Which of the following statements describe chemical properties?
a. oxygen gas supports combustion.
b.. hydrofluoric acid reacts with glass.
c. fertilizers to increase agricultural production.
d. water boils below 100°c on top of a mountain.
e. lead is denser than aluminum.

Explanation:

Chemical properties can only be observed when a chemical change, i.e. a change in the chemical composition, occurs.

On the other hand,  physical properties are observed or measured without altering the composition of the substances.

Typical examples of chemical properties are reactivity, flammability, combustion, oxidation states, acidity, among others that require to carry out a chemical reaction to be observed.

Typical examples of physical properties are freezing point, melting point, boiling point, density, solubility, corrosiveness, among others.

Thus, as for the choices you have:

A. Oxygen gas supports combustion: the only way to observe this property is by carrying out a combustion reaction, so this describes a chemical property.

B. Hydrofluoric acid reacts with glass: again, only by letting hydrofluoric acid react with glass this can be observed, so this is another example of a chemical property.

C. Fertilizers help to increase agricultural production: fertilizers are absorbed by the plants and converted by biochemical reactions, so this also describes a chemical property.

D. Water boils below 100°C on top of a mountain: boiling is a change of state, the water is H₂O and continues having the same composition after boiling, so this describes a physical property.

E. Lead is denser than aluminum: density is measured by physical media, measuring mass and volume; so this is a physical property.

substituting a hydrogen atom with a halogen in a hydrocarbon:

- the single bond remains single. a is wrong.

- the bond with hydrogen is as strong as with a halogen. c is wrong

- a hydrocarbon is saturated if there is no double bond. we do not know that so d is wrong.

- carbon will always have four valence electrons. e is wrong.

by elimination, the correct answer is b. the boiling point of the new compound increases.