TEAS Ready⚗️ Science · Chemistry · Lesson 6 of 6 · 6 min
Acids, bases, pH and buffers
🧪 Blood pH has to stay between about 7.35 and 7.45. A shift of a few tenths is a medical emergency, so the pH scale is worth knowing cold.
💡 Key idea
Below 7 is acid, above 7 is base, each pH unit is ten times, and buffers hold the line.
🧠 The idea
An ACID releases hydrogen ions (H⁺) in water, tastes sour and turns blue litmus red; examples are hydrochloric acid in the stomach and citric acid in lemons. A BASE accepts H⁺ or releases hydroxide ions (OH⁻), feels slippery and turns red litmus blue; examples are sodium hydroxide and ammonia. The PH SCALE runs from 0 to 14: below 7 is acidic, 7 is neutral (pure water), above 7 is basic. Each whole step is a tenfold change in H⁺, so pH 3 is 10 times more acidic than pH 4 and 100 times more acidic than pH 5. Lower pH means more H⁺. In NEUTRALIZATION, an acid and a base react to form water and a salt, for example HCl + NaOH → NaCl + H₂O. A BUFFER is a weak acid paired with its conjugate base that resists pH change when small amounts of acid or base are added; the bicarbonate buffer keeps blood near pH 7.4.
✏️ Worked example
Gastric juice is pH 2 and a sample of urine is pH 6. How many times more H⁺ does gastric juice have? Step 1: difference = 6 − 2 = 4 pH units. Step 2: each unit is a factor of 10, so 10 × 10 × 10 × 10 = 10,000. Step 3: gastric juice has 10,000 times the hydrogen ion concentration of the urine sample.
📌 Takeaways
- pH < 7 acidic, 7 neutral, > 7 basic; lower pH = more H⁺.
- One pH unit = a tenfold change in H⁺.
- Acid + base → salt + water; buffers resist pH change.
📖 Terms in this lesson
pH: A 0 to 14 scale of acidity: below 7 is acidic, 7 is neutral, above 7 is basic; for example, normal blood pH is about 7.4, slightly basic.
✅ Check yourself
Question 1 of 3
A solution has a pH of 9. It is
- strongly acidic
- neutral
- weakly acidic
- basic
Show the answer and rationale
Answer: D
Any pH above 7 is basic (alkaline), so pH 9 is a base. Acidic solutions are below 7, and neutral is exactly 7, the pH of pure water at room temperature.
Question 2 of 3
How many times more hydrogen ions does a solution at pH 3 have than one at pH 5?
- 1,000
- 20
- 2
- 100
Show the answer and rationale
Answer: D
The difference is 2 pH units, and each unit is a factor of 10: 10 × 10 = 100. Two treats the scale as linear, and 1,000 would require a difference of 3 units.
Question 3 of 3
What does a buffer do in a solution?
- Resists changes in pH
- Turns acids into bases
- Raises the pH to exactly 7
- Speeds up acid-base reactions
Show the answer and rationale
Answer: A
A buffer (a weak acid and its conjugate base) absorbs small additions of acid or base so the pH changes very little. It does not force pH to 7; the bicarbonate buffer, for example, holds blood near 7.4. Speeding reactions is what a catalyst does.
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