The half-life of a radioactive isotope is 10 days. What fraction of the original sample remains after 30 days? MCQ with Answer and Explanation

Explanation:
Physical changes alter the form, state, or appearance without changing chemical identity. They are usually easily reversible (e.g., freezing/melting). Chemical changes produce new substances and are often irreversible (e.g., burning). Physical: crushing, dissolving, boiling. Chemical: rusting, cooking, digestion.

Explanation:
Methane undergoes substitution reaction with chlorine in UV light: CH₄ + Cl₂ → CH₃Cl + HCl. Further substitution can give CH₂Cl₂, CHCl₃, CCl₄. This is a free radical mechanism. It does not produce methanol or ethane.

Explanation:
Natural gas is predominantly methane (CH₄), typically 70-95%. Ethane, propane, butane are minor components. CNG is compressed natural gas.