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What is the pH scale?

Class 10 ScienceNCERT chapter: Acids, Bases and Saltsइसे हिंदी में पढ़ें

Exam-ready answer

The pH scale is a scale from 0 to 14 used to measure the acidity or basicity of a solution. pH less than 7 means acidic, pH 7 is neutral, and pH greater than 7 means basic. It is calculated as pH = –log[H⁺].

Explanation

The pH scale measures how acidic or basic a solution is. It ranges from 0 to 14. A pH below 7 indicates an acidic solution, pH exactly 7 is neutral, and pH above 7 indicates a basic solution. The scale is based on the concentration of hydrogen ions (H⁺) in the solution.

The pH of a solution is defined as the negative logarithm of hydrogen ion concentration: pH = –log[H⁺]. For example, pure water has a pH of 7, so it is neutral. Lemon juice is acidic with pH about 2, while soap solution is basic with pH about 10. The lower the pH, the more acidic the solution.

The pH scale is important in everyday life. Our stomach produces hydrochloric acid with pH around 1.5 to help digest food. Tooth decay starts when the pH of the mouth falls below 5.5. Plants grow best in a specific pH range of soil. Thus, pH helps us understand and control acidity and basicity.

Points to remember

  1. pH scale ranges from 0 to 14.
  2. pH < 7: acidic; pH = 7: neutral; pH > 7: basic.
  3. Formula: pH = –log[H⁺].
  4. Lower pH means more H⁺ ions and more acidic.
  5. Examples: lemon juice pH ≈ 2, pure water pH = 7, soap solution pH ≈ 10.

Exam tip

Write the pH formula with correct units (none) and remember the range 0–14. Use examples like lemon juice (acidic) and soap solution (basic) to score full marks.

Questions students also ask

What is the pH of a neutral solution?
A neutral solution has a pH of exactly 7. Pure water is neutral with pH 7. At this pH, the concentrations of H⁺ and OH⁻ ions are equal.
How is pH related to hydrogen ion concentration?
pH is the negative logarithm of hydrogen ion concentration: pH = –log[H⁺]. Higher H⁺ concentration gives lower pH, making the solution more acidic.
Why is pH important in everyday life?
pH is important in digestion, tooth decay, soil fertility, and in many industrial processes. For example, tooth decay occurs when mouth pH falls below 5.5.

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