Physical and Chemical Changes

• Physical Change: The heat radiated from the candle flame causes solid paraffin wax around the base of the wick to melt into liquid wax. When the flame is extinguished, this molten wax cools and solidifies back into solid wax without any change in chemical composition ($C_n H_{2n+2}$). No new substance is formed, and the phase change is fully reversible.
• Chemical Change: Liquid wax is drawn up the porous wick by capillary action and vaporized. The wax vapor combusts with atmospheric oxygen ($O_2$), producing carbon dioxide ($CO_2$), water vapor ($H_2O$), carbon soot, heat, and light. New chemical substances with completely different properties are created, and the reaction cannot be reversed.
• Hence, a burning candle exhibits both physical and chemical changes simultaneously.

• (a) Rusting of iron: Chemical Change (iron reacts with moisture and oxygen to form a new brown substance, hydrated iron oxide $Fe_2O_3 \cdot xH_2O$, which cannot be reversed).
• (b) Dissolving sugar in water: Physical Change (no new substance is formed; sugar molecules merely disperse into intermolecular spaces, and solid sugar can be recovered by evaporating the water).
• (c) Digestion of food: Chemical Change (complex insoluble carbohydrates and proteins are broken down by digestive enzymes and acids into entirely new, simple soluble molecules like glucose and amino acids).
• (d) Glowing of an electric bulb: Physical Change (electric current heats the tungsten filament white-hot, but when current is switched off, the filament cools back to its original state with no change in chemical composition).

• Exothermic Change: A reaction that releases heat energy into the surrounding environment, raising the temperature.
Examples:
1. Slaking of lime: $CaO + H_2O \to Ca(OH)_2 + \text{Heat}$
2. Respiration: $C_6H_{12}O_6 + 6O_2 \to 6CO_2 + 6H_2O + \text{Energy}$
• Endothermic Change: A reaction that absorbs heat energy from the surrounding environment, causing the temperature to drop.
Examples:
1. Photosynthesis: $6CO_2 + 6H_2O + \text{Sunlight} \to C_6H_{12}O_6 + 6O_2$
2. Dissolving Ammonium Chloride in water: $NH_4Cl(s) + \text{Heat} \xrightarrow{H_2O} NH_4^+(aq) + Cl^-(aq)$.

• Definition: Rusting is the slow, destructive oxidation of metallic iron in the presence of air and water, forming a reddish-brown flaky coating of hydrated ferric oxide ($2Fe_2O_3 \cdot xH_2O$).
• Two Essential Conditions:
1. Presence of Oxygen ($O_2$)
2. Presence of Moisture / Water ($H_2O$)
(Rusting cannot occur in dry air alone or in boiled de-aerated water!)
• Two Prevention Methods:
1. Galvanization: Coating the iron surface with a thin protective layer of molten Zinc metal.
2. Painting / Greasing: Applying an impermeable barrier layer of paint or oil/grease that prevents oxygen and moisture from touching the bare iron.

• When bacteria (Lactobacillus) act on warm milk, they convert the milk sugar (lactose) into lactic acid.
• The acid denatures the milk protein (casein), coagulating it into a solid curdy mass with a sour taste.
• An entirely new chemical substance with totally different chemical properties is formed, and the curd can never be converted back into liquid milk by any physical or chemical method.

• (a) By Light (Photochemical): Photosynthesis in plants, or the darkening of silver bromide ($2AgBr \xrightarrow{h\nu} 2Ag + Br_2$) in photographic film.
• (b) By Electricity (Electrochemical): Electrolysis of acidified water into hydrogen and oxygen gases ($2H_2O \xrightarrow{\text{Electricity}} 2H_2 + O_2$).
• (c) By Pressure: Mixing dry potassium chlorate and sulfur explodes upon sudden mechanical impact/percussion, or synthesis of ammonia in the Haber process under $200\text{ atmospheres}$.

• Tearing Paper: Merely reduces the physical dimensions (size and shape) of the paper sheet. The microscopic cellulose molecules remain 100% chemically unaltered; no new substance is formed.
• Burning Paper: The cellulose combusts with oxygen, releasing heat, light, carbon dioxide gas, water vapor, and leaving behind gray mineral ash. New chemical substances are formed, and the ash cannot be turned back into paper.

• When heated, the magnesium ribbon burns with a dazzling, blinding white flame, combining vigorously with atmospheric oxygen to leave behind a powdery white ash of magnesium oxide ($MgO$).
• This is a Chemical Change because a new compound with completely distinct properties is formed with intense light and heat energy release.
• Word Equation:

$$\text{Magnesium} + \text{Oxygen} \xrightarrow{\text{Heat}} \text{Magnesium Oxide}$$

(

$$2Mg + O_2 \to 2MgO$$

).