रसायन विज्ञान की भाषा (Language of Chemistry)

• (a) Aluminum oxide:
Cation: $\text{Al}^{3+}$ (Valency 3), Anion: $\text{O}^{2-}$ (Valency 2).
Criss-cross: $\mathbf{\text{Al}_2\text{O}_3}$

• (b) Calcium phosphate:
Cation: $\text{Ca}^{2+}$ (Valency 2), Anion: $\text{PO}_4^{3-}$ (Valency 3).
Criss-cross: $\mathbf{\text{Ca}_3(\text{PO}_4)_2}$

• (c) Ammonium sulphate:
Cation: $\text{NH}_4^+$ (Valency 1), Anion: $\text{SO}_4^{2-}$ (Valency 2).
Criss-cross: $\mathbf{(\text{NH}_4)_2\text{SO}_4}$

• (d) Ferric hydroxide:
Cation: $\text{Fe}^{3+}$ (Ferric $=$ higher valency 3), Anion: $\text{OH}^-$ (Valency 1).
Criss-cross: $\mathbf{\text{Fe(OH)}_3}$.

• (a) $\text{Fe} + \text{H}_2\text{O} \to \text{Fe}_3\text{O}_4 + \text{H}_2$:
1. Balance Fe: Put 3 before Fe: $3\text{Fe} + \text{H}_2\text{O} \to \text{Fe}_3\text{O}_4 + \text{H}_2$
2. Balance O: 4 O on RHS $\implies$ Put 4 before $\text{H}_2\text{O}$: $3\text{Fe} + 4\text{H}_2\text{O} \to \text{Fe}_3\text{O}_4 + \text{H}_2$
3. Balance H: 8 H on LHS $\implies$ Put 4 before $\text{H}_2$:

$$\mathbf{3\text{Fe} + 4\text{H}_2\text{O} \to \text{Fe}_3\text{O}_4 + 4\text{H}_2}$$

• (b) $\text{KClO}_3 \to \text{KCl} + \text{O}_2$:
LCM of 3 and 2 is 6. Put 2 before $\text{KClO}_3$ and 3 before $\text{O}_2$:

$$\mathbf{2\text{KClO}_3 \to 2\text{KCl} + 3\text{O}_2}$$

• (c) $\text{Al} + \text{H}_2\text{SO}_4 \to \text{Al}_2(\text{SO}_4)_3 + \text{H}_2$:
Put 2 before Al, 3 before $\text{H}_2\text{SO}_4$, and 3 before $\text{H}_2$:

$$\mathbf{2\text{Al} + 3\text{H}_2\text{SO}_4 \to \text{Al}_2(\text{SO}_4)_3 + 3\text{H}_2}$$

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• Variable Valency: The phenomenon where an element exhibits more than one combining capacity (valency) in different chemical compounds.
• Cause: In transition metals, an unstable penultimate shell loses electrons along with the valence shell.
• Examples:
1. Iron (Fe):
• Lower valency ($+2$): Ferrous (e.g., $\text{FeCl}_2$, Ferrous chloride / Iron(II) chloride).
• Higher valency ($+3$): Ferric (e.g., $\text{FeCl}_3$, Ferric chloride / Iron(III) chloride).
2. Copper (Cu):
• Lower valency ($+1$): Cuprous (e.g., $\text{Cu}_2\text{O}$, Cuprous oxide / Copper(I) oxide).
• Higher valency ($+2$): Cupric (e.g., $\text{CuO}$, Cupric oxide / Copper(II) oxide).

• (a) Lead: Latin: Plumbum $\implies$ Symbol: $\mathbf{Pb}$
• (b) Gold: Latin: Aurum $\implies$ Symbol: $\mathbf{Au}$
• (c) Potassium: Latin: Kalium $\implies$ Symbol: $\mathbf{K}$
• (d) Tin: Latin: Stannum $\implies$ Symbol: $\mathbf{Sn}$
• (e) Mercury: Latin: Hydrargyrum $\implies$ Symbol: $\mathbf{Hg}$.

• Radical: An atom or a group of atoms of the same or different elements that behaves as a single unit with a positive or negative charge and maintains its identity in chemical reactions.
• Simple Radical: Formed from a single atom having an electrical charge.
Examples: $\text{Na}^+$ (sodium ion), $\text{Cl}^-$ (chloride ion), $\text{Mg}^{2+}$ (magnesium ion).
• Compound (Polyatomic) Radical: Formed from a group of different atoms bonded together carrying a net electrical charge.
Examples: $\text{NH}_4^+$ (ammonium), $\text{SO}_4^{2-}$ (sulphate), $\text{NO}_3^-$ (nitrate), $\text{CO}_3^{2-}$ (carbonate).

• The subscripts in a chemical formula (such as the '2' in $\text{H}_2\text{O}$) represent the fixed chemical stoichiometry and definite proportions by mass of that compound.
• Changing a subscript alters the chemical identity of the substance entirely (e.g., changing $\text{H}_2\text{O}$ to $\text{H}_2\text{O}_2$ converts life-giving water into toxic, caustic hydrogen peroxide!).
• Equations must be balanced exclusively by altering stoichiometric coefficients placed in front of the formulas.

• (a) $\text{\underline{N}H}_3$: Hydrogen has valency 1. Nitrogen combines with 3 hydrogen atoms $\implies \mathbf{\text{Valency of N} = 3}$ (Trivalent).
• (b) $\text{\underline{C}O}_2$: Oxygen has valency 2. Two oxygen atoms provide total combining capacity $2 \times 2 = 4 \implies \mathbf{\text{Valency of C} = 4}$ (Tetravalent).
• (c) $\text{\underline{P}Cl}_5$: Chlorine has valency 1. Phosphorus combines with 5 chlorine atoms $\implies \mathbf{\text{Valency of P} = 5}$ (Pentavalent).
• (d) $\text{H}_2\text{\underline{S}}$: Sulphur combines with 2 hydrogen atoms $\implies \mathbf{\text{Valency of S} = 2}$ (Divalent).

1. Physical States: A skeletal equation does not show whether substances are solid, liquid, or gas. Solution: Append state symbols $(s), (l), (g), (aq)$.
   2. Precipitation & Gas Evolution: Does not indicate if an insoluble solid or gas escapes. Solution: Use $\downarrow$ for precipitate and $\uparrow$ for evolved gas.
   3. Reaction Conditions: Does not state temperature, pressure, or catalyst. Solution: Write conditions above/below the arrow (e.g., $450^{\circ}\text{C}, 200\text{ atm}, \text{Fe catalyst}$).
   4. Thermal Changes: Does not indicate heat release or absorption. Solution: Add $+ \text{Heat}$ for exothermic or $- \text{Heat}$ (or $\Delta H$) for endothermic.