Grade 8
xx
Gateways to Science FeaturesINTRODUCTION to Gateways
Learning Goalprovides a brief outlook of lesson purpose.
Advance Preparationallows for preplanning to ensure lesson goes smoothly.
Safety Alert Iconreminds teacher and students of safe lab practices.
Lab Rotation Iconsignals students to prepare for lab station activities.
Station Descriptorssummarize for students lab activities by station.
Materials Listaids in lesson preparation.
5 Esare clearly labeled to help keep track of lesson progress.
ROTAT
ION
Stations
150
UNIT 2: Matter and EnergyLesson 11: Law of Conservation of Mass
EngagePage 82
Building Blocks
1. Arrange the objects in the plastic bag to model the reactants in the following equation.
NaOH + HCl NaCl + H2O
Notice that each color represents a different element in the equation.
Keyred = sodium
blue = hydrogen
yellow = oxygen
green = chlorine
2. Place all the reactant pieces on a balance to determine their mass. In your science notebook, record the mass.
Learning GoalDetermine if a chemical equation is balanced and how that relates to the law of conservation of mass.
Engage
Advance Preparation
• Gather the centigram cubes and place the correct amount in a resealable plastic bag for each group prior to class.
Teacher Instruction
• Distribute a plastic bag of centigram cubes to each group.
• Instruct students to follow the directions to complete the Engage activity and the science notebook entry.
Teacher Note• Centigram cubes can
be purchased from most educational supply companies. Many math departments use centigram cubes.
• Certain types of plastic such as toy building blocks or paper clips could be used instead of the centigram cubes, but the mass of individual blocks or paper clips may not be standardized.
MaterialsFor student groups• 1 red centigram
cube• 2 blue centigram
cubes• 1 yellow
centigram cube• 1 green
centigram cube • resealable
plastic bags• balance
1
SAMPLE
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Gateways to Science Features
Science Notebook Icon quickly indicates the need for students to use a science notebook.
Facilitation Questionsassist in guiding and scaffolding instruction.
Reproducible Master (RM) Snapshots and Answer Keysoffer an at-a-glance view. RM Answer Keys are placed in lesson to reduce the amount of printed materials.
Teacher Noteprovides helpful tips and information.
Student Pagesare embedded in all lessons for ease of use and include answers to student edition.
157© REGION 4 EDUCATION SERVICE CENTER. ALL RIGHTS RESERVED.
Unit 2: M
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Figure 2.41. Engage Equation Model
NaOHsodium
hydroxide+
HClhydrochloric
acid
NaClsodium chloride
+H2O
water
Na O H + ClH Na Cl +H H
O
Science
100 Sheets • 200 Pages • Wide Ruled
9 �/� x 7 �/� in. • 24.7 x 19.0 cm
Reflection
How are the activities in Explore similar to Figure 2.41?
Facilitation Questions
• What is the law of conservation of mass? Matter cannot be created or destroyed. The numbers of atoms in the reactants and the products are the same, but they are rearranged chemically.
• How does the Engage activity support the law of conservation of mass? The number of atoms in the reactants and products are the same. The mass of the reactants and the products are equal.
• How do the Explore activities support the law of conservation of mass? The mass of the reactants and products did not change.
Explain, Part 3
Teacher Instruction
• Read and discuss “Applying the Law of Conservation of Mass” to help students understand how to determine whether an equation is balanced or unbalanced.
• Display RM 60: ERP to model an example. Then guide students through several examples before releasing students to work independently. Allow students to compare, discuss, and revise their work in small-group settings.
• Instruct students to revisit their equations from Lesson 9, Elaborate, and identify which equations are balanced and which are not.
Teacher Note• Students should not
balance equations. Students only need to recognize whether an equation is balanced; this relates to the law of conservation of mass. Having students balance equations now is counterproductive to learning and can actually develop student misconceptions.
• Coloring the elements, coefficients, and subscripts with different colored map pencils prior to ERP (Elements, Reactant, Product) may help students identify important parts of the equation that could easily be missed.
