Which of the following does NOT involve a chemical change?
A. Evaporation of water
B. Souring of milk
C. Fermentation of grapes
D. Making curd from milk
Answer: Option A
Solution (By JKExamLibrary)
Evaporation of water is a physical change—liquid water changes to water vapor, but the chemical composition (H₂O) remains unchanged. Souring of milk and curd formation involve lactic acid bacteria converting lactose to lactic acid (chemical change). Fermentation of grapes involves yeast converting sugars to ethanol and CO₂. Chemical changes produce new substances. Physical changes only alter the state or form.
Explanation:
In the liver, methanol is oxidized to formaldehyde, then to formic acid, which damages the optic nerve and can cause blindness or death. Ethanol is used as an antidote by competing for the enzyme alcohol dehydrogenase.
Explanation:
CH₄ + 2O₂ → CO₂ + 2H₂O + heat. Complete combustion of any hydrocarbon with sufficient oxygen always produces carbon dioxide and water. Incomplete combustion due to limited oxygen produces carbon monoxide (CO) and/or soot (carbon particles). Methane is the main component of natural gas and biogas. CO is a poisonous gas. The combustion of alkanes is highly exothermic.
Explanation:
Bauxite (Al₂O₃·2H₂O) is the principal ore of aluminium. Haematite is Fe₂O₃, galena is PbS, cinnabar is HgS. Bauxite is refined by Bayer's process to produce alumina, then electrolytically reduced (Hall-Héroult process).
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