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Acids, Bases and Salts - Concept of pH scale

Grade 10CBSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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The pH scale is a scale for measuring hydrogen ion concentration, [H+][H^+], in a solution.

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The 'p' in pH stands for 'potenz' in German, which means power.

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The pH scale ranges from 00 (very acidic) to 1414 (very alkaline/basic).

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A neutral solution has a pH=7pH = 7. Examples include pure H2OH_2O and NaClNaCl solution.

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Acidic solutions have a pH<7pH < 7. The lower the pH value, the higher the concentration of H+H^+ (or H3O+H_3O^+) ions.

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Basic solutions have a pH>7pH > 7. The higher the pH value, the higher the concentration of OH−OH^- ions and lower the concentration of H+H^+ ions.

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The strength of acids and bases depends on the number of H+H^+ ions and OH−OH^- ions produced, respectively. Strong acids like HClHCl dissociate completely, while weak acids like CH3COOHCH_3COOH dissociate partially.

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Universal Indicator is used to determine pH by showing different colors at different concentrations of hydrogen ions in a solution.

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Importance of pH in daily life: Plants and animals are pH sensitive. Our body works within the pH range of 7.07.0 to 7.87.8. Acid rain has a pH less than 5.65.6.

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pH in the digestive system: The stomach produces HClHCl with a pH of about 1.21.2. Excess acid causes indigestion, which is treated with antacids like Mg(OH)2Mg(OH)_2 (Milk of Magnesia).

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Tooth decay starts when the pH of the mouth is lower than 5.55.5. Bacteria produce acids by degradation of sugar and food particles.

📐Formulae

pH=−log⁡10[H+]pH = -\log_{10}[H^+]

[H+][OH−]=10−14 at 25∘C[H^+][OH^-] = 10^{-14} \text{ at } 25^\circ C

If [H+]=10−x, then pH=x\text{If } [H^+] = 10^{-x}, \text{ then } pH = x

pH+pOH=14pH + pOH = 14

💡Examples

Problem 1:

You have two solutions, A and B. The pH of solution A is 66 and the pH of solution B is 88. Which solution has more hydrogen ion concentration? Which of this is acidic and which one is basic?

Solution:

Solution A (pH=6pH = 6) has a higher hydrogen ion concentration than solution B (pH=8pH = 8). Solution A is acidic and solution B is basic.

Explanation:

On the pH scale, values less than 77 represent acidic nature and values greater than 77 represent basic nature. Since pHpH is the negative logarithm of [H+][H^+], a lower pH value indicates a higher concentration of H+H^+ ions. Solution A has [H+]=10−6 mol/L[H^+] = 10^{-6} \text{ mol/L} while Solution B has [H+]=10−8 mol/L[H^+] = 10^{-8} \text{ mol/L}.

Problem 2:

Five solutions A, B, C, D and E when tested with universal indicator showed pH as 4,1,11,74, 1, 11, 7 and 99 respectively. Which solution is: (a) neutral (b) strongly alkaline (c) strongly acidic (d) weakly acidic (e) weakly alkaline?

Solution:

(a) D (pH=7pH = 7), (b) C (pH=11pH = 11), (c) B (pH=1pH = 1), (d) A (pH=4pH = 4), (e) E (pH=9pH = 9).

Explanation:

According to the pH scale: pH=7pH = 7 is neutral. pH=1pH = 1 is the lowest value here, making it strongly acidic. pH=11pH = 11 is the highest, making it strongly alkaline. pH=4pH = 4 is closer to neutral than 11, so it is weakly acidic. pH=9pH = 9 is closer to neutral than 1111, so it is weakly alkaline.

Problem 3:

A milkman adds a very small amount of baking soda (NaHCO3NaHCO_3) to fresh milk. Why does he shift the pH of the fresh milk from 66 to slightly alkaline?

Solution:

He shifts the pH to prevent the milk from setting as curd quickly.

Explanation:

Fresh milk has a pH of 66 and is slightly acidic. As it sits, bacteria produce lactic acid, which lowers the pH and causes the milk to curdle. By adding baking soda (NaHCO3NaHCO_3), which is basic, the milkman increases the pH (making it alkaline). This alkaline environment neutralizes the lactic acid produced, delaying the souring and curdling process.