. If one reactant is left over, the result may remain acidic or basic.
A buffer can handle a limited addition of acid or base. Once its components are used up, pH can change sharply.
Optional supporting detail
A conjugate acid–base pair differs by one H+. For a weak acid HA and its conjugate base A−, added H+ is taken up by A− to make HA. Added OH− is consumed when HA donates H+, producing water and more A−. In the carbonic acid/bicarbonate pair, added base consumes carbonic acid and produces bicarbonate and water. Do not reverse this response: the acid component counters added base.
Worked buffer reasoning: if a small base addition makes an unbuffered sample jump from pH 6 to 9 but a buffered sample only rises to 6.2, the buffer resisted—not prevented—all change. At larger additions it can run out of its acid component. No logarithm formula or buffer-equation memorization is needed for this qualitative explanation.
Terms to remember
- acid
- A substance that increases hydrogen ion concentration in water.
- base
- A substance that accepts hydrogen ions or increases hydroxide ions in water.
- pH
- A scale that describes hydrogen ion concentration.
- neutralization
- A reaction between an acid and a base.
- buffer
- A system that resists sudden pH changes.
- indicator
- A substance that changes color over a pH range.
Your quick summary
- Acids raise H+ concentration.
- Bases accept H+ or raise OH− concentration.
- pH below 7 is acidic and above 7 is basic at room temperature.
- Each pH unit represents a tenfold change in hydrogen-ion concentration.
- Buffers resist pH change.
Lesson quiz
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