Physical and Chemical Changes

• (a) Burning of magnesium ribbon: Chemical Change (Magnesium reacts with oxygen to form a completely new substance, white magnesium oxide powder: $2\text{Mg} + \text{O}_2 \to 2\text{MgO}$).
• (b) Sublimation of camphor: Physical Change (Solid camphor turns into vapor and re-crystallizes without chemical decomposition).
• (c) Curdling of milk: Chemical Change (Bacteria convert lactose into lactic acid, coagulating casein protein permanently).
• (d) Dissolution of sugar in water: Physical Change (Sugar molecules disperse in water; water can be evaporated to recover original sugar).
• (e) Rusting of an iron almirah: Chemical Change (Iron permanently oxidizes into reddish-brown hydrated ferric oxide).

• Physical Change Aspect: The heat of the flame causes the solid paraffin wax to melt into liquid wax. Some of this melted wax flows down the side of the candle and cools down, solidifying back into wax. No new chemical substance is formed, and the chemical composition of wax remains unchanged.
• Chemical Change Aspect: The liquid wax is drawn up through the wick by capillary action, vaporizes, and burns chemically in atmospheric oxygen. It undergoes combustion to form entirely new substances—carbon dioxide gas ($\text{CO}_2$) and water vapor ($\text{H}_2\text{O}$)—along with the release of heat and light. This burnt portion cannot be reversed back into wax.

• Exothermic Reaction: A chemical reaction accompanied by the release of heat energy into the surrounding environment.
Example: Slaking of quicklime:

$$\mathbf{\text{CaO (s)} + \text{H}_2\text{O (l)} \to \text{Ca(OH)}_2\text{ (aq)} + \text{Heat}}$$

• Endothermic Reaction: A chemical reaction accompanied by the absorption of heat energy from the surroundings.
Example: Thermal decomposition of limestone:

$$\mathbf{\text{CaCO}_3\text{ (s)} \xrightarrow{\Delta} \text{CaO (s)} + \text{CO}_2\text{ (g)} \uparrow}$$

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• Two Essential Conditions:
1. Presence of Oxygen (Air).
2. Presence of Moisture (Water or Water Vapor).
• Galvanisation: The industrial process of depositing a thin, uniform protective coating of Zinc metal onto the surface of iron articles.
• Mechanism of Protection: Zinc forms an impermeable barrier preventing air and moisture from touching the underlying iron. Even if the zinc coating is scratched, zinc is chemically more reactive than iron, so it corrodes sacrificially, protecting the iron from rusting.

• This is an Exothermic Chemical Combination Reaction.
• Explanation: Calcium oxide reacts vigorously with water to form calcium hydroxide (slaked lime):

$$\text{CaO} + \text{H}_2\text{O} \to \text{Ca(OH)}_2 + \text{Heat}$$

• The formation of strong new chemical bonds releases a massive quantity of thermal energy, boiling pockets of water to make a hissing sound and making the glass beaker intensely hot to the touch.

• The dissolution of ammonium chloride ($\text{NH}_4\text{Cl}$) in water is an Endothermic Physical/Chemical Process.
• When solid ammonium chloride dissolves, it absorbs thermal energy directly from the surrounding water molecules to break apart its ionic crystal lattice.
• Because thermal energy is continuously extracted from the water, the average kinetic energy of the water drops, making the solution and beaker feel noticeably cold to the touch.

1. Evolution of a Gas: Bubbles/effervescence (e.g., $\text{Zn} + 2\text{HCl} \to \text{ZnCl}_2 + \text{H}_2 \uparrow$).
   2. Change in Color: E.g., blue copper sulphate turning green when iron nails are added.
   3. Formation of a Precipitate: Insoluble solid settling out (e.g., white $\text{BaSO}_4$ precipitate).
   4. Change in Temperature: Release of heat (exothermic) or absorption of heat (endothermic).

• It is primarily a Chemical Change.
• Justification:
Complex insoluble organic biomolecules (proteins, starches, fats) are enzymatically cleaved and hydrolyzed by stomach acid ($\text{HCl}$) and digestive enzymes (pepsin, amylase) into completely new, simpler soluble substances (amino acids, glucose, fatty acids).
These new nutrients have different chemical structures and cannot be reconstituted back into the original food.