Question

instructional science review quiz (term 2)
kankakee high school

1. which of these is a molecule?

a. h₂ b. co₂ c. h₂o d. nacl

1. which of the following is a compound made of two elements?

a. o₂ b. h₂o c. n₂ d. he

1. what holds atoms together in a molecule?

a. gravity b chemical bond c. heat d. air

1. what happens when elements chemically combine?

a. they form a mixture
b they form a compound
c. they separate
d. they melt

1. a student mixes iron and sulfur but can still see both substances. this is a

____.
a. compound b. mixture c. molecule d. gas

Explanation:

• Question 11: A molecule is formed when two or more atoms bond together. \(H_2\) (two hydrogen atoms), \(CO_2\) (one carbon and two oxygen atoms), and \(H_2O\) (two hydrogen and one oxygen atoms) are molecules. \(NaCl\) is an ionic compound, not a molecule in the traditional covalent - bonded sense.
• Question 12: A compound made of two elements. \(O_2\) and \(N_2\) are di - atomic elements (not compounds), \(He\) is a noble gas (an element). \(H_2O\) is a compound made of hydrogen and oxygen.
• Question 13: Chemical bonds (such as covalent, ionic, or metallic bonds) hold atoms together in a molecule. Gravity is a force between large masses, heat is a form of energy, and air is a mixture of gases.
• Question 14: When elements chemically combine, they form a compound. A mixture is a physical combination (no chemical reaction), separation is the opposite of combination, and melting is a physical change of state.
• Question 15: If two substances are mixed and can still be seen (no chemical reaction), it is a mixture. A compound is a chemical combination, a molecule is a small unit of a substance (either an element or a compound), and gas is a state of matter.

Answer:

1. a. \(H_2\), b. \(CO_2\), c. \(H_2O\)
2. b. \(H_2O\)
3. b. Chemical bond
4. b. They form a compound
5. b. Mixture