Ch 02 - Acids, Bases and Salts10th Science

Natural Indicators

• Litmus: Turns Red in acidic solutions, Blue in basic solutions.
• Turmeric: Remains Yellow in acidic solutions, turns Reddish-Brown in basic solutions.

Synthetic Indicators

• Phenolphthalein: Remains Colourless in acidic solutions, turns Pink in basic solutions.
• Methyl Orange: Turns Red in acidic solutions, Yellow in basic solutions.

• Definition: Substances whose odour (smell) changes depending on whether they are in an acidic or basic medium.
• Examples: Vanilla extract, Onion, and Clove oil.
• Reaction Mechanism: They retain their characteristic smell in acidic solutions but completely lose their smell in basic solutions.

Variables & Units:

• Zn: Zinc granules (Solid)
• H2SO4: Dilute Sulphuric Acid (Aqueous)
• ZnSO4: Zinc Sulphate (Aqueous Salt)
• H2: Hydrogen gas

Conditions/Notes:

• The metal actively displaces hydrogen atoms from the acid to form a salt and release gas.

• Red Cabbage Extract: Remains Red in acidic solutions, but turns Green in basic solutions.
• Hydrangea Flowers: Naturally Blue in acidic soil/solutions, but turn Pink in basic environments.

• Composition: It is a precisely formulated mixture of several indicators, not a single dye.
• Function: It shows different colours at different concentrations of hydrogen ($H^+$) ions, rather than just identifying an acid or base.
• Use: It helps quantify the exact pH strength of the solution on a scale of 0 to 14.

Variables:

• NaOH: Sodium Hydroxide (Strong Base)
• Na2ZnO2: Sodium Zincate (Salt)
• H2: Hydrogen Gas

Conditions/Notes:

• The reaction requires heating ($\Delta$).
• Crucial Boundary Condition: Such reactions are not possible with all metals, only amphoteric ones like Zinc and Aluminium.

Variables:

• Na2CO3: Sodium Carbonate (Solid)
• HCl: Hydrochloric Acid (Aqueous)
• NaCl: Sodium Chloride (Aqueous Salt)
• CO2: Carbon Dioxide gas

Conditions/Notes:

• The reaction produces brisk effervescence due to the rapid release of Carbon Dioxide gas.

• Procedure: Pass the evolved gas through a soap solution so that gas-filled bubbles rise into the air. Bring a burning candle near one of these bubbles.
• Observation: The gas combusts with a characteristic 'pop' sound (a mini-explosion).
• Conclusion: This observation strictly confirms the presence of highly combustible Hydrogen ($H_2$) gas.

Variables:

• Ca(OH)2: Calcium Hydroxide / Lime water (Aqueous)
• CaCO3: Calcium Carbonate (Solid)

Conditions/Notes:

• The lime water turns milky.
• This milkiness is caused by the formation of a white precipitate of insoluble Calcium Carbonate.

Variables:

• Ca(HCO3)2: Calcium Hydrogen Carbonate (Aqueous)

Conditions/Notes:

• The milkiness strictly disappears.
• This happens because Calcium Hydrogen Carbonate is highly soluble in water, turning the solution clear again.

Variables:

• NaOH: Sodium Hydroxide (Strong Base)
• HCl: Hydrochloric Acid (Strong Acid)
• NaCl: Sodium Chloride (Neutral Salt)

Conditions/Notes:

• The effect of a base is perfectly nullified by an acid, and vice-versa, resulting in neutral pH products.

• Composition: Egg shells are primarily made of Calcium Carbonate ($CaCO_3$).
• Reaction: When reacted with an acid (like $HCl$), the carbonates release Carbon Dioxide ($CO_2$) gas.
• This evolved $CO_2$ is strictly what turns the lime water milky, confirming the carbonate nature of the shells.

Variables:

• CuO: Copper(II) Oxide (Black colour)
• CuCl2: Copper(II) Chloride

Conditions/Notes:

• The solution transforms to a blue-green colour due to the formation of Copper(II) Chloride.
• Because they react with acids to yield salt and water, metallic oxides are classified as basic in nature.

Variables:

• H+: Hydrogen ion
• H3O+: Hydronium ion

Conditions/Notes:

• Hydrogen ions cannot exist alone or independently in water. They immediately hydrate by attaching to water molecules to form the Hydronium ion ($H_3O^+$).

• Reaction: Non-metallic oxides react directly with bases to form a salt and water (e.g., $CO_2$ reacting with $Ca(OH)_2$).
• Chemical Conclusion: Because this reaction perfectly mimics an acid-base neutralisation, it proves that non-metallic oxides are acidic in nature.

