Comprehensive Notes on Acids, Bases, and Salts

Introduction to Acids, Bases, and Salts

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- Bases: These substances possess a bitter taste and feel soapy to the touch. They change the colour of red litmus paper to blue.
- Acidity Remedy: If someone suffers from acidity after overeating, a solution of baking soda is suggested as a remedy because of the ability of bases to nullify the effect of acids.
Chemical Indicators:
- Natural Indicators:
  - Litmus: A natural indicator which is a purple dye extracted from lichen, a plant belonging to the division Thallophyta. In a neutral medium (neither acidic nor basic), its colour is purple.
  - Turmeric: Another natural indicator. A yellow curry stain (containing turmeric) on a white cloth turns reddish-brown when scrubbed with soap (which is basic). It turns yellow again when washed with plenty of water.
  - Other Plant Materials: Red cabbage leaves and the coloured petals of certain flowers such as Hydrangea, Petunia, and Geranium serve as acid-base indicators.
- Synthetic Indicators: Examples include Methyl orange and Phenolphthalein.
- Olfactory Indicators: Substances whose odour changes in acidic or basic media. Examples include onion, vanilla essence, and clove oil.

Chemical Properties of Acids and Bases

Reaction of Acids with Metals:
- General Reaction: $\text{Acid} + \text{Metal} \rightarrow \text{Salt} + \text{Hydrogen gas}$
- Activity 2.3: When zinc granules are added to dilute sulphuric acid ( $H_2SO_4$ ), hydrogen gas bubbles are formed on the surface of the zinc.
- Chemical Equation: $Zn(s) + H_2SO_4(aq) \rightarrow ZnSO_4(aq) + H_2(g)$
- Testing Hydrogen Gas: When a burning candle is brought near a bubble filled with the evolved gas, the gas burns with a "pop" sound.
Reaction of Bases with Metals:
- Activity 2.4: Strong bases like sodium hydroxide react with certain metals like zinc to produce hydrogen gas and a salt.
- Chemical Equation: $2NaOH(aq) + Zn(s) \rightarrow Na_2ZnO_2(s) + H_2(g)$
- Product Name: The salt formed is called Sodium zincate.
- Limitation: These reactions are not possible with all metals.
Reaction of Metal Carbonates and Hydrogencarbonates with Acids:
- General Reaction: $\text{Metal carbonate/Metal hydrogencarbonate} + \text{Acid} \rightarrow \text{Salt} + \text{Carbon dioxide} + \text{Water}$
- Activity 2.5 (Sodium Carbonate): $Na_2CO_3(s) + 2HCl(aq) \rightarrow 2NaCl(aq) + H_2O(l) + CO_2(g)$
- Activity 2.5 (Sodium Hydrogencarbonate): $NaHCO_3(s) + HCl(aq) \rightarrow NaCl(aq) + H_2O(l) + CO_2(g)$
- Testing for Carbon Dioxide: On passing the evolved gas through lime water (calcium hydroxide solution), the lime water turns milky due to the formation of a white precipitate of calcium carbonate ( $CaCO_3$ ).
  - $Ca(OH)_2(aq) + CO_2(g) \rightarrow CaCO_3(s) + H_2O(l)$
- Excess Carbon Dioxide Reaction: If excess $CO_2$ is passed, the milkiness disappears because soluble calcium hydrogencarbonate is formed.
  - $CaCO_3(s) + H_2O(l) + CO_2(g) \rightarrow Ca(HCO_3)_2(aq)$
- Natural Forms of Calcium Carbonate: Limestone, chalk, and marble.
Reaction Between Acids and Bases (Neutralisation):
- Definition: The reaction between an acid and a base to give a salt and water is known as a neutralisation reaction.
- General Formula: $\text{Base} + \text{Acid} \rightarrow \text{Salt} + \text{Water}$
- Example Activity 2.6: $NaOH(aq) + HCl(aq) \rightarrow NaCl(aq) + H_2O(l)$
- Indicator Behavior: Phenolphthalein turns pink in NaOH. Adding HCl dropwise eventually turns the solution colourless once the base is neutralised. Adding more NaOH restores the pink colour.
Reaction of Metallic Oxides with Acids:
- General Reaction: $\text{Metal oxide} + \text{Acid} \rightarrow \text{Salt} + \text{Water}$
- Activity 2.7: Adding dilute hydrochloric acid to copper oxide ( $CuO$ ) results in a blue-green solution.
- Specific Reaction: $CuO(s) + 2HCl(aq) \rightarrow CuCl_2(aq) + H_2O(l)$
- Observation: The blue-green colour is due to the formation of copper(II) chloride.
- Nature of Oxides: Metallic oxides are considered Basic Oxides because they react with acids to form salt and water.
Reaction of Non-metallic Oxides with Bases:
- Observation: Non-metallic oxides (like $CO_2$ ) react with bases (like $Ca(OH)_2$ ) to produce salt and water.
- Conclusion: Non-metallic oxides are acidic in nature.

