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NABTEB Chemistry Past Questions

Published on Sep 5, 2025 • Education

Introduction

Hello students, and welcome to today’s Chemistry class. In this lesson, we will explore two important topics that are often tested in exams such as NABTEB, WAEC, and NECO:

1. Differences between Physical and Chemical Change
2. Electrovalency

These concepts form the foundation for understanding many reactions in Chemistry. After the explanations, we will also provide practice multiple-choice questions to help you prepare better.

Physical vs. Chemical Change

Key Differences

1. Formation of New Substances
   • Physical Change: No new substances are formed.
   • Chemical Change: New substances are formed.
2. Reversibility
   • Physical Change: Easily reversible (e.g., melting of ice).
   • Chemical Change: Not easily reversible (e.g., rusting of iron).
3. Mass of Substance
   • Physical Change: No change in mass of the substance.
   • Chemical Change: Change in mass occurs due to reaction.
4. Energy Involved
   • Physical Change: Accompanied by a small energy change.
   • Chemical Change: Involves a large energy change.
5. Separation of Components
   • Physical Change: Components can be separated by physical means (e.g., evaporation of salt solution).
   • Chemical Change: Components cannot be separated by physical means (e.g., burning paper).

Examples

• Physical Change: Dissolving salt in water, melting ice.
• Chemical Change: Burning wood, cooking food, rusting of iron.

Electrovalency

Definition

Electrovalency is the number of electrons an atom loses or gains to form an ion during a chemical reaction.

• It refers to the combining capacity of an atom when it forms an ionic bond.
• Electrovalency is always expressed as a positive whole number.

Examples

• Sodium (Na) loses 1 electron to form Na+Na^+Na+. Its electrovalency is +1.
• Magnesium (Mg) loses 2 electrons to form Mg2+Mg^{2+}Mg2+. Its electrovalency is +2.
• Chlorine (Cl) gains 1 electron to form Cl−Cl^-Cl−. Its electrovalency is –1.

Practice Questions

Question 1

Which of the following is a physical change?

A. Burning of wood
B. Rusting of iron
C. Melting of ice
D. Cooking of rice

Answer: C. Melting of ice

Question 2

A chemical change is usually:

A. Easily reversible
B. Accompanied by a small energy change
C. Accompanied by a large energy change
D. Independent of mass change

Answer: C. Accompanied by a large energy change

Question 3

Electrovalency refers to:

A. The number of protons lost by an atom
B. The number of neutrons gained by an atom
C. The number of electrons lost or gained by an atom
D. The number of bonds formed in a molecule

Answer: C. The number of electrons lost or gained by an atom

Question 4

A. 1
B. 2
C. 3
D. 4

Answer: B. 2

Question 5

Which of the following is a chemical change?

A. Dissolving sugar in water
B. Melting of candle wax
C. Evaporation of water
D. Combustion of petrol

Answer: D. Combustion of petrol

Question 6 – Reversibility

Which of the following changes is not easily reversible?

A. Freezing of water
B. Boiling of water
C. Burning of paper
D. Condensation of steam

Answer: C. Burning of paper

Question 7 – Ionic Bond

An ionic bond is formed when:

A. Electrons are shared between atoms
B. Protons are transferred between atoms
C. Neutrons are transferred between atoms
D. Electrons are transferred from one atom to another

Answer: D. Electrons are transferred from one atom to another

Question 8 – Example of Electrovalency

The electrovalency of sodium in NaClNaClNaCl is:

A. 1
B. 2
C. 3
D. 4

Answer: A. 1

Question 9 – Separation

Which method can best separate a mixture of salt and water?

A. Filtration
B. Distillation
C. Chromatography
D. Sublimation

Answer: B. Distillation

Question 10 – Exam Application

Which statement best describes a physical change?

A. It always involves the formation of a new substance.
B. It is always irreversible.
C. It does not involve the formation of new substances.
D. It is always accompanied by a large energy change.

Answer: C. It does not involve the formation of new substances.

Watch the full tutorial here: FULL TUTORIAL

Conclusion

In summary, a physical change does not produce a new substance and is usually reversible, while a chemical change produces a new substance and is irreversible.

Electrovalency explains how atoms combine by losing or gaining electrons to form ions, and it is fundamental to understanding ionic bonding.

Practicing questions on these topics will strengthen your preparation for NABTEB, WAEC, NECO, and JAMB Chemistry exams.

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