Chapter 2 · Question 7
What is the pH scale? How does it help classify solutions? Distinguish between strong acids and weak acids, giving one example of each. Does a basic solution contain ions?
Q7
What is the pH scale? How does it help classify solutions? Distinguish between strong acids and weak acids, giving one example of each. Does a basic solution contain ions?
Answer Revealed
Direct Answer:
The pH scale is a scale from 0 to 14 that measures the hydrogen ion concentration in a solution, where 'p' stands for 'potenz' (German for power). A pH of 7 is neutral; values below 7 are acidic; values above 7 are basic. A strong acid produces a higher concentration of ions (e.g., HCl) while a weak acid produces a lower concentration (e.g., ) — even at the same molar concentration. Yes, basic solutions do contain ions (from the self-ionisation of water), but the ion concentration far exceeds the ion concentration, making the solution basic. pH measures the balance: higher means lower pH.
Simple Explanation
The pH scale runs from 0 (very acidic) to 14 (very basic), with 7 in the middle being neutral — like pure water. Think of it as a number that tells you how crowded the solution is with ions: the more , the lower the pH. Strong acids like hydrochloric acid release almost all their ions; weak acids like vinegar (acetic acid) release only a few. And yes — even basic solutions have some ions floating around, just very few compared to the ones. A universal indicator paper changes through a rainbow of colours across the scale.
Exam-Ready Structure
The pH scale is the quantitative tool introduced in NCERT Class 10 to move beyond the qualitative 'acid or base' distinction and classify solutions by their hydrogen ion concentration. 1. Definition: pH is a scale used to measure the hydrogen ion concentration in a solution. The 'p' stands for 'potenz,' a German word meaning power. The scale ranges from 0 to 14. 2. Classification: (a) pH = 7: neutral solution (for example, pure water). (b) pH < 7: acidic solution — the lower the pH, the higher the concentration and the stronger the acidic nature. (c) pH > 7: basic/alkaline solution — the higher the pH, the stronger the basic nature. 3. As shown in Figure 2.6, there is an inverse relationship between pH and concentration: as hydrogen ion concentration increases, pH decreases. 4. Measuring pH: Commonly done using pH paper impregnated with a universal indicator — a mixture of several indicators that shows a range of colours across the pH scale. 5. Strong and weak acids: If hydrochloric acid and acetic acid of the same concentration are compared, they produce different amounts of hydrogen ions. Acids that give rise to more ions are called strong acids; acids that give fewer ions are called weak acids. HCl is a strong acid example, while is a weak acid example. 6. ions in basic solutions: Basic solutions also contain ions, but their basic nature is due to the greater concentration of ions compared with ions.
Key Points
- pH scale: 0-14; pH=7 neutral, pH<7 acidic, pH>7 basic/alkaline
- Lower pH higher concentration (inverse relationship)
- Strong acid (e.g., HCl) — high concentration; weak acid (e.g., ) — low concentration
- Strength (degree of dissociation) ≠ concentration (amount per unit volume)
- Basic solutions contain some ions but concentration exceeds concentration
- Universal indicator is a mixture of several indicators giving different colours across the pH range
Common Mistakes
- Confusing strength and concentration — a concentrated weak acid may not produce many ions; a dilute strong acid may still produce more
- Saying basic solutions have no ions — every aqueous solution has both and ions