Common Characteristics of Acids and Bases

Conduction of Electricity:
- Activity 2.8: Dilute solutions of hydrochloric acid and sulphuric acid conduct electricity, causing a bulb in a circuit to glow. Conversely, solutions of glucose and alcohol do not conduct electricity.
- Reasoning: Conduction is due to the flow of ions.
- Ions in Acids: Acids contain $H^+$ as a cation and anions like $Cl^-$ in HCl, $NO_3^-$ in $HNO_3$ , $SO_4^{2-}$ in $H_2SO_4$ , and $CH_3COO^-$ in $CH_3COOH$ .
- Acidic Properties: The presence of hydrogen ions, $H^+(aq)$ , in solution is responsible for the acidic properties of a substance.
Acids and Bases in Aqueous Solutions:
- Activity 2.9: Dry HCl gas does not change the colour of dry litmus paper. It only changes the colour of wet litmus paper.
- Hydronium Ion Formation: Hydrogen ions cannot exist alone; they exist after combining with water molecules.
  - $HCl + H_2O \rightarrow H_3O^+ + Cl^-$
  - $H^+ + H_2O \rightarrow H_3O^+$
- Bases in Water: Bases generate hydroxide ( $OH^-$ ) ions in water.
  - $NaOH(s) \xrightarrow{H_2O} Na^+(aq) + OH^-(aq)$
  - $KOH(s) \xrightarrow{H_2O} K^+(aq) + OH^-(aq)$
  - $Mg(OH)_2(s) \xrightarrow{H_2O} Mg^{2+}(aq) + 2OH^-(aq)$
- Definition of Alkali: A base that dissolves in water is called an alkali. Alkalis are soapy to touch, bitter, and corrosive.
Dilution Process:
- Definition: Mixing an acid or base with water results in a decrease in the concentration of ions ( $H_3O^+$ or $OH^-$ ) per unit volume. This is called dilution.
- Exothermic Nature: Dissolving a concentrated acid (especially nitric or sulphuric acid) or base in water is highly exothermic.
- Safety Warning: Always add acid slowly to water with constant stirring. Never add water to concentrated acid, as the heat generated may cause the mixture to splash out (causing burns) or the glass container to break.

The pH Scale and Strength of Solutions

Definition of pH Scale: A scale developed for measuring hydrogen ion concentration in a solution. The "p" in pH stands for potenz, German for "power".
- Range: 0 (very acidic) to 14 (very alkaline/basic).
- Neutral Point: pH 7 represents a neutral solution.
- Acidic Range: pH values less than 7.
- Basic Range: pH values from 7 to 14.
- Inversely Proportional: Higher hydronium ion concentration results in a lower pH value.
Strong vs. Weak Acids/Bases:
- Strong Acids: Acids that give rise to more $H^+$ ions (e.g., Hydrochloric acid).
- Weak Acids: Acids that give rise to fewer $H^+$ ions (e.g., Acetic acid).
- Strong/Weak Bases: Depends on the number of $OH^-$ ions produced.
Universal Indicator: A mixture of several indicators that shows different colours at different concentrations of hydrogen ions.

Importance of pH in Everyday Life

Biological Systems:
- Human Body: Works within a pH range of 7.0 to 7.8.
- Acid Rain: When the pH of rain water is less than 5.6. It lowers the pH of river water, threatening aquatic life.
- Venus Atmosphere: Composed of thick white and yellowish clouds of sulphuric acid, making life unlikely.
Agriculture:
- Plants require a specific pH range for healthy growth. Soil can be tested using universal indicator paper.
Digestive System:
- The stomach produces hydrochloric acid ( $HCl$ ) to aid digestion. Indigestion causes excess acid production leading to pain. Antacids (bases) like Magnesium hydroxide (Milk of Magnesia) are used to neutralise the acid.
Tooth Decay:
- Begins when the pH in the mouth is lower than 5.5.
- Tooth enamel is made of calcium hydroxyapatite (crystalline calcium phosphate), the hardest substance in the body.
- Bacteria degrade sugar to produce acid. Prevention involves using basic toothpaste to neutralise acid.
Self-Defence in Nature:
- Honey-bee sting: Injects acid causing pain; treated with a mild base like baking soda.
- Nettle leaves: Stinging hairs inject methanoic acid. A traditional remedy is rubbing the area with the leaf of a Dock plant (which is basic).
- Natural Acids Table:
  - Vinegar: Acetic acid
  - Orange/Lemon: Citric acid
  - Tamarind: Tartaric acid
  - Tomato: Oxalic acid
  - Sour milk (Curd): Lactic acid
  - Ant sting/Nettle sting: Methanoic acid

