Acids, Bases and Salts – Comprehensive Study Notes

Everyday Substances & Their Tastes

• Daily-use items such as lemon, tamarind, common salt, sugar, cucumber, grapes, unripe mango, etc. have distinctly different tastes.
  • Sour: lemon juice, orange juice, vinegar, curd, amla, tamarind.
  • Bitter: baking soda solution, coffee.
  • Salty: common salt solution.
  • Sweet: sugar solution.
• Cautionary points stressed in the text:
  • Do not taste or touch laboratory/unknown substances unless specifically instructed.
  • Acids and bases can be corrosive or irritating to skin.

Acids – Nature, Definition & Examples

• Latin root "acere" = sour.
• Characteristic properties:
  • Sour in taste.
  • Turn blue litmus red.
  • React with bases to give salts + water with evolution of heat.
• Natural acids encountered in food (all occur naturally):
  • Acetic acid → vinegar.
  • Formic acid → ant sting nettle.
  • Citric acid → citrus fruits (lemon, orange).
  • Lactic acid → curd/milk products.
  • Oxalic acid → spinach.
  • Ascorbic acid (Vitamin C) → amla & citrus fruits.
  • Tartaric acid → tamarind, grapes, unripe mango.
• Laboratory / mineral acids mentioned in activities:
  • Hydrochloric acid $(\text{HCl})$.
  • Sulphuric acid $(\text{H}<em>2\text{SO}</em>4)$.
  • Nitric acid $(\text{HNO}_3)$.

Bases – Nature, Definition & Examples

• Substances bitter in taste, slippery/soapy to touch.
• Turn red litmus blue.
• Neutralise acids.
• Common bases and their household/lab sources:
  • Calcium hydroxide $(\text{Ca(OH)}_2)$ → lime water, white-wash.
  • Ammonium hydroxide $(\text{NH}_4\text{OH})$ → window cleaners.
  • Sodium hydroxide $(\text{NaOH})$ & Potassium hydroxide $(\text{KOH})$ → soaps.
  • Magnesium hydroxide $(\text{Mg(OH)}_2)$ → milk of magnesia (antacid).
  • Sodium hydrogencarbonate $(\text{NaHCO}_3)$ → baking soda (mild base, used in cooking & ant-bite remedy).

Neutral Substances

• Solutions that do not affect either red or blue litmus.
• Examples from activities: distilled water, most sugar solutions, common-salt solution (if pure), etc.

Indicators – Detecting Acidic or Basic Nature

• Definition: substances that show different colours in acidic, basic and neutral media.

Litmus (Natural Dye)

• Extracted from lichens; mauve/purple in distilled water.
• Colour changes:
  • Acidic → red.
  • Basic → blue.
• Available as solution or red/blue litmus paper strips.

Turmeric

• Component curcumin works as indicator.
• Procedure (Activity 4.2): prepare paste, coat filter paper, dry → turmeric paper.
• Colour change: yellow → red-brown in basic solution; no change in acidic or neutral solutions.
• Practical example: turmeric stains on white clothes turn reddish when washed with soapy (basic) water.

China Rose (Gudhal) Petal Extract

• Preparation (Activity 4.3): soak petals in warm water to obtain coloured indicator.
• Behaviour:
  • Acidic solution → dark pink/magenta.
  • Basic solution → green.
  • Neutral → retains original faint colour.

Phenolphthalein (Synthetic)

• Colourless in acidic/neutral medium.
• Bright pink in basic medium.
• Key reagent for demonstrating neutralisation (Activity 4.5).

Additional Natural Indicators Mentioned

• Red-cabbage juice, beet-root juice (used in projects/extended learning).

