Model Answers for Worksheets Grade 9
CHAPTER 2: SECTION 1, 2 AND 3
Concept Reviews
SECTION: CLASSIFYING MATTER
1. a. E d. E
b. O e. O
c. E f. O
2. An atom is the smallest unit of an element that maintains the properties of that element, while a molecule is the smallest particle of a substance that has the chemical properties of that substance. Molecules consist of one atom or two or more atoms bonded together.
3. A pure substance is made up of matter that has a fixed composition and definite properties. Although a homogeneous mixture is uniformly mixed, it is a combination of more than one pure substance and does not necessarily have a fixed composition.
4. a. C d. E
b. E e. C
c. C f. E
5. Elements are pure substances because each has a fixed composition of protons, neutrons, and electrons and particular characteristic properties. Compounds are pure substances because each has a fixed composition of atoms and definite properties.
6. a. M d. S
b. S e. M
c. S f. M
7. Copper and carbon are classified as elements because they are made of only one kind of atom and cannot be broken down by chemical means.
SECTION: PROPERTIES OF MATTER
1. a. physical
b. chemical
c. physical
d. physical
e. chemical
2. a. physical
b. chemical
c. physical
d. physical
e. physical
3. Aluminum is light and durable (physical properties) and nonreactive (chemical property).
4. 136.3 g silver
m = DV
= (10.49 g/cm3)(12.99 cm3) = 136.3 g
5. 2.3 g/cm3
D = m/V
= (820 g)/(350 cm3) = 2.3 g/cm3
Chapter Assessment
TEST A
1. b 11. c
2. b 12. b
3. d 13. properties
4. c 14. oxygen
5. c 15. Cl2
6. c 16. miscible
7. d 17. reactive
8. b 18. less dense
9. d 19. physical
10. b
20. Density affects how heavy something is, as well as whether something floats in a particular liquid. Boats float because the air inside the vessel is less dense than the water the vessel sits in. Cream can be separated from milk because it is less dense than milk and floats to the top.
CHAPTER6: SECTION 1 AND 2
SECTION 1 COMPOUNDS AND MOLECULES
Review
1. Different models show different characteristics of the compound’s structure. One model cannot show every characteristic.
2. The ball-and-stick model shows bond angles and bond lengths. The ball-and-stick model uses balls to represent atoms and sticks to represent bonds.
3.
4. The particles in liquids are more strongly attracted to one another than particles in gases. Thus, molecules in rubbing alcohol are more strongly attracted to one another than molecules in methane gas.
5. The attractions between particles are relatively weak.
Concept Reviews
SECTION: COMPOUNDS AND
MOLECULES
1. Compounds are held together by chemical bonds.
2. A ball-and-stick model gives you a better idea of bond lengths and bond angles. A space-filling model gives you a better idea of the space occupied by atoms.
3. The model would be more accurate because bonds can bend, stretch, and
rotate without breaking.
4. Substances with network structures have strong bonds holding the atoms or ions together. Much energy (a higher temperature) is needed to break these
bonds.
5. Table salt has a strong network structure consisting of very tightly bonded sodium cations and chloride anions. Table sugar is made of individual molecules. The bonds within each molecule are strong, but there are no bonds (just slight attractions) between molecules.
6. Because the boiling point of the compound is relatively low, the compound is likely to be in the form of individual molecules.
SECTION 2 IONIC AND COVALENT BONDING
Review
1. No, Na and K are both metals. They would not form ions with opposite charges. Thus, they would not form an ionic compound.
2. Atoms of the same element that are bonded together share electrons equally. Atoms of different elements that are bonded together do not share electrons equally.
3. Top row: 4; double; polar bottom row: 2 per bond; single; polar
4. The bond between O and H is covalent. The bonds between the two OH-
5. Aluminum foil and dissolved KOH; aluminum is a metal, and metals can conduct electric current as solids. KOH is an ionic compound. Ionic compounds can conduct electric current when they are dissolved.
Concept Review
SECTION: IONIC AND COVALENT
BONDING
1. Atoms join to form bonds so that each atom has a more stable electron
configuration.
2. An ionic bond is formed by the attraction between oppositely charged ions. A covalent bond is formed when atoms share one or more pairs of electrons.
3. A triple bond is stronger than a double bond because more energy is required to break a triple bond with 3 pairs of shared electrons as opposed to 2 pairs of shared electrons in a double bond.
4. A compound that contains one or more polyatomic ions has both ionic and covalent bonds. The atoms making up the polyatomic ion are covalently bonded. The polyatomic ion forms an ionic bond with an oppositely charged ion.
5. Gold has metallic bonds in which the electron clouds of the gold atoms overlap. This overlap allows electrons to be transferred from atom to atom easily; therefore, gold is a good conductor of electricity.
