How acids, bases, pH and buffers work
TEAS 7 outline: Acids and bases
Also covers HESI A2 Chemistry (Acids and bases)
- Acids raise H+ and have a pH below 7; bases are above 7.
- Each pH unit is a tenfold change in H+.
- Strong acids and bases ionize completely; weak ones ionize only partly.
- Acid plus base gives salt plus water (neutralization).
- Buffers resist pH change; normal blood pH is 7.35 to 7.45.
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Normal arterial blood pH stays in a narrow range, 7.35 to 7.45. A key blood buffer is carbonic acid, a weak acid, paired with bicarbonate, its base partner.
Key facts
- The pH scale runs from 0 to 14: below 7 is acidic, 7 is neutral (equal H+ and OH- at 25 °C), and above 7 is basic (alkaline).
- pH is a logarithmic scale, so a drop of one pH unit means ten times more hydrogen ions.
- An acid is a proton (H+) donor and a base is a proton acceptor; a base can lower H+ by supplying OH- or by binding H+.
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- Strong acids such as hydrochloric acid and strong bases such as sodium hydroxide ionize completely in water; weak acids, such as the acid in vinegar, ionize only partly.
- Neutralization: acid + base produces a salt + water, as when an antacid such as magnesium hydroxide reacts with stomach acid.
- A buffer is a weak acid and its conjugate base (or a weak base and its conjugate acid) that resists pH change when small amounts of acid or base are added.
- Normal arterial blood pH is 7.35 to 7.45; the carbonic acid-bicarbonate buffer, CO2 exhaled by the lungs, and bicarbonate control by the kidneys keep it there.
Quick check1 question · 2 more at the end
Which of these is a weak acid that ionizes only partly when dissolved in water?
Correct answer: D.
Acid strength describes how completely an acid ionizes in water. Strong acids such as hydrochloric, nitric and sulfuric acid ionize completely, while weak acids such as acetic acid ionize only partly, so most of their molecules stay intact. Strength is about the degree of ionization, not about how concentrated the solution is.
A Hydrochloric acid is a strong acid that ionizes completely into hydrogen and chloride ions.
B Nitric acid is on the standard list of strong acids, which ionize completely in water.
C Sulfuric acid is also a strong acid; students sometimes assume an acid with two hydrogens must be weaker.
D Correct: acetic acid, the acid in vinegar, is a weak acid that ionizes only partly.
Full lesson
Each part starts with its main idea. Open a part when you want the details.
The short answer: Acids raise the hydrogen ion (H+) concentration and have a pH below 7; bases lower it, often by adding hydroxide (OH-), and have a pH above 7. Each pH unit is a tenfold change in H+. Strong acids and bases ionize completely, weak ones partly. Acid plus base gives salt plus water; buffers resist pH change.
1 What makes something an acid or a base?An acid is a substance that increases the concentration of hydrogen ions (H+) in a solution. 1 min
An acid is a substance that increases the concentration of hydrogen ions (H+) in a solution. In water, that hydrogen ion actually rides on a water molecule as a hydronium ion (H3O+). A base lowers the hydrogen ion concentration, either by releasing hydroxide ions (OH-) or by grabbing hydrogen ions directly.
A second, very useful way to say this: an acid is a proton donor and a base is a proton acceptor (a hydrogen ion is just a proton). Water can do both, which is why even pure water contains a tiny amount of H3O+ and OH-.