Variables:

• OH-: Hydroxide ion (anion responsible for basic character)

Conditions/Notes:

• Bases that are highly soluble in water are specifically termed Alkalis.
• All bases generate $OH^-$ ions when dissolved in water.

• Acids: Dissociate extensively in aqueous solutions to produce Hydrogen ions ($H^+$) (or Hydronium ions), which act as mobile charge carriers for the current.
• Glucose/Alcohol: Although they structurally contain hydrogen atoms, their covalent bonds do not break in water. They do not ionise to release $H^+$ ions, hence the circuit remains incomplete.

• The Core Concept: The separation of $H^+$ ions from HCl molecules can only occur in the presence of water.
• Conclusion: Without moisture, HCl gas remains strictly molecular and cannot exhibit acidic properties. It will only turn moist blue litmus paper red, as the water facilitates ionisation into $H^+$ ions.

• Nature of Reaction: Dissolving a concentrated acid in water is a highly exothermic (heat-producing) process.
• The Danger: If water is added to acid, the sudden, massive heat generated can cause the mixture to rapidly boil and splash out, causing severe acid burns.
• Secondary Risk: The localized extreme heat can instantly shatter the glass container.

• Definition: A scale for measuring hydrogen ion concentration. 'p' stands for potenz (power in German).
• Range (0 to 14):
  • < 7 (Acidic): Lower number = higher $H^+$ concentration / stronger acid.
  • 7 (Neutral): Perfectly Neutral (e.g., pure water, neutral salts).
  • > 7 (Basic/Alkaline): Higher number = higher $OH^-$ concentration / stronger base.

• Strong Acids: They completely ionise (dissociate) in aqueous solutions, generating a massive concentration of $H^+$ ions. (Examples: $HCl, H_2SO_4, HNO_3$)
• Weak Acids: They only partially ionise in water, generating a substantially lower concentration of $H^+$ ions. (Examples: Acetic acid $CH_3COOH$, Citric acid).

Organic Acid Matchup:

• Tomato: Oxalic Acid
• Tamarind: Tartaric Acid
• Curd (Sour Milk): Lactic Acid
• Vinegar: Acetic Acid

Note: These are all weak acids that occur naturally in plant and animal materials.

• Chemical Reason: Sour foods contain acids (e.g., Lactic acid in curd).
• The Reaction: These acids react directly with the reactive metals (copper or zinc in brass).
• Consequence: This reaction forms toxic metal salts which contaminate the food, causing food poisoning and severely damaging human health.

Variables & Products:

• NaCl (Brine): Aqueous Sodium Chloride
• NaOH: Sodium Hydroxide (Formed near cathode)
• Cl2: Chlorine gas (Formed at anode)
• H2: Hydrogen gas (Formed at cathode)

Conditions/Notes:

• The process is named "Chlor-Alkali" purely based on its core products: Chlor (for Chlorine) and Alkali (for Sodium Hydroxide).

• Range: The human body works normally strictly within a pH range of 7.0 to 7.8.
• Significance: Any slight shift outside this narrow window disrupts severe metabolic and biochemical activities. Survival becomes impossible if the pH deviates too much.

Variables:

• Ca(OH)2: Dry slaked lime
• Cl2: Chlorine gas
• CaOCl2: Bleaching Powder (Calcium Oxychloride)

Conditions/Notes:

• While $CaOCl_2$ represents the basic formula, the actual chemical composition of industrial bleaching powder is quite complex.

• Stomach Environment: The stomach naturally produces Hydrochloric Acid ($HCl$) to facilitate digestion without harming the mucosal lining.
• Indigestion: Overeating triggers excessive acid production, causing acute pain and irritation.
• Chemical Remedy: Patients take Antacids (mild bases like Magnesium Hydroxide / 'Milk of Magnesia' or Baking Soda) which neutralise the excess stomach acid.

Variables:

• NaCl: Sodium Chloride (in aqueous Brine)
• NH3: Ammonia
• NH4Cl: Ammonium Chloride
• NaHCO3: Sodium Hydrogencarbonate (Baking Soda)

Conditions/Notes:

• Baking soda is a mild, non-corrosive basic salt.

• Threshold: Tooth decay physically initiates when the mouth's pH drops below 5.5.
• Mechanism: Bacteria in the mouth degrade leftover food sugars into acids.
• These acids actively corrode the tooth enamel, which is made of ultra-hard Calcium Hydroxyapatite (a crystalline calcium phosphate).
• Prevention: Basic toothpastes neutralise this acid build-up.

Variables:

• Na2CO3: Sodium Carbonate
• CO2: Carbon Dioxide gas

Conditions/Notes:

• The release of Carbon Dioxide gas gets trapped in the dough, causing bread or cake to rise and become soft and spongy.

