Chemistry Test 213

Both (A) and (R) are true but (R) is not the correct explanation of (A)

The melting and boiling points of noble gases are very low in comparison to those of other substances of comparable atomic and molecular masses. This indicates that only weak van der Waals forces or weak London dispersion forces are present between the atoms of the noble gases in the liquid or the solid state.

The van der Waals force increases with the increase in the size of the atom, and therefore, in general, the boiling and melting points increase from He to Rn.

Helium boils at −269 ^∘C. Argon has larger mass than helium and have larger dispersion forces. Because of larger size the outer electrons are less tightly held in the larger atoms so that instantaneous dipoles are more easily induced resulting in greater interaction between argon atoms. Therefore, its boiling point (−186 ^∘C) is more than that of He.

Similarly, because of increased dispersion forces, the boiling and melting points of monoatomic noble gases increase from helium to radon.

Due to larger size of iodine atom, it can accommodate up to seven small fluorine atoms around, while due to smaller sizes of chlorine and bromine atoms cannot accommodate seven fluorine atoms, i.e., steric factor dominate in case of chlorine and bromine.

PbO dissolves in HNO₃ as it forms Pb(NO₃)₂ which is soluble.

HNO₃ is a strong acid in which N-atom is in its maximum oxidation state +5, so it can act only as oxidising agent not as reducing agent.

SiC is covalent compound also known as carborundum. It has semiconducting properties.

Al(OH)₃ dissolves in NaOH solution to give AlOH₄^- ion which is supposed to have the octahedral complex species [Al(H₂O₂)(OH)₄]^- in aqueous solution.

A buffer solution is an aqueous solution consisting of a mixture of a weak acid and its conjugate base, or vice versa.

Aqueous solution of borax tribasic acid (H₃BO₃) and strong base. It contains weak acid and its salt with strong base.Hence borax act as buffer.

Most pesticides contain elements, the type and number of which depend on the pesticide desired. Chlorine, oxygen, sulfur, phosphorus, nitrogen, and bromine are most common. Inert ingredients can be many substances, dependent on the type of pesticide. Liquid pesticides have traditionally used kerosene or some other petroleum distillate as a carrier, though water has recently begun to replace kerosene.