Core Experiments & Observations

• Activity 4.1: Test variety of household liquids with red & blue litmus; classify as acid/base/neutral.
• Activity 4.2: Turmeric indicator paper turns reddish with soap (base).
• Activity 4.3: China-rose indicator differentiates acids and bases by magenta/green colour.
• Activity 4.4: Comparative test of laboratory acids & bases with three indicators—litmus, turmeric & China-rose—tabulated for reference.
• Activity 4.5 (Teacher demo):
  1. Take dilute $\text{HCl}$; add phenolphthalein → remains colourless (acidic).
  2. Add $\text{NaOH}$ dropwise while stirring → solution turns pink (basic dominance).
  3. Add one extra drop of $\text{HCl}$ → back to colourless (acidic again).
  4. Conclusion: phenolphthalein signals crossing of neutral point; reaction is exothermic.

Neutralisation Reaction – Concept & Chemistry

• General word equation:
  $\text{Acid} + \text{Base} \;\rightarrow\; \text{Salt} + \text{Water} + \text{Heat}$
• Symbolic example used in text:
  $\text{HCl} + \text{NaOH} \;\rightarrow\; \text{NaCl} + \text{H}_2\text{O}$
• Observations:
  • Acidic and basic properties are cancelled (solution becomes neutral).
  • Temperature rises; reaction is exothermic.
  • Salt produced may itself be acidic, basic, or neutral depending on parent acid/base.

Everyday Applications of Neutralisation

• Indigestion:
  • Stomach contains $\text{HCl}$. Excess secretion → acidity.
  • Antacid (e.g.
    $\text{Mg(OH)}_2$ in milk of magnesia) neutralises excess acid.
• Ant-bite or nettle sting:
  • Injects formic acid.
  • Rubbing moist baking soda $(\text{NaHCO}_3)$ or calamine (zinc carbonate) provides relief by neutralising the acid.
• Soil Treatment in Agriculture:
  • Overuse of fertilisers → soil becomes acidic.
  • Treated with quick lime $(\text{CaO})$ or slaked lime $(\text{Ca(OH)}_2)$.
  • Conversely, basic soils are amended with organic matter (compost) that releases acids.
• Factory Waste Management:
  • Industrial effluents often acidic; must be neutralised with bases before discharge to prevent aquatic life damage.

Acid Rain – Environmental Relevance

• Pollutant gases $\text{CO}<em>2$, $\text{SO}</em>2$, $\text{NO}<em>2$ dissolve in rain water → $\text{H}</em>2\text{CO}<em>3$, $\text{H}</em>2\text{SO}<em>4$, $\text{HNO}</em>3$ respectively.
• Leads to corrosion of monuments, soil imbalance, harm to flora/fauna.

Safety & Ethical Considerations

• Laboratory acids/bases are corrosive; wear gloves, goggles, handle with care.
• Awareness of environmental responsibility: neutralise factory waste, curb pollutants causing acid rain.
• Ethical science practice: follow cautionary notes; no unnecessary tasting/skin contact.

Consolidated Comparisons (Acids vs Bases)

• Taste: sour vs bitter & soapy.
• Litmus effect: blue→red vs red→blue.
• Presence in daily life: fruits/curd/vinegar vs soap, milk of magnesia, lime water.
• pH perspective (link to previous lessons if covered): $\text{pH} < 7$ acidic; $\text{pH} > 7$ basic; $\text{pH} = 7$ neutral.

Practice / Reflection Questions Highlighted in Text

• Identify nature of ammonia (turns red litmus blue ⇒ basic).
• Distilled water’s neutrality verification.
• Reasoning tasks: Why antacid, calamine, soil treatment, etc.
• Logical Multiple-choice: Dependence of colour change on indicator type.

Extended Learning & Projects

• Secret message with baking-soda ink, revealed by beet-root juice (acid-base colour play).
• Prepare red-cabbage indicator; tabulate observations for various solutions.
• Test local soil pH; interact with farmers about amendments.
• Medical inquiry: consult doctor about anti-acidity medicines & preventive measures.

Did You Know? (Biological Link)

• DNA (deoxyribonucleic acid) is an acid present in every cell; governs genetic traits.
• Proteins are composed of amino acids; fats include fatty acids.

Keywords Recap

• Acid, Base, Acidic, Basic, Indicator, Neutral, Neutralisation, Salt, Antacid, Acid Rain, Exothermic.