pH Scale (Concept) MCQs Quiz | Class 10
This Class X Science quiz covers Unit I: Chemical Substances – Nature and Behaviour, specifically focusing on the pH Scale concept. Questions test your understanding of pH meaning, the acidic and basic ranges, neutral substances, and the relationship between hydrogen ion concentration and pH values. Submit your answers to check your score and download the PDF solution.
Understanding the pH Scale
The pH scale is a measure of how acidic or basic a water-based solution is. It generally ranges from 0 to 14. The term pH stands for “power of hydrogen” or “potential of hydrogen” (from the German word ‘Potenz’). It measures the concentration of hydrogen ions (H+) in a solution.
Key Zones on the pH Scale
- Neutral (pH = 7): Pure water has a pH of 7. It is neither acidic nor basic. Green is the standard color for neutral on a universal indicator.
- Acidic (pH < 7): Solutions with a pH less than 7 are acidic. The lower the number, the stronger the acid. For example, gastric juice (pH about 1.2) is a strong acid, while lemon juice (pH about 2.2) is moderately acidic.
- Basic / Alkaline (pH > 7): Solutions with a pH greater than 7 are basic. The higher the number, the stronger the base. Sodium hydroxide (pH 14) is a very strong base.
H+ Ion Concentration vs. pH Value
There is an inverse relationship between the concentration of hydrogen ions and the pH value:
- High H+ concentration results in a low pH (Acidic).
- Low H+ concentration results in a high pH (Basic).
Universal Indicator Colors
A universal indicator is a mixture of dyes that changes color across the entire pH range:
| pH Range | Nature | Approximate Color |
|---|---|---|
| 0 – 3 | Strong Acid | Red |
| 3 – 6 | Weak Acid | Orange / Yellow |
| 7 | Neutral | Green |
| 8 – 11 | Weak Base | Blue |
| 12 – 14 | Strong Base | Violet / Purple |
Quick Revision Points
- Acids turn blue litmus red. Bases turn red litmus blue.
- Mixing an acid and a base results in neutralization, moving the pH towards 7.
- Our body works within the pH range of 7.0 to 7.8.
- Rainwater with a pH less than 5.6 is called acid rain.
- Tooth decay starts when the pH of the mouth is lower than 5.5.
Practice Questions
- What is the pH of pure water at 25 degrees Celsius? (Ans: 7)
- Which ion is responsible for acidic nature? (Ans: H+)
- Which ion is responsible for basic nature? (Ans: OH-)
- Does a solution with pH 13 have H+ ions? (Ans: Yes, but concentration is very low compared to OH- ions)
- What happens to pH when an acid is diluted? (Ans: pH increases towards 7)
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