2 How does the pH scale work?pH measures how acidic or basic a solution is. 1 min
pH measures how acidic or basic a solution is. The scale runs from 0 to 14:
- Below 7: acidic (more H+ than OH-)
- Exactly 7: neutral (equal H+ and OH-, as in pure water at 25 °C)
- Above 7: basic, also called alkaline (more OH- than H+)
The scale is logarithmic and runs backward from what you might expect. A lower pH means more hydrogen ions, and each one-unit step is a tenfold change. A solution at pH 3 has ten times the H+ of one at pH 4, and a hundred times the H+ of one at pH 5.
| Substance | Approximate pH |
|---|---|
| Gastric juice | 1.5 |
| Lime juice | 2 |
| Black coffee | 5 |
| Pure water | 7 |
| Blood | 7.4 |
| Milk of magnesia | 10.5 |
| Household ammonia | 12 |
You can measure pH precisely with a pH meter, or estimate it with an indicator: a dye that changes color with pH. Litmus is a familiar indicator, and pH paper is a strip of paper with indicator dyes built in.
3 What is the difference between strong and weak?Strength describes how completely an acid or base breaks apart (ionizes) in water. 1 min
Strength describes how completely an acid or base breaks apart (ionizes) in water. A strong acid such as hydrochloric acid (HCl) ionizes completely into H+ and Cl-. A strong base such as sodium hydroxide (NaOH) releases its hydroxide completely.
A weak acid ionizes only partly, so most of its molecules stay intact. The acids in vinegar and tomato juice are weak acids. Strength is not the same as concentration: "strong" is about how much of the acid ionizes, not how much acid is in the bottle.
4 What happens when an acid meets a base?The "salt" is the ionic compound made from the base's positive ion and the acid's negative ion. 1 min
They neutralize each other. The general pattern is:
acid + base → salt + water
The "salt" is the ionic compound made from the base's positive ion and the acid's negative ion. Antacids are a real example. Milk of magnesia (magnesium hydroxide) reacts with stomach acid (hydrochloric acid) to form magnesium chloride and water. Another common antacid, calcium carbonate, also neutralizes stomach acid and releases carbon dioxide gas.
5 How do buffers keep pH steady?A buffer is a pair made of a weak acid and its conjugate base (or a weak base and its conjugate acid). 1 min
A buffer is a pair made of a weak acid and its conjugate base (or a weak base and its conjugate acid). It has a partner ready for each job, so it can soak up extra H+ or extra OH-. As a result, the pH barely changes when small amounts of acid or base are added.
Your blood is the classic example. Normal arterial blood pH stays in a narrow range, 7.35 to 7.45. Below 7.35 is called acidosis; above 7.45 is alkalosis. A key buffer is the carbonic acid-bicarbonate pair:
- If extra acid enters the blood, bicarbonate (HCO3-) picks up the extra H+ and becomes carbonic acid (H2CO3), a weak acid.
- If extra base enters, carbonic acid gives up H+ to neutralize it, forming bicarbonate and water.
- Carbonic acid can break down into carbon dioxide and water, and the lungs breathe the CO2 out.
That third step links breathing to pH. Breathing out more CO2 removes acid and raises blood pH within minutes. The kidneys adjust bicarbonate levels too, but that takes hours to days. Normally there is about 20 times more bicarbonate than carbonic acid, so this buffer is best at stopping the blood from becoming too acidic.
Common mistakes
The traps students fall into most. Read these before you practice.
Watch out: A lower pH means more acidic and more H+, not less.
Watch out: One pH unit is a tenfold change in H+, not a change of one unit or double.
Watch out: "Strong" means the acid ionizes completely, not that the solution is concentrated.
Watch out: Breathing out more CO2 makes blood less acidic (pH rises), not more acidic, because CO2 forms carbonic acid in blood.
Watch out: Bicarbonate is the buffer partner that picks up extra H+; carbonic acid is the partner that neutralizes extra base.
How to remember it
Memory hook: "Low pH, lots of H." The lower the number, the more hydrogen ions. And "BAse Binds H+": bicarbonate is the base half of the blood buffer, so it binds the extra acid.
Self-check:
- A solution goes from pH 6 to pH 4. Did it become more or less acidic, and by how many times?
- Write the word equation for neutralization and name the salt formed when hydrochloric acid reacts with sodium hydroxide.
- Explain what happens to blood pH if a person breathes out less CO2 than usual.