MaterialsFor teacher• RM 60
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Gateways to Science, STAAR Edition, Grade 8 Page 1 of 2
ERP
UNIT 2: Matter and Energy RM 60Lesson 11: Law of Conservation of Mass
C + O2 CO2
C + O O OO C
reactants product
Elements Found in Equation
Reactant Atoms
(number of)
Product Atoms
(number of)
Lesson 11: Law of Conservation of Mass
128
UNIT 2: Matter and EnergyLesson 8: Analyzing Chemical Formulas
Evaluate
Teacher Instruction
• Instruct students to use the Periodic Table to complete RM 46: Assessment—Analyzing Chemical Formulas.
RM 46 Answer Key
1. D
2. G
3. D
4. F
5. A
MaterialsFor each student• RM 1• RM 46
RM 46 Answer Key
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Gateways to Science, STAAR Edition, Grade 8
Lesson 1: Properties of MatterUNIT 2: Matter and Energy RM 1
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Gateways to Science, STAAR Edition, Grade 8 Page 1 of 2
Choose the best answer for each question.
1 Which element is found in all of these compounds?
C2H6 NaHCO3 C6H12O6 CO2
A Oxygen
B Sodium
C Hydrogen
D Carbon
2 How many atoms of carbon are found in glucose, C6H12O6?
F 3
G 6
H 12
J 24
3 A molecule that contains two or more elements is called —
A an atom
B a mixture
C an ion
D a compound
Assessment—Analyzing Chemical Formulas
UNIT 2: Matter and Energy RM 46Lesson 8: Analyzing Chemical Formulas
SAMPLE
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Unit 2: M
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d En
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Lesson 8: Analyzing Chemical Formulas
EngagePage 64
Science
100 Sheets • 200 Pages • Wide Ruled
9 �/� x 7 �/� in. • 24.7 x 19.0 cm
Which Does Not Belong?
In your science notebook, copy the list below. Circle the one word that does not belong in the group. Finish the statement that explains why it does not belong.
1. silver, nickel, cobalt, iron (not in period 4)
2. neon, nitrogen, helium, argon (not a noble gas)
3. group, period, seven, horizontal rows (not related to the horizontal rows on the Periodic Table)
4. vertical column, group, period, valence electrons (not related to the vertical columns on the Periodic Table)
5. sodium chloride, carbon dioxide, lead, lithium sulfide (not a compound; only has 1 element listed)
6. Cu, Ag, H2O, Au (not an element; has 2 elements listed; not a metal)
7. CH4, NaOH, C6H6, C6H12O6 (not an organic compound; does not contain carbon)
Learning GoalDetermine the number of atoms of each element in chemical formulas.
Engage
Teacher Instruction
• Instruct students to copy Items 1–7 in Engage into their science notebooks, circle the one word that does not belong in each list, and explain why it does not belong. Review the example in the student edition if necessary.
Teacher NoteItems 5 and 7 check students’ prior knowledge. A complete understanding of the concept is not expected at this time. The concept will be further developed in this lesson.SAMPLE
118
UNIT 2: Matter and EnergyLesson 8: Analyzing Chemical Formulas
Explore
Advance Preparation
• Copy RM 40: Matching Cards in color on cardstock. Laminate, cut out, and place in plastic bags.
Teacher Instruction
• Instruct students to use the materials in the plastic bag to complete the Explore activity.
• Circulate among the working groups while making formative assessments and redirecting student thinking as needed.
• Allow students to participate in a gallery walk to compare results.
• Allow students to return cards to plastic bag after the Explain discussion.
ExplorePage 64
Matching Game
Working in small groups, match the graphic card model with a possible chemical formula.
RM 40 Answer Key
Refer to RM 40 for correct placement of cards.
MaterialsFor student pairs• RM 40• sandwich-size
resealable plastic bag
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Gateways to Science, STAAR Edition, Grade 8
Matching Cards
UNIT 2: Matter and Energy RM 40Lesson 8: Analyzing Chemical Formulas
SAMPLE
119© REGION 4 EDUCATION SERVICE CENTER. ALL RIGHTS RESERVED.
