Chemistry · Class 9-10 · CBSE
Acids, Bases, Salts and the Everyday pH Scale
Master Arrhenius definitions of acids and bases, indicator colour shifts, universal indicator & pH scale, importance of pH in living organisms, and preparation & chemistry of Bleaching Powder, Baking Soda, Washing Soda and Plaster of Paris.
1Properties of Acids and Bases
According to Arrhenius theory,Acidsproduce hydrogen ions H⁺ (or hydronium ions H₃O⁺) in aqueous solution and taste sour.Basesproduce hydroxide ions OH⁻ in aqueous solution, taste bitter, and feel soapy to touch. Water-soluble bases are calledAlkalis.
Chemical Indicators & Colour Response
| Indicator | Acidic Medium | Neutral Medium | Basic Medium |
|---|---|---|---|
| Blue Litmus | Turns Red | Remains Blue | Remains Blue |
| Red Litmus | Remains Red | Remains Red | Turns Blue |
| Phenolphthalein | Colourless | Colourless | Vibrant Pink |
| Methyl Orange | Reddish Pink | Orange | Yellow |
| Turmeric (Natural) | Yellow | Yellow | Reddish Brown (e.g. soap on curry stain) |
| Olfactory (Vanilla/Onion) | Characteristic smell retained | Smell unchanged | Smell completely destroyed |
Blue Litmus
- Acidic Medium
- Turns Red
- Neutral Medium
- Remains Blue
- Basic Medium
- Remains Blue
Red Litmus
- Acidic Medium
- Remains Red
- Neutral Medium
- Remains Red
- Basic Medium
- Turns Blue
Phenolphthalein
- Acidic Medium
- Colourless
- Neutral Medium
- Colourless
- Basic Medium
- Vibrant Pink
Methyl Orange
- Acidic Medium
- Reddish Pink
- Neutral Medium
- Orange
- Basic Medium
- Yellow
Turmeric (Natural)
- Acidic Medium
- Yellow
- Neutral Medium
- Yellow
- Basic Medium
- Reddish Brown (e.g. soap on curry stain)
Olfactory (Vanilla/Onion)
- Acidic Medium
- Characteristic smell retained
- Neutral Medium
- Smell unchanged
- Basic Medium
- Smell completely destroyed
2The pH Scale and Importance in Daily Life
ThepH Scale(potenz = power of hydrogen) quantifies hydronium ion concentration from 0 to
14. Neutral pure water has pH =
7. Acidic solutions have pH < 7, and basic solutions have pH > 7.
pH in Everyday Living Systems
| System / Organism | Ideal pH Range | Consequences of Imbalance & Remedy |
|---|---|---|
| Human Blood / Body | 7.35 – 7.45 (slightly alkaline) | Enzymes denature if blood pH deviates outside this narrow band |
| Human Stomach | 1.2 – 3.0 (Dilute HCl) | Excess acid causes acidity; treated with mild bases (Antacids: Mg(OH)₂ / Milk of Magnesia) |
| Tooth Decay | Enamel erodes when pH < 5.5 | Bacteria ferment sugar acids eroding calcium hydroxyapatite; basic toothpastes neutralize acid |
| Soil for Crops | 6.0 – 7.5 | Too acidic soil treated with quicklime (CaO) or slaked lime Ca(OH)₂; too alkaline treated with gypsum |
| Acid Rain | Rainwater pH < 5.6 | Dissolved atmospheric SO₂ & NO₂ acidify river waters, killing aquatic fauna |
Human Blood / Body
- Ideal pH Range
- 7.35 – 7.45 (slightly alkaline)
- Consequences of Imbalance & Remedy
- Enzymes denature if blood pH deviates outside this narrow band
Human Stomach
- Ideal pH Range
- 1.2 – 3.0 (Dilute HCl)
- Consequences of Imbalance & Remedy
- Excess acid causes acidity; treated with mild bases (Antacids: Mg(OH)₂ / Milk of Magnesia)
Tooth Decay
- Ideal pH Range
- Enamel erodes when pH < 5.5
- Consequences of Imbalance & Remedy
- Bacteria ferment sugar acids eroding calcium hydroxyapatite; basic toothpastes neutralize acid
Soil for Crops
- Ideal pH Range
- 6.0 – 7.5
- Consequences of Imbalance & Remedy
- Too acidic soil treated with quicklime (CaO) or slaked lime Ca(OH)₂; too alkaline treated with gypsum
Acid Rain
- Ideal pH Range
- Rainwater pH < 5.6
- Consequences of Imbalance & Remedy
- Dissolved atmospheric SO₂ & NO₂ acidify river waters, killing aquatic fauna
