12. a teacher wants to demonstrate a chemical reaction for her students. which activity below would best demonstrate a chemical reaction?
a. placing metal in an acid to create gas
b. measuring the densities of water and ice
c. melting a solid piece of aluminum
d. filling a balloon with helium gas

13. when a sugar reacts with potassium chlorate, it releases light into the environment. this type of reaction is
a. endothermic c. endergonic
b. exothermic d. exergonic

14. which of the following observations does not indicate that a chemical reaction has occurred?
a. formation of a precipitate c. state of matter change
b. production of a gas d. heat and light released

15. what is the general form for a single - replacement reaction?
a. ax + by → ay + bx c. a + b → ab
b. ab → a + b d. a + bx → ax + b

16. what is the general form for a double - displacement reaction?
a. ax + by → ay + bx c. a + b → ab
b. ab → a + b d. a + bx → ax + b

17. which of the following is a correctly balanced chemical equation?
a. cl₂ + 2nai → 2nacl + i₂ c. cl₂ + nai → nacl + i₂
b. cl₂ + nai → nacl + 2i₂ d. 2cl₂ + nai → 2nacl + i₂

18. which situation provides the best evidence that a chemical reaction is taking place?
a. a metal strip bends when heated
b. a solution freezes in the freezer
c. two solutions mixed in a beaker produce a solid
d. a chemical is dissolved in water

19. in a reaction in which hydrogen reacts with oxygen to produce water, which substances are the reactants?
a. hydrogen only c. water only
b. oxygen only d. both hydrogen and oxygen

20. which of the following is not an example of a chemical change?
a. iron rusting c. an apple ripening
b. cutting paper d. a piece of wood burning

21. which of the following equations best represents “magnesium reacting with chlorine to produce magnesium chloride?”
a. mg → cl₂ + mgcl₂ c. mgcl₂ + mg → cl₂
b. mgcl₂ + mg → cl₂ d. mg + cl₂ → mgcl₂

22. how many atoms of oxygen are represented in 2ca(no₃)₂?
a. 3 b. 6 c. 10 d. 12

23. the symbol (s) written after a formula in a chemical equation stands for
a. solid b. soluble c. solution d. synthesis

24. a solid produced from two liquids in a double displacement chemical reaction is called a
a. reactant b. precipitate c. crystal d. molecule

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Question

12. a teacher wants to demonstrate a chemical reaction for her students. which activity below would best demonstrate a chemical reaction?
a. placing metal in an acid to create gas
b. measuring the densities of water and ice
c. melting a solid piece of aluminum
d. filling a balloon with helium gas

13. when a sugar reacts with potassium chlorate, it releases light into the environment. this type of reaction is
a. endothermic 			c. endergonic
b. exothermic 			d. exergonic

14. which of the following observations does not indicate that a chemical reaction has occurred?
a. formation of a precipitate 	c. state of matter change
b. production of a gas 		d. heat and light released

15. what is the general form for a single - replacement reaction?
a. ax + by → ay + bx 	c. a + b → ab
b. ab → a + b 			d. a + bx → ax + b

16. what is the general form for a double - displacement reaction?
a. ax + by → ay + bx 	c. a + b → ab
b. ab → a + b 			d. a + bx → ax + b

17. which of the following is a correctly balanced chemical equation?
a. cl₂ + 2nai → 2nacl + i₂ 			c. cl₂ + nai → nacl + i₂
b. cl₂ + nai → nacl + 2i₂ 			d. 2cl₂ + nai → 2nacl + i₂

18. which situation provides the best evidence that a chemical reaction is taking place?
a. a metal strip bends when heated
b. a solution freezes in the freezer
c. two solutions mixed in a beaker produce a solid
d. a chemical is dissolved in water

19. in a reaction in which hydrogen reacts with oxygen to produce water, which substances are the reactants?
a. hydrogen only 		c. water only
b. oxygen only 			d. both hydrogen and oxygen

20. which of the following is not an example of a chemical change?
a. iron rusting 			c. an apple ripening
b. cutting paper 			d. a piece of wood burning

21. which of the following equations best represents “magnesium reacting with chlorine to produce magnesium chloride?”
a. mg → cl₂ + mgcl₂ 		c. mgcl₂ + mg → cl₂
b. mgcl₂ + mg → cl₂ 		d. mg + cl₂ → mgcl₂

22. how many atoms of oxygen are represented in 2ca(no₃)₂?
a. 3 		b. 6 		c. 10 		d. 12

23. the symbol (s) written after a formula in a chemical equation stands for
a. solid 		b. soluble 		c. solution 		d. synthesis

24. a solid produced from two liquids in a double displacement chemical reaction is called a
a. reactant 		b. precipitate 		c. crystal 		d. molecule

Accepted Answer

### Question 12
# Brief Explanations:
To determine the best activity to demonstrate a chemical reaction, we analyze each option:
- Option A: Placing metal in acid creates gas (a new substance), indicating a chemical reaction.
- Option B: Measuring densities of water and ice is a physical property comparison, no chemical change.
- Option C: Melting aluminum is a phase change (physical change).
- Option D: Filling a balloon with helium is a physical process (gas transfer), no reaction.
So, A involves a chemical reaction.
# Answer:
A. Placing metal in an acid to create gas

