HS CHEMISTRY PRACTICE QUESTIONS III

1. Which of the examples is NOT an example of a chemical change?

(Refer to the picture for options.)

• A. Calcium carbonate --> Calcium oxide + Carbon dioxide

•  B. Saltwater --> Water vapor + Salt

• C. Magnesium + Oxygen --> Magnesium oxide

• D. Water --> Hydrogen + Oxygen

Answer explanation

When a substance undergoes a physical change, its composition remains the same despite its molecules being rearranged.

Some examples of physical change are freezing of water, melting of wax, boiling of water.

When a substance undergoes a chemical change, its molecular composition is changed entirely.

examples of chemical change are digestion of food, burning of coal, rusting, etc.

2. Which of these chemical equations is INCORRECT?

(Refer to the diagram for the options)

• Option A

• Option B

• Option C

• Option D

Answer explanation

law of conservation of mass, states that during a chemical reaction, the total mass of the products must be equal to the total mass of the reactants. 

3.Which of the following equations are examples of combination reaction?

• A. I and II only

• B. I and III only

• C. II and IV only

• D. I, II and III only

Answer explanation

A combination reaction, also known as a synthesis reaction, is a reaction in which two or more substances combine to form a single new substance.

The general form of a combination reaction is:

A+B→AB

4. Which chemical equation is correctly balanced where x=1, y=2, z= 3 ?

(Refer to the diagram for option)

• Option A

• Option B

• Option C

• Option D

Answer explanation

x Ca(OH)₂ + y HNO₃ --> x Ca(NO₃)₂ + y H₂O

(x=1, y=2, z=3)

For number 5-7, refer to the chemical equation :

5. The skeleton equation represents a chemical reaction.  Which of these are the reactants?

• A. Zn and HCl

• B. ZnCl₂ and H₂

• C. HCl and ZnCl₂

• D. Zn and H₂

Answer explanation

All chemical reactions—including a candle burning—involve reactants and products.

Reactants are substances that start a chemical reaction.

Products are substances that are produced in the reaction.

Reactants → Products

For number 5-7, refer to the chemical equation :

6.The chemical reaction represented by the equation would be classified as a —

• A. synthesis reaction      

• B. decomposition reaction

• C. single-replacement reaction

• D. double-replacement reaction

Answer explanation

A single-replacement reaction is a reaction in which one element replaces a similar element in a compound

The general form of a single-replacement (also called single-displacement) reaction is:

A+BC→AC+B

Zn+ 2 HCl --> ZnCl_² + H₂

For number 5-7, refer to the chemical equation :

7.The skeleton equation for this chemical reaction violates the law of conservation of mass. Which of these is the correct balanced chemical equation?

• 
• 
• 
• 

Answer explanation

Zn + 2 HCl --> ZnCl₂ + H₂

1 - Zn - 1

2 - Cl - 2

2 - H - 2

8. What are the missing coefficients for the skeleton equation below?

• A. 4,6,6,2     

• B. 2,3,2,3

•    C.2,3,3,2

• D.1,3,3,1

Answer explanation

Reactants vs Products

2 -Cr- 2

3 -Fe- 3

6=2*3 -N- 3*2 = 6

6 = 3*2-O- 2*3= 6

9. When heated in a closed container in the presence of a catalyst, potassium chlorate decomposes into potassium chloride and oxygen gas via the following reaction:

2KClO₃ (s) → 2KCl(s) + 3O₂ (g)

If 12.25 g of potassium chlorate decomposes, how many grams of oxygen gas will be generated?

• A. 1.60 g

• B. 3.20 g

• C. 4.80 g

• D. 18.37 g

Answer explanation

10. In a redox reaction, which of the following is true for the substance that is reduced?

• A. It is also the reducing agent.

• B. It always gains electrons.

• C. It never contains oxygen

• D. It must contain hydrogen.

Answer explanation

The ion or molecule that accepts electrons is called the oxidizing agent - by accepting electrons it oxidizes other species.

The ion or molecule that donates electrons is called the reducing agent - by giving electrons it reduces the other species.

Hence, what is oxidized is the reducing agent and what is reduced is the oxidizing agent.