• The Toxin: Both bee stings and stinging nettle plant hairs inject Methanoic Acid (also known as Formic acid), causing intense burning pain.
• The Remedies:
  • Bee Sting: Apply a mild household base like Baking Soda to the skin.
  • Nettle Sting: Rub the area with the leaf of a Dock plant, a naturally basic plant that symbiotically grows right next to nettles in the wild.

• Acid Rain: Rain is classified as acid rain strictly when its pH falls below 5.6. It makes survival of aquatic life highly difficult when it runs into rivers.
• Venus Profile: The atmosphere of Venus is entirely enveloped in thick, yellowish clouds of pure Sulphuric Acid ($H_2SO_4$), making biological life as we know it impossible.

• Strong Acid + Strong Base $\rightarrow$ Neutral Salt (pH = 7)
• Strong Acid + Weak Base $\rightarrow$ Acidic Salt (pH < 7)
• Weak Acid + Strong Base $\rightarrow$ Basic Salt (pH > 7)

Note: Weak Acid + Weak Base combinations require complex equilibrium calculations and are usually close to neutral.

Variables:

• Na2CO3: Sodium Carbonate (obtained by heating baking soda)
• Na2CO3·10H2O: Washing Soda (Sodium Carbonate Decahydrate)

Conditions/Notes:

• Recrystallisation in water yields washing soda. It is fundamentally a basic salt.

• Definition: Rock salt refers to beds of solid, large sodium chloride ($NaCl$) crystals formed when ancient seas dried up. It is mined similarly to coal.
• Colour: The large crystals often appear brown strictly due to the presence of trapped impurities within the salt matrix.

• Hydrogen ($H_2$): Used as fuels, in margarine, and to synthesize ammonia for fertilisers.
• Chlorine ($Cl_2$): Used for water treatment, swimming pools, PVC, disinfectants, and pesticides.
• Sodium Hydroxide ($NaOH$): Used for de-greasing metals, manufacturing soaps and detergents, and artificial fibres / paper making.

Variables:

• CaSO4·2H2O: Gypsum (Calcium Sulphate Dihydrate)
• CaSO4·1/2H2O: Plaster of Paris (Calcium Sulphate Hemihydrate)

Conditions/Notes:

• Must be heated precisely at 373 K (100 °C).
• Exception: If heated above this temperature, all water is lost (forming 'dead burnt plaster') and it permanently loses its setting property.

Variables:

• CaSO4·1/2H2O: Plaster of Paris (White powder)
• CaSO4·2H2O: Gypsum (Hard solid mass)

Conditions/Notes:

• Only half a water molecule is shown as water of crystallisation because two formula units of $CaSO_4$ share one molecule of water.
• Uses: Used by doctors to support fractured bones, and by artisans to make toys, casts, and smooth decorative ceilings.

• Baking Soda: Pure Sodium Hydrogencarbonate ($NaHCO_3$).
• Baking Powder: A precise mixture of Baking Soda and a mild edible acid (like Tartaric Acid).
• The Role of Acid: When heated, baking soda produces Sodium Carbonate, which has a bitter taste. The $H^+$ ions from the tartaric acid neutralise this basic salt, forming a pleasant-tasting sodium salt of the acid instead.

• Medicine (Antacid): Being alkaline, it neutralises excess stomach acid to relieve indigestion.
• Baking: Used to make baking powder, which releases $CO_2$ to make baked goods fluffy.
• Safety: Acts as the active ingredient in soda-acid fire extinguishers due to its rapid $CO_2$ release when mixed with acid.

• Manufacturing: Extensively used in the glass, soap, and paper industries.
• Chemical Production: Used to manufacture sodium compounds like Borax.
• Cleaning: Acts as a potent domestic cleaning agent.
• Water Treatment: Specifically used to remove the permanent hardness of water.

• Definition: Water of crystallisation is the fixed number of water molecules chemically attached to one formula unit of a salt within a crystal lattice.
• The Copper Sulphate Proof: Hydrated Copper Sulphate ($CuSO_4 \cdot 5H_2O$) appears as dry blue crystals but contains 5 water molecules.
• Heating Effect: When heated, it loses this water and turns into a white powder (anhydrous copper sulphate). If moistened again, the blue colour instantly reappears.

Variables:

• NaHCO3: Sodium Hydrogencarbonate (Baking soda)
• H2SO4: Dilute Sulphuric Acid
• Na2SO4: Sodium Sulphate (Salt)
• CO2: Carbon dioxide gas

Conditions/Notes:

• The extinguisher contains a solution of baking soda and a separate glass ignition tube of acid.
• When broken, they mix and cause a massive, rapid release of $CO_2$. Because $CO_2$ is heavier than oxygen, it physically blankets the fire, cutting off its oxygen supply.