HESI Chemistry Questions with Answers and Explanations

HESI Chemistry Questions with Answers and Explanations
1. Which of the following substances allows for the fastest diffusion?
A. gas
B. solid
C. liquid
D. plasma
Explanation: Diffusion is fastest through gases. The next fastest medium for diffusion is liquid,
followed by plasma, and then solids. In chemistry, diffusion is defined as the movement of
matter by the random motions of molecules. In a gas or a liquid, the molecules are in perpetual
motion. For instance, in a quantity of seemingly immobile air, molecules of nitrogen and oxygen
are constantly bouncing off each other. There is even some miniscule degree of diffusion in
solids, which rises in proportion to the temperature of the substance.
2. What is the oxidation number of hydrogen in CaH2?
A. +1
B. –1
C. 0
D. +2
Explanation: The oxidation number of the hydrogen in CaH2 is –1. The oxidation number is the
positive or negative charge of a monoatomic ion. In other words, the oxidation number is the
numerical charge on an ion. An ion is a charged version of an element. Oxidation number is often
referred to as oxidation state. Oxidation number is sometimes used to describe the number of
electrons that must be added or removed from an atom in order to convert the atom to its
elemental form.