Fundamental Concepts of Acids and Bases
In aqueous chemistry, acids and bases are identified by their distinctive chemical properties and ionic dissociations:
- Acids: Chemical substances that produce hydronium ions ($H_3O^+$ or $H^+$) in aqueous solution. Sour taste, turn blue litmus red, $\text{pH} < 7$.
- Bases: Chemical substances that produce hydroxide ions ($OH^-$) in aqueous solution. Alkalis are water-soluble bases ($NaOH$, $KOH$). Bitter taste, soapy feel, turn red litmus blue, $\text{pH} > 7$.
Universal & Natural Acid-Base Indicators:
| Indicator Type | Name | Original Color | In Acidic Medium | In Basic Medium |
|---|---|---|---|---|
| Natural | Litmus Paper/Solution | Purple | Red | Blue |
| Natural | Turmeric Paste | Yellow | Yellow (No change) | Reddish-brown |
| Natural | Red Cabbage Extract | Purple | Red | Greenish-yellow |
| Synthetic | Phenolphthalein | Colorless | Colorless | Vivid Pink |
| Synthetic | Methyl Orange | Orange | Red | Yellow |
| Olfactory | Vanilla Extract | Sweet Odor | Retains Sweet Odor | Odor Vanishes |
| Olfactory | Clove Oil / Onion | Pungent Odor | Retains Smell | Smell Destroyed |
Chemical Properties of Acids and Bases
1. Reaction with Metals:
$$\text{Acid} + \text{Metal} \rightarrow \text{Salt} + H_2(g) \uparrow$$
$$Zn(s) + 2HCl(aq) \rightarrow ZnCl_2(aq) + H_2(g) \uparrow$$
$$Zn(s) + 2NaOH(aq) \xrightarrow{\Delta} Na_2ZnO_2(aq)\text{ [Sodium Zincate]} + H_2(g) \uparrow$$
2. Reaction with Carbonates & Bicarbonates:
$$Na_2CO_3(s) + 2HCl(aq) \rightarrow 2NaCl(aq) + H_2O(l) + CO_2(g) \uparrow$$
$$Ca(OH)_2(aq) + CO_2(g) \rightarrow CaCO_3(s)\downarrow\text{ [Milky]} + H_2O(l)$$
With excess $CO_2$, milkiness disappears:
$$CaCO_3(s) + H_2O(l) + CO_2(g) \rightarrow Ca(HCO_3)_2(aq)\text{ [Soluble Clear]}$$
3. Neutralisation Reaction:
$$HCl(aq) + NaOH(aq) \rightarrow NaCl(aq) + H_2O(l) + \text{Heat}$$
The pH Scale & Everyday Significance
$$\text{pH} = -\log_{10}[H^+]$$
- $\text{pH} < 7$: Acidic
- $\text{pH} = 7$: Neutral (Pure water at $25^\circ\text{C}$)
- $\text{pH} > 7$: Basic / Alkaline
Real-Life Applications of pH:
- Human Digestive System: Stomach secretes $HCl$ ($\text{pH} \approx 1.2 - 1.4$). In indigestion, antacids like Milk of Magnesia ($Mg(OH)_2$) neutralize excess acidity.
- Tooth Decay: Tooth enamel is Calcium hydroxyapatite. When mouth $\text{pH} < 5.5$, bacterial acid corrodes enamel. Toothpastes are basic to neutralize it.
- Soil pH: Most crops require neutral or slightly basic soil ($\text{pH} \approx 6.5 - 7.5$). Acidic soils are treated with quicklime ($CaO$) or slaked lime ($Ca(OH)_2$).
- Self Defence by Insects: Bee stings inject methanoic acid (formic acid), treated with mild base like baking soda. Wasp stings are alkaline, neutralized with vinegar.
Important Industrial Salts and Compounds
1. Bleaching Powder ($CaOCl_2$):
Produced by action of dry chlorine gas on dry slaked lime:
$$Ca(OH)_2 + Cl_2 \rightarrow CaOCl_2 + H_2O$$
Uses: Bleaching cotton/linen in textile industries, disinfectant for drinking water.
2. Baking Soda ($NaHCO_3$):
Solvay process:
$$NaCl + H_2O + CO_2 + NH_3 \rightarrow NH_4Cl + NaHCO_3 \downarrow$$
Heating Baking Soda:
$$2NaHCO_3 \xrightarrow{\Delta} Na_2CO_3 + H_2O + CO_2 \uparrow$$
Baking Powder: Mixture of baking soda and mild edible acid like tartaric acid. On heating, $CO_2$ gas released makes cakes fluffy and soft.
3. Washing Soda ($Na_2CO_3 \cdot 10H_2O$):
Recrystallisation of sodium carbonate:
$$Na_2CO_3 + 10H_2O \rightarrow Na_2CO_3 \cdot 10H_2O$$
Uses: Glass, soap, paper industries; permanent hardness removal of water.
4. Plaster of Paris ($CaSO_4 \cdot \frac{1}{2}H_2O$):
Heating Gypsum at $373\text{ K}$ ($100^\circ\text{C}$):
$$CaSO_4 \cdot 2H_2O \xrightarrow{373\text{ K}} CaSO_4 \cdot \frac{1}{2}H_2O + 1\frac{1}{2}H_2O$$
Setting of POP: On mixing with water, it sets into a hard solid mass of gypsum within 15 minutes.
