Question 1

What is pH?

Accepted Answer

pH is a logarithmic scale used to specify the acidity or basicity of an aqueous solution. It is defined as the negative base-10 logarithm of the hydrogen ion activity: pH = -log[H⁺]. Because the scale is logarithmic, each whole number change represents a tenfold change in hydrogen ion concentration. A solution with pH 3 is ten times more acidic than one with pH 4.

Question 2

What does the pH scale measure?

Accepted Answer

The pH scale typically ranges from 0 to 14 at standard conditions (25°C). A pH of 7 is considered neutral, representing pure water where the concentrations of hydrogen ions [H⁺] and hydroxide ions [OH⁻] are equal at 1×10⁻⁷ mol/L. Values below 7 indicate acidity; values above 7 indicate alkalinity (basicity). The scale can extend beyond 0–14 for extremely concentrated strong acids or bases.

Question 3

What is the difference between pH and pOH?

Accepted Answer

pH measures hydrogen ion concentration, while pOH measures hydroxide ion concentration. They are related by the ion product of water: at 25°C, pH + pOH = 14. In neutral water, both pH and pOH equal 7. As a solution becomes more acidic, pH decreases and pOH increases; as it becomes more basic, pH increases and pOH decreases.

Question 4

Why is the pH of blood strictly regulated?

Accepted Answer

Human blood is normally maintained within a narrow pH range of 7.35 to 7.45. Deviations below 7.35 cause acidosis, while deviations above 7.45 cause alkalosis. Both conditions can be life-threatening because enzymes and proteins are highly sensitive to pH changes. The body maintains blood pH through buffer systems (bicarbonate, phosphate, proteins), respiratory regulation of CO₂, and renal excretion of acids and bases.

Question 5

What are buffer solutions?

Accepted Answer

A buffer solution resists changes in pH when small amounts of acid or base are added. Buffers typically consist of a weak acid and its conjugate base (or a weak base and its conjugate acid). The bicarbonate buffer system (H₂CO₃/HCO₃⁻) is critically important in human blood, helping to maintain pH within the narrow range required for survival.

Question 6

What is the pH of common substances?

Accepted Answer

Stomach acid (hydrochloric acid) has a pH of approximately 1–2. Lemon juice is about pH 2–3. Black coffee is roughly pH 5. Pure water is pH 7. Human blood is about pH 7.4. Baking soda solution is around pH 9. Household bleach is approximately pH 12–13. Drain cleaners can reach pH 14. These values illustrate the enormous range of chemical environments in everyday life.

Question 7

How does temperature affect pH?

Accepted Answer

The pH of pure water changes with temperature because the ion product of water (Kw) is temperature-dependent. At 25°C, Kw = 1.0×10⁻¹⁴ and neutral pH is 7.0. At 100°C, Kw increases to approximately 5.1×10⁻¹³, making neutral pH about 6.14. Therefore, pH measurements should ideally specify the temperature, and pH meters often include automatic temperature compensation.

Question 8

What is acidosis and alkalosis?

Accepted Answer

Acidosis and alkalosis are clinical terms describing abnormal blood pH. Metabolic acidosis results from excessive acid production or bicarbonate loss (e.g., diabetic ketoacidosis). Respiratory acidosis occurs when CO₂ accumulates due to inadequate ventilation. Metabolic alkalosis involves excess bicarbonate, while respiratory alkalosis results from hyperventilation and excessive CO₂ loss. Prompt medical treatment is essential.

Question 9

How is pH measured in a laboratory?

Accepted Answer

Laboratory pH measurement typically uses a glass electrode pH meter, which measures the electrical potential difference between a reference electrode and a hydrogen-ion-sensitive glass electrode. Indicator papers (litmus, universal indicator) provide approximate pH values through color change. For high-precision work, pH meters are calibrated using standard buffer solutions of known pH (commonly pH 4.01, 7.00, and 10.01).

Question 10

What is the relationship between pH and hydrogen ion concentration?

Accepted Answer

The relationship is logarithmic and inverse: pH = -log₁₀[H⁺], where [H⁺] is the molar concentration of hydrogen ions in moles per liter. To find [H⁺] from pH, use the inverse formula: [H⁺] = 10^(-pH). This logarithmic relationship means that small pH changes correspond to large changes in ion concentration, which is why biological systems invest significant energy in pH homeostasis.

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The pH Scale and Acid-Base Chemistry

pH of Common Substances

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