Which of the following gases diffuses fastest under identical conditions? MCQ with Answer and Explanation

Explanation:
Sodium is a soft alkali metal that can be easily cut with a knife, revealing a silvery surface that tarnishes quickly in air. Potassium is also soft. Iron, zinc, and copper are much harder. Alkali metals have low density and low melting points. They are stored under kerosene to prevent reaction with moisture and oxygen.

Explanation:
Mendeleev arranged elements in order of increasing atomic mass and grouped elements with similar chemical properties. He left gaps for undiscovered elements like eka-aluminium (gallium) and eka-silicon (germanium), predicting their properties. The modern table uses atomic number as the basis. Mendeleev's table had anomalies like placing Ar (40) before K (39) due to mass order, later resolved by atomic number concept.

Explanation:
Crude petroleum is heated and the vapors are passed through a fractionating column where they condense at different levels according to boiling points. Fractions: petrol (40-120°C), kerosene (150-250°C), diesel (250-350°C), lubricating oil, etc. This is a continuous process. The column has trays and maintains temperature gradient. Simple distillation cannot separate close boiling mixtures.