9 when you put ice in a glass of water and put the glass in the refrigerator, what will the ice do? does this represent a physical or chemical change?
law of conservation of mass
determine if the following chemical reactions are balanced (b) or unbalanced (u)
10. ______ 4fes + 7o₃ → 2fe₂o₃ + 4so₃
11. ______ pcl₅ + 4h₂o → h₃po₄ + 5hcl
12. ______ 8co + 4h₂ → c₈h₃ + 2h₂o
13. ______ 2cu + o₁ → 2cuo
14. what is missing from the following equation, and how do you know?
6co₂ + 6h₂o → 6o₂ + c₆h₁₂o₆
45g 20g → 14g ?
15. which equations best represents the chemical reaction below
a) 2cuo → 2cu + o₂
b) 2cu + o₂ → 2cuo
c) 2cu + 2o → cuo
d) cuo → 2cu + 2o
16. an experiment is conducted in an open test tube. the mass of the reactants is 20 grams. after the reaction, the mass of the products was 15 grams. what could account for the missing mass? was the law of conservation of mass broken? how could this problem be eliminated?

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Accepted Answer

### Question 9
# Answer:
The ice will freeze (or the water will freeze more, and the ice may grow as the water freezes); this is a physical change.
# Brief Explanations:
When ice is in water and placed in a refrigerator (cold environment), the water temperature drops. If the temperature goes below $0^\circ\text{C}$, the water will freeze, and the ice can grow as liquid water turns to solid ice. A physical change occurs because the substance (water/ice) remains $H_2O$ throughout—only the state (solid/liquid) changes, with no new substance formed.

### Question 10
# Answer:
Unbalanced (U)
# Brief Explanations:
- **Step 1: Count Fe atoms**
  Left: $4$ Fe (from $4\text{FeS}$); Right: $2\times2 = 4$ Fe (from $2\text{Fe}_2\text{O}_3$) → Fe is balanced.
- **Step 2: Count S atoms**
  Left: $4$ S (from $4\text{FeS}$); Right: $4$ S (from $4\text{SO}_3$) → S is balanced.
- **Step 3: Count O atoms**
  Left: $7\times2 = 14$ O (from $7\text{O}_2$); Right: $(2\times3)+(4\times3)=6 + 12 = 18$ O (from $2\text{Fe}_2\text{O}_3$ and $4\text{SO}_3$) → O is unbalanced.

### Question 11
# Answer:
Balanced (B)
# Brief Explanations:
- **Step 1: Count P atoms**
  Left: $1$ P (from $\text{PCl}_5$); Right: $1$ P (from $\text{H}_3\text{PO}_4$) → P is balanced.
- **Step 2: Count Cl atoms**
  Left: $5$ Cl (from $\text{PCl}_5$); Right: $5$ Cl (from $5\text{HCl}$) → Cl is balanced.
- **Step 3: Count H atoms**
  Left: $4\times2 = 8$ H (from $4\text{H}_2\text{O}$); Right: $3 + 5 = 8$ H (from $\text{H}_3\text{PO}_4$ and $5\text{HCl}$) → H is balanced.
- **Step 4: Count O atoms**
  Left: $4\times1 = 4$ O (from $4\text{H}_2\text{O}$); Right: $4$ O (from $\text{H}_3\text{PO}_4$) → O is balanced.

### Question 12
# Answer:
Unbalanced (U)
# Brief Explanations:
- **Step 1: Count C atoms**
  Left: $8$ C (from $8\text{CO}$); Right: $8$ C (from $\text{C}_8\text{H}_3$) → C is balanced.
- **Step 2: Count O atoms**
  Left: $8$ O (from $8\text{CO}$); Right: $2$ O (from $2\text{H}_2\text{O}$) → O is unbalanced.
- **Step 3: Count H atoms**
  Left: $4\times2 = 8$ H (from $4\text{H}_2$); Right: $3 + 2\times2 = 7$ H (from $\text{C}_8\text{H}_3$ and $2\text{H}_2\text{O}$) → H is unbalanced.

### Question 13
# Answer:
Balanced (B)
# Brief Explanations:
- **Step 1: Count Cu atoms**
  Left: $2$ Cu (from $2\text{Cu}$); Right: $2$ Cu (from $2\text{CuO}$) → Cu is balanced.
- **Step 2: Count O atoms**
  Left: $2$ O (from $\text{O}_2$); Right: $2$ O (from $2\text{CuO}$) → O is balanced.

### Question 14
# Answer:
A reactant or product mass (or the reaction condition "light/energy" for photosynthesis); we know via the Law of Conservation of Mass.
# Brief Explanations:
The equation represents photosynthesis ($6\text{CO}_2 + 6\text{H}_2\text{O}\xrightarrow{\text{light}}6\text{O}_2 + \text{C}_6\text{H}_{12}\text{O}_6$), so "light energy" is missing (reaction condition). For mass: Total reactant mass = $45 + 20 = 65\,\text{g}$; total product mass = $14 +?$. By conservation of mass, the missing mass of $\text{C}_6\text{H}_{12}\text{O}_6$ should be $65 - 14 = 51\,\text{g}$ (or the reaction needs light to proceed).

### Question 15
# Answer:
B. $2\text{Cu} + \text{O}_2
ightarrow 2\text{CuO}$
# Brief Explanations:
- The diagram shows $2$ Cu atoms (single circles) + $1$ $\text{O}_2$ molecule (double circle) → $2$ $\text{CuO}$ units (each CuO is a Cu - O pair, and there are $2$ pairs, forming $2$ $\text{CuO}$ molecules).
- Option A is decomposition (CuO → Cu + $\text{O}_2$), opposite of the diagram.
- Option C uses "O" (atoms, not $\text{O}_2$ molecules) and is unbalanced.
- Option B matches: $2$ Cu + $\text{O}_2$ → $2$ CuO (reactants: $2$ Cu, $1$ $\text{O}_2$; products: $2$ CuO).

### Question 16
# Answer:
Missing mass: Gaseous products escaped (e.g., $\text{CO}_2$, $\text{H}_2$ vapor); Law of Conservation of Mass is not broken; Eliminate by conducting the reaction in a closed system.
# Brief Explanations:
- In an open test tube, gaseous products (e.g., $\text{CO}_2$, $\text{H}_2$ from reactions) escape into the air, reducing the measured product mass.
- The Law of Conservation of Mass holds for closed systems (mass is conserved in the entire universe, but open systems lose mass to the surroundings).
- To eliminate this, perform the reaction in a sealed container (closed system) to trap all products.

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