Some metals of lose their Shine and appear because of the action of air and moisture on them some matter as a result of oxidation with the atmosphere oxygen and lose their lustre (shine) and become dull.

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Reasons for Metal losing lustre and becoming dull:
Metal surfaces can lose their lustre and become dull due to various reasons. Some of the common factors include:
1. Oxidation: When metals react with oxygen in the air, they form metal oxides on the surface. This can make the metal lose its shine and appear dull.
2. Corrosion: Exposure to moisture, acids, or salts can cause metal to corrode, leading to the formation of a layer of corrosion products on the surface. This can also contribute to the dull appearance of the metal.
3. Physical Wear: Continuous rubbing or scratching of the metal surface can remove the top layer of the metal, exposing a rough and dull surface underneath.
4. Chemical Reactions: Metals can react with other substances present in the environment, leading to the formation of compounds that may not have a shiny appearance.
5. Surface Contamination: Dust, dirt, oils, and other contaminants can accumulate on the metal surface over time, masking its natural lustre and making it look dull.

NCERT Solution - Separation of Substances:
Separation of substances is the process of isolating one component from a mixture without changing the chemical properties of the individual substances. This process is essential in various fields, including chemistry, environmental science, and industry. There are several methods of separating substances, such as:
1. Filtration: This method is used to separate insoluble solids from liquids by passing the mixture through a filter paper. The solid particles are retained on the filter paper, while the liquid passes through.
2. Evaporation: This technique involves heating a solution to evaporate the solvent, leaving behind the solute in its solid form.
3. Distillation: Distillation is used to separate components of a liquid mixture based on their boiling points. The liquid mixture is heated, and the vapors are condensed and collected.
4. Chromatography: This method is used to separate and analyze mixtures by passing them through a stationary phase, where different components move at different rates.
5. Magnetic Separation: This technique is used to separate magnetic materials from non-magnetic substances using a magnet.
These methods of separation are crucial in various scientific experiments and industrial processes to obtain pure substances for further analysis or use.