3Chemistry of Common Industrial Salts
Important Salts from Common Salt (NaCl)
| Commercial Name | Chemical Formula | Preparation Reaction | Key Uses |
|---|---|---|---|
| Sodium Hydroxide (Caustic Soda) | NaOH | Chlor-Alkali: 2NaCl + 2H₂O → 2NaOH + Cl₂↑ + H₂↑ | Soaps, paper manufacturing, degreasing metals |
| Bleaching Powder | CaOCl₂ | Dry slaked lime + chlorine: Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O | Disinfecting drinking water, textile bleaching agent |
| Baking Soda | NaHCO₃ | NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃ (Solvay process) | Cooking, baking powder (with tartaric acid), fire extinguisher |
| Washing Soda | Na₂CO₃·10H₂O | Recrystallization of sodium carbonate: Na₂CO₃ + 10H₂O | Glass, soap, paper, removing permanent hardness of water |
| Plaster of Paris (POP) | CaSO₄·½H₂O | Heating gypsum at 373 K: CaSO₄·2H₂O → CaSO₄·½H₂O + 1½H₂O | Bone fracture casts, making toys, false ceilings, statues |
Sodium Hydroxide (Caustic Soda)
- Chemical Formula
- NaOH
- Preparation Reaction
- Chlor-Alkali: 2NaCl + 2H₂O → 2NaOH + Cl₂↑ + H₂↑
- Key Uses
- Soaps, paper manufacturing, degreasing metals
Bleaching Powder
- Chemical Formula
- CaOCl₂
- Preparation Reaction
- Dry slaked lime + chlorine: Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O
- Key Uses
- Disinfecting drinking water, textile bleaching agent
Baking Soda
- Chemical Formula
- NaHCO₃
- Preparation Reaction
- NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃ (Solvay process)
- Key Uses
- Cooking, baking powder (with tartaric acid), fire extinguisher
Washing Soda
- Chemical Formula
- Na₂CO₃·10H₂O
- Preparation Reaction
- Recrystallization of sodium carbonate: Na₂CO₃ + 10H₂O
- Key Uses
- Glass, soap, paper, removing permanent hardness of water
Plaster of Paris (POP)
- Chemical Formula
- CaSO₄·½H₂O
- Preparation Reaction
- Heating gypsum at 373 K: CaSO₄·2H₂O → CaSO₄·½H₂O + 1½H₂O
- Key Uses
- Bone fracture casts, making toys, false ceilings, statues
4Exam Corner: Tips, Mistakes & Board Questions
Common Board Traps & Explanations
- Never add water to concentrated acid! Always add acid slowly to water with constant stirring, because hydration is violently exothermic and can cause acid splashing.
- Baking powder is a mixture of baking soda (NaHCO₃) and a mild edible acid like tartaric acid. Tartaric acid neutralizes the bitter-tasting sodium carbonate produced during heating.
- Dry HCl gas does not change the colour of dry litmus paper because H⁺ ions only dissociate in the presence of water to form H₃O⁺.
Exam question · CBSE Class 10 Board
What is the chemical formula of Plaster of Paris and why should it be stored in moisture-proof containers?
- A.CaSO₄·2H₂O; because it evaporates
- B.CaSO₄·½H₂O; because it absorbs moisture and sets into hard solid Gypsum
- C.CaCO₃; because it turns into lime
- D.CaOCl₂; because it releases chlorine gas
- Formula of Plaster of Paris is calcium sulphate hemihydrate: CaSO₄·½H₂O.
- When exposed to atmospheric moisture, it reacts: CaSO₄·½H₂O + 1½ H₂O → CaSO₄·2H₂O (Gypsum).
- Gypsum is a hard rock-like solid that destroys the moulding utility of POP.
- Correct answer is option B.
Answer: B. CaSO₄·½H₂O; because it absorbs moisture and sets into hard solid Gypsum
Remember
- Acids releaseH⁺ / H₃O⁺ions; Bases releaseOH⁻ions in aqueous medium.
- pH scale:0 (strong acid) to 14 (strong base); 7 is neutral.
- Antacids likeMg(OH)₂ (Milk of Magnesia)treat stomach hyperacidity.
- Plaster of Paris isCaSO₄·½H₂O; hydrates into hardGypsum (CaSO₄·2H₂O)
Check yourself
Question 1
Tooth enamel is the hardest substance in the human body made of calcium hydroxyapatite. It starts corroding when mouth pH falls below: