The modern periodic table is based on: MCQ with Answer and Explanation

Explanation:
Physical changes affect form or state but not chemical composition. They can be reversible (melting/freezing). No new substances form. They may be endothermic or exothermic. Example: dissolving salt in water – salt is still NaCl.

Explanation:
Isotopes are atoms of the same element (same atomic number, same number of protons) but different mass numbers due to varying number of neutrons. For example, ¹²C, ¹³C, ¹⁴C. Their chemical properties are similar but physical properties (mass, density, rate of diffusion) differ. Isobars have same mass number but different atomic numbers.

Explanation:
Aqua regia (royal water) is a 3:1 mixture by volume of concentrated HCl and HNO₃. It generates nascent chlorine and nitrosyl chloride, which oxidize gold and platinum to soluble chloro complexes (e.g., HAuCl₄). Neither acid alone can dissolve these noble metals. Used for cleaning glassware and in etching. Must be prepared fresh and handled with extreme care.