The season that is starting is winter.
The answer to 21 is (4)
LiF or lithium fluoride is the non covalent molecule or ionic compound.
Option 3.
<h3><u>Explanation:</u></h3>
Covalent molecules are those molecules which do have actual bonds between the atoms present in the molecule by sharing of the electrons. But in ionic molecules, there's no actual bonds between the atoms, but the oppositely charged ions are attracted towards each other by means of electrostatic force of attraction.
The molecules that are formed by the atoms with high electronegativity and electropositivity are actually ionic because the atoms with high electronegativity are able to actually gain electron readily and the atoms with high electropositivity are actually ready to give the electrons to the electronegative elements.
Lithium is highly electropositive and fluoride is highly electronegative. So they establish an ionic bond. But other molecules like fluorine molecule has both the electronegative elements, Carbon monoxide has carbon which isn't electropositive highly, and ammonia has hydrogen which isn't electropositive.
So lithium fluoride is the ionic compound.
The exposure of a person to colloids majorly present in industrial area may results in the accumulation in the body tissue which causes dark deposit in the skin. They are not safe and is dangerous for health which may cause turning of skin blue permanently and it is life long.
Whereas, the colloid of silver nitrate is used to treat skin burns and ulcers but its overuse may result in adverse effect.