Elements & Compounds
Question: Which is an element?
A. H₂O
B. CO₂
C. O₂
D. NaCl
Answer: C — O₂
Question: How many oxygen atoms are in:
CO₂
Answer: 2 oxygen atoms
Question: Which is a physical change?
A. Wood burning
B. Ice melting
C. Metal rusting
D. Milk souring
Answer: B — Ice melting.
Question: What is the most important question when determining whether a chemical change occurred?
Answer: Did a new substance form?
Question: In saltwater, which substance is the solvent?
Water
Question: Which correctly describes CO?
A. One carbon atom only
B. One cobalt atom
C. Carbon and oxygen bonded together
D. Two oxygen atoms bonded together
Answer: C — Carbon and oxygen bonded together.
Question: How many total atoms are in:
H₂SO₄
Answer: 7
2 H + 1 S + 4 O = 7
Question: A large salt crystal is crushed into powder.
Physical or chemical change? Why?
Answer: Physical. Its size and shape changed, but it remains salt.
Question: Two clear liquids are mixed and an unexpected solid forms.
What is the solid called?
Answer: A precipitate.
Question: Sugar is dissolved in water.
Identify:
Solute = ?
Solvent = ?
Solution = ?
Answer:
Solute = Sugar
Solvent = Water
Solution = Sugar-water solution
Question: A student says:
“N₂ is a compound because it contains two atoms.”
Is the student correct? Explain.
Answer: No. N₂ is an element because it contains only one type of atom—nitrogen.
Question: Analyze:
Ca(OH)₂
How many different elements and total atoms are present?
Answer: 3 elements, 5 atoms
Ca = 1
O = 2
H = 2
Question: Water changes:
H₂O(s) → H₂O(l)
What happens to the water molecules?
Answer: Their arrangement and movement change, but their molecular identity remains H₂O.
Question: A liquid begins bubbling.
A student immediately concludes:
“Chemical change!”
Why isn't that enough information?
Answer: Bubbles can occur during a physical change, such as boiling. More evidence/context is needed to determine whether a new substance formed
Question: What makes a solution an aqueous solution?
Water is the solvent
Question: Classify each as an element or compound:
Cl₂ | CaCl₂ | Mg | MgO
Answer:
Cl₂ = Element
CaCl₂ = Compound
Mg = Element
MgO = Compound
Question: Analyze:
Mg₃(PO₄)₂
Determine the number of:
Mg = ?
P = ?
O = ?
Total atoms = ?
Answer:
Mg = 3
P = 2
O = 8
Total = 13 atoms
Question: Sugar dissolves completely in water.
A student argues:
“The sugar disappeared, so it underwent a chemical change.”
Explain the error.
Answer: The sugar didn't disappear. Its particles became distributed throughout the water. There isn't sufficient evidence that a new substance formed.
Question: Two liquids are mixed.
Observations:
Which conclusion is best supported?
Answer: A chemical change is strongly supported because multiple unexpected observations indicate that new substances may have formed.
Question: What vocabulary term describes decreasing the concentration by adding more solvent?
Dilution
Question: Two students disagree.
Student A: “If a substance contains more than one atom, it must be a compound.”
Student B: “The number of atoms doesn't determine whether something is a compound.”
Who has the stronger argument? Use O₂ and CO₂
Answer: Student B. O₂ contains two atoms but only one type of atom, so it is an element. CO₂ contains carbon and oxygen, so it is a compound
Question: A student analyzes:
Al₂(SO₄)₃
and writes:
Al = 2
S = 1
O = 12
Total = 15
Find the student's mistake and determine the correct total number of atoms.
Answer: The outside subscript 3 multiplies everything inside the parentheses, including sulfur.
Al = 2
S = 3
O = 12
Total = 17 atoms
Question: A substance changes from a rigid solid into a flowing liquid. Its particles become farther apart and move more freely, but the molecules before and after the change are identical.
Determine the type of change
Answer: Physical change.
Evidence:
Question: Examine the particle model:
Before:
AB + AB
After:
A₂ + B₂
A student argues:
“This is physical because the same number and types of atoms are still present.”
Evaluate the student's claim.
Answer: The claim is incorrect. The atoms were conserved, but they formed different bonding arrangements, producing different substances. Therefore, this represents a chemical change.
Question: A solution contains 15 g of solute. More water is added, but no solution is removed.
What happens to the amount of solute and the concentration?
Answer:
Amount of solute → Stays the same
Concentration → Decreases