Lesson 8: Analyzing Chemical Formulas
Unit 2: M
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d En
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y
Explain, Part 1
Teacher Instruction
• Debrief Explore by guiding students to recognize the relationships between the chemical formulas and the graphic cards.
• Guide students to recognize that the letters represent certain shapes and the number of shapes is equivalent to the subscript.
• Ask students to determine the total number of shapes in each graphic card. Later in the lesson, relate this number to the total number of atoms in a compound.
Explain, Part 2
Teacher Instruction
• Instruct students to read and discuss “Analyzing Chemical Formulas.”
• Display RM 41: Practice—Determining Atoms to model how to determine the atoms per element in a compound.
• Scaffold instruction by modeling several examples first. Use guided practice before students begin to work independently.
• Provide time for students to re-examine their Explore results, allowing revisions. Ensure answers are correct before students disassemble the materials and store them in the plastic bag.
• Instruct student pairs to complete the “Atoms per Element” science notebook entry.
• Review the answers with students, allowing them to revise their answers based on the class discussion.
• Extension Activity: If students are having difficulty determining the number of elements found in a compound or the number of atoms in each element, utilize RM 42: Additional Practice for remediation or additional practice
Explain, Part 1Page 65
Review the Explore card matching activity in a class discussion.
• What do the letters represent?
• What is the total number of shapes on each card?
MaterialsFor student pairs• Matching Cards
arrangement
MaterialsFor teacher• RM 41 • RM 42 (optional)
For each student• RM 41 (optional)• RM 42 (optional)
For student pairs• Matching Cards
arrangement
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Gateways to Science, STAAR Edition, Grade 8
NaCl CH4
Fe2O3 H2SO4
HCl Mg(OH)2
Practice—Determining Atoms
UNIT 2: Matter and Energy RM 41Lesson 8: Analyzing Chemical Formulas
Pb
O
O
O
N
N
N
C
O
O
OCaSAMPLE
120
UNIT 2: Matter and EnergyLesson 8: Analyzing Chemical Formulas
Explain, Part 2Pages 65–66
Analyzing Chemical Formulas
All compounds have definite compositions that can be represented by a chemical formula. Remember that chemical formulas use symbols or a combination of symbols and subscripts to show the elements and the number of atoms of each element that make up the molecule.
For example, the formula for oxygen gas is O2. The subscript indicates two oxygen atoms are bonded in an oxygen molecule as shown in Figure 2.32.
Figure 2.32. Oxygen Molecule
O O
Another common chemical formula is H2O. As shown in Figure 2.33, H2 represents 2 atoms of hydrogen and O represents 1 atom of oxygen. H2O is the chemical formula for the inorganic compound known as water.
Figure 2.33. Molecule of Water
H H
O
The inorganic compound magnesium hydroxide is written as Mg(OH)2. Notice the subscript after the parentheses in Figure 2.34. The subscript indicates there are 2 sets of the OH molecule.
Chemical formulas are a shorthand way to communicate the parts of a molecule. Subscripts are used in formulas to identify how many atoms of a particular element are present. In some cases, parentheses are used to indicate elements grouped together as a molecule within the compound.
Figure 2.34. Magnesium Hydroxide
Mg(OH)2
H HO OMg
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Gateways to Science, STAAR Edition, Grade 8
1. bromine—Br2
2. calcium chloride—CaCl2
3. potassium iodide—KI
4. benzene—C6H6
5. beryllium hydroxide—Be(OH)2
6. copper carbonate—CuCO3
7. magnesium nitrate—Mg(NO3)2
8. magnesium hydroxide—Mg(OH)2
9. silver oxide—Ag2O
10. ammonium hydroxide—NH4OH
11. sodium carbonate—Na2CO3
12. calcium nitrite —Ca(NO2)2
Additional Practice
UNIT 2: Matter and Energy RM 42Lesson 8: Analyzing Chemical Formulas
SAMPLE
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Lesson 8: Analyzing Chemical Formulas
Unit 2: M
atte
r an
d En
erg
y
Science
100 Sheets • 200 Pages • Wide Ruled
9 �/� x 7 �/� in. • 24.7 x 19.0 cm
Atoms per Element
Identify the elements found in each of the following compounds. Then determine the number of atoms in each element.