### Question 13
# Brief Explanations:
Reactions releasing energy (like light) are exergonic (releases free energy) and exothermic (releases heat/light). Endothermic/endergonic absorb energy. Since the reaction releases light, it's exergonic (and exothermic, but exergonic relates to free energy). Among options, D (exergonic) fits as it releases energy.
# Answer:
D. exergonic

### Question 14
# Brief Explanations:
Chemical reaction indicators: precipitate (A), gas production (B), heat/light (D) are signs. A state of matter change (C, like melting) can be physical (e.g., ice to water) without a chemical reaction. So C does not indicate a chemical reaction.
# Answer:
C. state of matter change

### Question 15
# Brief Explanations:
Single - replacement reaction: one element replaces another in a compound. The general form is $ A + BX
ightarrow AX + B $.
- Option A: Double - displacement (swap ions).
- Option B: Decomposition.
- Option C: Synthesis.
- Option D: Matches single - replacement.
# Answer:
D. $ A + BX
ightarrow AX + B $

### Question 16
# Brief Explanations:
Double - displacement reaction: ions of two compounds swap. General form: $ AX + BY
ightarrow AY + BX $.
- Option A: Matches double - displacement.
- Option B: Decomposition.
- Option C: Synthesis.
- Option D: Single - replacement.
# Answer:
A. $ AX + BY
ightarrow AY + BX $

### Question 17
# Brief Explanations:
To balance $ Cl_2 + NaI
ightarrow NaCl + I_2 $:
- Balance Cl: $ Cl_2 $ has 2 Cl, so $ 2NaCl $.
- Balance Na: 2 Na from $ 2NaCl $ means 2 $ NaI $.
- Balance I: 2 I from $ 2NaI $ gives $ I_2 $. So the balanced equation is $ Cl_2 + 2NaI
ightarrow 2NaCl + I_2 $ (Option A). Other options have unbalanced atoms (e.g., B has unbalanced I, C unbalanced Cl/Na, D unbalanced Cl/Na).
# Answer:
A. $ Cl_2 + 2NaI
ightarrow 2NaCl + I_2 $

### Question 18
# Brief Explanations:
Analyze each option:
- Option A: Metal bending when heated is a physical change (shape change, no new substance).
- Option B: Solution freezing is a phase change (physical).
- Option C: Two solutions mixing to produce a solid (precipitate) is a chemical reaction (new substance).
- Option D: Dissolving is a physical process (no new substance).
So, C shows a chemical reaction.
# Answer:
C. Two solutions mixed in a beaker produce a solid

### Question 19
# Brief Explanations:
In a chemical reaction, reactants are the substances that react to form products. In $ H_2 + O_2
ightarrow H_2O $, hydrogen ($ H_2 $) and oxygen ($ O_2 $) are reactants, water ($ H_2O $) is the product. So both hydrogen and oxygen are reactants (Option D).
# Answer:
D. Both hydrogen and oxygen

### Question 20
# Brief Explanations:
Chemical changes involve new substances:
- Option A: Iron rusting (new substance, $ Fe_2O_3 $).
- Option B: Cutting paper only changes shape (physical change, no new substance).
- Option C: Apple ripening (chemical changes in enzymes/starches).
- Option D: Wood burning (new substances like $ CO_2 $).
So B is not a chemical change.
# Answer:
B. Cutting paper

### Question 21
# Brief Explanations:
The reaction is magnesium ($ Mg $) reacting with chlorine ($ Cl_2 $) to form magnesium chloride ($ MgCl_2 $). The correct equation is $ Mg + Cl_2
ightarrow MgCl_2 $ (Option D).
- Option A: Incorrect products.
- Option B: Incorrect reaction (reverse).
- Option C: Incorrect reaction (reverse).
# Answer:
D. $ Mg + Cl_2
ightarrow MgCl_2 $

### Question 22
# Explanation:
## Step 1: Analyze the formula $ Ca(NO_3)_2 $
In $ Ca(NO_3)_2 $, for the nitrate ion ($ NO_3^- $), each $ NO_3^- $ has 3 O atoms. And there are 2 $ NO_3^- $ groups in $ Ca(NO_3)_2 $, so O atoms in one $ Ca(NO_3)_2 $ are $ 2\times3 = 6 $.
## Step 2: Account for the coefficient 2
The coefficient 2 in $ 2Ca(NO_3)_2 $ means we have 2 units of $ Ca(NO_3)_2 $. So total O atoms are $ 2\times6=12 $.
# Answer:
D. 12

### Question 23
# Brief Explanations:
In chemical equations, (s) stands for solid, (l) for liquid, (g) for gas, (aq) for aqueous. So (s) means solid (Option A). Soluble (B) is not indicated by (s); solution (C) is (aq); synthesis (D) is a reaction type, not a state.
# Answer:
A. solid

### Question 24
# Brief Explanations:
In a double - displacement reaction, a solid formed from two liquids is called a precipitate (Option B). Reactant (A) is a starting substance, crystal (C) is a solid structure, molecule (D) is a particle. So B is correct.
# Answer:
B. precipitate

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