Question

are elements considered to be substances?
a yes, because they have a definite composition
b no, because they are generally unreactive
c no, because they have a variable composition
d yes. because they can be either homogeneous or heterogeneous

Explanation:

A substance is a form of matter that has a definite composition. Elements are pure substances as they consist of only one type of atom and have a definite composition. Option b is incorrect as reactivity is not the criterion for being a substance. Option c is wrong because elements have a definite, not variable, composition. Option d is incorrect as homogeneity/heterogeneity is a property of mixtures, not elements (which are pure substances).

Answer:

A. Yes, because they have a definite composition