1. carbon dioxide—CO2
• 1 atom of carbon
• 2 atoms of oxygen
2. glucose—C6H12O6
• 6 atoms of carbon
• 12 atoms of hydrogen
• 6 atoms of oxygen
3. calcium hydroxide—Ca(OH)2
• 1 atom of calcium
• 1 x 2 = 2 atoms of oxygen
• 1 x 2 = 2 atoms of hydrogen
RM 41 Answer Key
CaCO3 Pb(NO)3
NaCl CH4
1 atom of sodium1 atom of chlorine
1 atom of carbon4 atoms of hydrogen
Fe2O3 H2SO4
2 atoms of iron3 atoms of oxygen
2 atoms of hydrogen1 atom of sulfur4 atoms of oxygen
HCl Mg(OH)2
1 atom of hydrogen1 atom of chlorine
1 atom of magnesium2 atoms of oxygen2 atoms of hydrogen
SAMPLE
122
UNIT 2: Matter and EnergyLesson 8: Analyzing Chemical Formulas
RM 42 Answer Key
1. bromine—Br2
• Molecule (not a compound) —1 element
• 2 atoms of bromine
2. calcium chloride—CaCl2
• compound—2 elements
• 1 atom of calcium
• 2 atoms of chlorine
3. potassium iodide—KI
• compound—2 elements
• 1 atom of potassium
• 1 atom of iodine
4. benzene—C6H6
• compound—2 elements
• 6 atoms of carbon
• 6 atoms of hydrogen
5. beryllium hydroxide—Be(OH)2
• compound—3 elements
• 1 atom of beryllium
• 2 atoms of oxygen
• 2 atoms of hydrogen
6. copper carbonate—CuCO3
• compound—3 elements
• 1 atom of copper
• 1 atom of carbon
• 3 atoms of oxygen
7. magnesium nitrate—Mg(NO3)2
• compound—3 elements
• 1 atom of magnesium
• 2 atoms of nitrogen
• 6 atoms of oxygen
8. magnesium hydroxide Mg(OH)2
• compound—3 elements
• 1 atom of magnesium
• 2 atoms of oxygen
• 2 atoms of hydrogen
9. silver oxide Ag2O
• compound—2 elements
• 2 atoms of silver
• 1 atom of oxygen
10. ammonium hydroxide NH4OH
• compound—3 elements
• 1 atom of nitrogen
• 5 atoms of hydrogen
• 1 atom of oxygen
11. sodium carbonate Na2CO3
• compound—3 elements
• 2 atoms of sodium
• 1 atom of carbon
• 3 atoms of oxygen
12. calcium nitrite Ca(NO2)2
• compound—3 elements
• 1 atom of calcium
• 2 atoms of nitrogen
• 4 atoms of oxygen
SAMPLE
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Lesson 8: Analyzing Chemical Formulas
Unit 2: M
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d En
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y
Explain, Part 3
Teacher Instruction
• Read and discuss “Using Coefficients and Subscripts.”
• Display RM 43: Using Coefficients to model how to determine the number of atoms in compounds with coefficients. Scaffold student instruction and involvement as described in previous lessons.
• Check for understanding by directing students to complete the science notebook entry “Chemical Formulas.”
• Display RM 44: Chemical Formulas to discuss the answers after students have had ample time to respond.
• Allow students to revise their answers after discussion.
Explain, Part 3Pages 67–68
Using Coefficients and Subscripts
A coefficient indicates the number of molecules of a compound or element. For example, 6 molecules of water is written as 6H2O.
Figure 2.35. Parts of a Compound
6H2OCoefficient—There are 6 molecules of water.
Subscript—There are 2 atoms of hydrogen in each of the 6 molecules of water.
No subscript—There is only 1 atom of oxygen in each of the 6 molecules of water.