Information on Salts

Family of Salts: Salts having the same positive or negative radicals belong to the same family (e.g., NaCl and $Na_2SO_4$ are sodium salts; NaCl and KCl are chloride salts).
pH of Salts:
- Strong Acid + Strong Base: Neutral salt (pH 7).
- Strong Acid + Weak Base: Acidic salt (pH < 7).
- Weak Acid + Strong Base: Basic salt (pH > 7).
Common Salt (Sodium Chloride - $NaCl$ ):
- Formed by $HCl + NaOH$ .
- Found in seawater and as deposits of solid rock salt (often brown due to impurities).
- Was an important symbol in India's struggle for freedom (Dandi March).
Chemicals from Common Salt:
1. Sodium Hydroxide ( $NaOH$ ): Produced via the Chlor-alkali process where electricity is passed through brine (aqueous $NaCl$ ).
  - $2NaCl(aq) + 2H_2O(l) \rightarrow 2NaOH(aq) + Cl_2(g) + H_2(g)$
  - $Cl_2$ is produced at the anode; $H_2$ at the cathode; $NaOH$ near the cathode.
2. Bleaching Powder ( $Ca(ClO)_2$ ): Produced by the action of chlorine gas on dry slaked lime.
  - $Ca(OH)_2 + Cl_2 \rightarrow Ca(ClO)_2 + H_2O$
  - Uses: Bleaching textiles/paper, oxidising agent, disinfecting drinking water.
3. Baking Soda (Sodium Hydrogencarbonate - $NaHCO_3$ ): Produced using $NaCl$ , $H_2O$ , $CO_2$ , and $NH_3$ .
  - $NaCl + H_2O + CO_2 + NH_3 \rightarrow NH_4Cl + NaHCO_3$
  - Heating Reaction: $2NaHCO_3 \xrightarrow{\text{Heat}} Na_2CO_3 + H_2O + CO_2$
  - Uses: Making baking powder (baking soda + tartaric acid), antacids, soda-acid fire extinguishers.
4. Washing Soda ( $Na_2CO_3 \cdot 10H_2O$ ): Obtained by recrystallisation of sodium carbonate.
  - $Na_2CO_3 + 10H_2O \rightarrow Na_2CO_3 \cdot 10H_2O$
  - Uses: Glass, soap, paper industries; manufacture of borax; cleaning agent; removing water hardness.

Water of Crystallisation

Definition: The fixed number of water molecules present in one formula unit of a salt.
Hydrated Copper Sulphate ( $CuSO_4 \cdot 5H_2O$ ): Blue crystals that turn white when heated (water is removed). Colour returns when water is added.
Gypsum ( $CaSO_4 \cdot 2H_2O$ ): Contains two water molecules.
Plaster of Paris (Calcium Sulphate Hemihydrate - $CaSO_4 \cdot \frac{1}{2}H_2O$ ):
- Obtained by heating gypsum at 373 K.
- Reaction with water: $CaSO_4 \cdot \frac{1}{2}H_2O + 1\frac{1}{2}H_2O \rightarrow CaSO_4 \cdot 2H_2O$ (returns to hard gypsum).
- Uses: Supporting fractured bones, making toys, decoration, smoothing surfaces.

Questions & Discussion

Q: How to identify contents of three test tubes (distilled water, acidic, basic) using only red litmus paper?
- A: Dip red litmus in all. The one that turns it blue is the basic solution. Now use this blue litmus to test the remaining two. The one that turns it red is acidic. The one that has no effect is distilled water.
Q: Why should curd not be kept in brass or copper vessels?
- A: Curd contains acids which react with the metals to produce toxic salts, making the food poisonous.
Q: Which gas is liberated when acid reacts with metal?
- A: Hydrogen gas ( $H_2$ ), tested by the pop sound with a burning candle.
Q: Metal compound A reacts with dilute HCl to produce effervescence and a gas that extinguishes a candle. One product is calcium chloride.
- A: Compound A is Calcium Carbonate ( $CaCO_3$ ). The gas is $CO_2$ .
- Equation: $CaCO_3(s) + 2HCl(aq) \rightarrow CaCl_2(aq) + H_2O(l) + CO_2(g)$
Q: Why does distilled water not conduct electricity but rain water does?
- A: Distilled water is pure and contains no ions. Rain water contains dissolved gases like $CO_2$ which form acids and dissociate into ions.
Q: Why does fresh milk take a long time to set as curd when baking soda is added?
- A: The milk becomes more alkaline, so the lactic acid produced by bacteria first has to neutralise the added base before lowering the pH enough to form curd.