CBSE Class 10 Answered

(a) Metals like Na, Ca and Mg are very reactive metals, they react with oxygen even at room temperature therefore they are never found in free state in nature.

(b) Solder is an alloy which has a low melting point hence it is used for welding electrical circuits.

(c) Silver ornaments turn blackish after some time due to the formation of a thin silver sulphide layer on their surface by the action of hydrogen sulphide gas present in air.

(d) Gold is used for making jewellery because of its bright shiny surface and high resistance to corrosion.

(e) Gallium has a very low melting point (28^oC) hence it starts melting when placed on hand due to the  heat of  our body.