A visual can also be used to help determine the total number of atoms. The coefficient indicates there are 6 molecules of water.
MaterialsFor teacher• RM 43• RM 44
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Gateways to Science, STAAR Edition, Grade 8
Using Coefficients
UNIT 2: Matter and Energy RM 43
6H2O
3CaCl2
Coefficient —There are 6 molecules of water.
Lesson 8: Analyzing Chemical Formulas
Subscript—There are 2 atoms of hydrogen.
No subscript—There is only 1 atom of oxygen.
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Gateways to Science, STAAR Edition, Grade 8
Chemical Formulas
UNIT 2: Matter and Energy RM 44
1. HO
2. HO
3. CHCl
4. 3CO2
5. 2CaSO3
6. 2 Ca(OH)2
Lesson 8: Analyzing Chemical Formulas
H HO O
H H
O
H H
O
H H
O
C Cl
H
H
H C Cl
H
H
HSAMPLE
124
UNIT 2: Matter and EnergyLesson 8: Analyzing Chemical Formulas
Figure 2.36. Visual Model of a Compound
H H
OH H
O
H H
OH H
O
H H
OH H
O
Here’s another way to determine the number of atoms of each element:.
To determine the number of atoms in 6H2O:
6 molecules x 2 hydrogen atoms = 12 hydrogen atoms
6 molecules x 1 oxygen atom = 6 oxygen atoms
Science
100 Sheets • 200 Pages • Wide Ruled
9 �/� x 7 �/� in. • 24.7 x 19.0 cm
Chemical Formulas
In your science notebook, write the chemical formula for each of the following. Place the proper coefficient in the blank and any subscripts needed in the appropriate boxes.
1. H
O
3H2O
2. H
O
H2O2
3. C
H
Cl
2CH3Cl
H HO O
H H
O
H H
O
H H
O
C Cl
H
H
H C Cl
H
H
H
SAMPLE
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Lesson 8: Analyzing Chemical Formulas
Unit 2: M
atte
r an
d En
erg
y
Copy the following formulas into your science notebook. Determine the number of atoms per element represented in each of the following substances. Justify your answers mathematically or with a drawing.
4. 3CO2
3 x 1 carbon = 3 atoms3 x 2 oxygen = 6 atoms
5. 2CaSO3
2 x 1 calcium = 2 atoms2 x 1 sulfur = 2 atoms2 x 3 oxygen = 6 atoms
6. 2Ca(OH)2
2 x 1 calcium = 2 atoms2 x (1 x 2) oxygen = 4 atoms2 x (1 x 2) hydrogen = 4 atom
Facilitation Questions
• What is a coefficient and what does it tell you? A coefficient is a number written in front of a chemical formula indicating how many of those molecules are involved in a chemical reaction.
• What is a subscript and what does it tell you? A subscript is a number within a chemical formula indicating the number of atoms of an element in that molecule.
SAMPLE
126
UNIT 2: Matter and EnergyLesson 8: Analyzing Chemical Formulas
Elaborate
Teacher Instruction
• Instruct students to complete RM 45: Composition of Compounds.
• Once completed, utilize RM 45 to discuss student answers. Allow students to revise answers after discussion.
• Instruct students to complete the science notebook entry “Reflection.”
• Allow students to share their reflections and to revise if needed.
ElaboratePage 69
Follow your teacher’s instructions to complete the Composition of Compounds handout.
Science
100 Sheets • 200 Pages • Wide Ruled
9 �/� x 7 �/� in. • 24.7 x 19.0 cm
Reflection
A friend is having difficulty writing chemical formulas. Create a list of guidelines to help your friend write chemical formulas. Student answers will vary. Possible answers may include: The first letter of an element symbol is always capitalized and the second letter is written lower case. Subscripts are used when there are two or more atoms of the element and are written to the lower right of the element. Subscripts are not used if only one atom of an element is present.
MaterialsFor teacher• RM 45
For each student• RM 45
© REGION 4 EDUCATION SERVICE CENTER. ALL RIGHTS RESERVED.
Gateways to Science, STAAR Edition, Grade 8
Mo
del
Nam
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UNIT 2: Matter and Energy RM 45
Composition of Compounds
Lesson 8: Analyzing Chemical Formulas
SAMPLE
127© REGION 4 EDUCATION SERVICE CENTER. ALL RIGHTS RESERVED.
Lesson 8: Analyzing Chemical Formulas
Unit 2: M
atte
r an
d En
erg
y
RM 45 Answer Key
Model NameElement, Molecule,
CompoundFormula
Total Number
of Elements
Names of Elements
Number of Atoms of Each Element
H H
Owater compound H2O 2
hydrogenoxygen
H = 2 atomsO = 1 atom
HONa sodium hydroxide
compound and molecule
NaOH 3sodiumoxygen
hydrogen
Na = 1 atomO = 1 atomH = 1 atom
Na
1 valenceelectron
CI
7 valenceelectrons
sodium chloride
(salt)compound NaCI 2
sodiumchlorine
Na = 1 atomCl = 1 atom
Ca
2 valenceelectrons
CI
7 valenceelectrons
CI
7 valenceelectrons
calcium chloride
compound CaCl2 2calciumchlorine
Ca = 1 atomCl = 2 atoms
Mg
2 valenceelectrons
O
6 valenceelectrons
magnesiumoxide
compound MgO 2magnesium
oxygenMg = 1 atomO = 1 atom
SAMPLE
128
UNIT 2: Matter and EnergyLesson 8: Analyzing Chemical Formulas
Evaluate
Teacher Instruction
• Instruct students to use the Periodic Table to complete RM 46: Assessment—Analyzing Chemical Formulas.
RM 46 Answer Key
1. D
2. G
3. D
4. F
5. A
MaterialsFor each student• RM 1• RM 46
© REGION 4 EDUCATION SERVICE CENTER. ALL RIGHTS RESERVED.
Gateways to Science, STAAR Edition, Grade 8
Lesson 1: Properties of MatterUNIT 2: Matter and Energy RM 1
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© REGION 4 EDUCATION SERVICE CENTER. ALL RIGHTS RESERVED.
Gateways to Science, STAAR Edition, Grade 8 Page 1 of 2
Choose the best answer for each question.
1 Which element is found in all of these compounds?
C2H6 NaHCO3 C6H12O6 CO2
A Oxygen
B Sodium
C Hydrogen
D Carbon
2 How many atoms of carbon are found in glucose, C6H12O6?
F 3
G 6
H 12
J 24
3 A molecule that contains two or more elements is called —
A an atom
B a mixture
C an ion
D a compound
Assessment—Analyzing Chemical Formulas
UNIT 2: Matter and Energy RM 46Lesson 8: Analyzing Chemical Formulas
SAMPLE
64
UNIT 2: Matter and EnergyLesson 8: Analyzing Chemical Formulas
Engage
Science
100 Sheets • 200 Pages • Wide Ruled
9 �/� x 7 �/� in. • 24.7 x 19.0 cm
Which Does Not Belong?
In your science notebook, copy the list below. Circle the one word that does not belong in the group. Finish the statement that explains why it does not belong.
1. silver, nickel, cobalt, iron (not . . . )
2. neon, nitrogen, helium, argon (not . . . )
3. group, period, seven, horizontal rows (not . . . )
4. vertical column, group, period, valence electrons (not . . . )
5. sodium chloride, carbon dioxide, lead, lithium sulfide (not . . . )
6. Cu, Ag, H2O, Au (not . . . )
7. CH4, NaOH, C6H6, C6H12O6 (not . . . )
Explore
Matching Game
Working in small groups, match the graphic card model with a possible chemical formula.
SAMPLE
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65
Lesson 8: Analyzing Chemical Formulas
Explain, Part 1
Review the Explore card matching activity in a class discussion.
• What do the letters represent?
• What is the total number of shapes on each card?
Explain, Part 2
Analyzing Chemical Formulas
All compounds have definite compositions that can be represented by a chemical formula. Remember that chemical formulas use symbols or a combination of symbols and subscripts to show the elements and the number of atoms of each element that make up the molecule.
For example, the formula for oxygen gas is O2. The subscript indicates two oxygen atoms are bonded in an oxygen molecule as shown in Figure 2.32.
Figure 2.32. Oxygen Molecule
O O
Another common chemical formula is H2O. As shown in Figure 2.33, H2 represents 2 atoms of hydrogen and O represents 1 atom of oxygen. H2O is the chemical formula for the inorganic compound known as water.
Figure 2.33. Molecule of Water
H H
O SAMPLE
66
UNIT 2: Matter and EnergyLesson 8: Analyzing Chemical Formulas
The inorganic compound magnesium hydroxide is written as Mg(OH)2. Notice the subscript after the parentheses in Figure 2.34. The subscript indicates there are 2 sets of the OH molecule.
Chemical formulas are a shorthand way to communicate the parts of a molecule. Subscripts are used in formulas to identify how many atoms of a particular element are present. In some cases, parentheses are used to indicate elements grouped together as a molecule within the compound.
Figure 2.34. Magnesium Hydroxide
Mg(OH)2
H HO OMg
Science
100 Sheets • 200 Pages • Wide Ruled
9 �/� x 7 �/� in. • 24.7 x 19.0 cm
Atoms per Element
Identify the elements found in each of the following compounds. Then determine the number of atoms in each element.
1. carbon dioxide—CO2
2. glucose—C6H12O6
3. calcium hydroxide—Ca(OH)2
SAMPLE
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67© REGION 4 EDUCATION SERVICE CENTER. ALL RIGHTS RESERVED.
67
Lesson 8: Analyzing Chemical Formulas
Explain, Part 3
Using Coefficients and Subscripts
A coefficient indicates the number of molecules of a compound or element. For example, 6 molecules of water is written as 6H2O.
Figure 2.35. Parts of a Compound
6H2OCoefficient—There are 6 molecules of water.
Subscript—There are 2 atoms of hydrogen in each of the 6 molecules of water.
No subscript—There is only 1 atom of oxygen in each of the 6 molecules of water.
A visual can also be used to help determine the total number of atoms. The coefficient indicates there are 6 molecules of water.
Figure 2.36. Visual Model of a Compound
H H
OH H
O
H H
OH H
O
H H
OH H
O
Here’s another way to determine the number of atoms of each element:.
To determine the number of atoms in 6H2O:
6 molecules x 2 hydrogen atoms = 12 hydrogen atoms
6 molecules x 1 oxygen atom = 6 oxygen atoms
SAMPLE
68
UNIT 2: Matter and EnergyLesson 8: Analyzing Chemical Formulas
Science
100 Sheets • 200 Pages • Wide Ruled
9 �/� x 7 �/� in. • 24.7 x 19.0 cm
Chemical Formulas
In your science notebook, write the chemical formula for each of the following. Place the proper coefficient in the blank and any subscripts needed in the appropriate boxes.
1. H
O
2. H
O
3. C
H
Cl
Copy the following formulas into your science notebook. Determine the number of atoms per element represented in each of the following substances. Justify your answers mathematically or with a drawing.
4. 3CO2
5. 2CaSO3
6. 2Ca(OH)2
H HO O
H H
O
H H
O
H H
O
C Cl
H
H
H C Cl
H
H
H
SAMPLE
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69© REGION 4 EDUCATION SERVICE CENTER. ALL RIGHTS RESERVED.
69
Lesson 8: Analyzing Chemical Formulas
Elaborate
Follow your teacher’s instructions to complete the Composition of Compounds handout.
Science
100 Sheets • 200 Pages • Wide Ruled
9 �/� x 7 �/� in. • 24.7 x 19.0 cm
Reflection
A friend is having difficulty writing chemical formulas. Create a list of guidelines to help your friend write chemical formulas.
Evaluate
Use the Periodic Table and your knowledge of compounds and chemical formulas to complete the assessment.
SAMPLE