Are you ready to test your knowledge of Chemical Combination? This Elements & Compounds Test 3 covers important topics such as valency, writing chemical formulae, ions, cations and anions, common compounds, and the laws of chemical combination. Take this TNPSC Chemistry test to check your understanding and see how well you can answer questions from these important concepts. After completing the test, go through the explanations to revise the topics and strengthen your TNPSC preparation.
Note: Detailed explanations for all questions are available below the test, allowing you to review your responses and strengthen your subject knowledge.
Valency, Formulae & Chemical Combination – Elements & Compounds Test 3 : Important Details
In this section, you can go through the detailed explanations for all 20 questions in the TNPSC Chemistry Elements & Compounds Test 3 – Chemical Combination. The explanations cover important concepts such as valency, chemical formulae, ions, cations and anions, common compounds, and the laws of chemical combination. Reading the explanations after taking the test will help you understand why each answer is correct, clear up any doubts, and quickly revise the important points needed for TNPSC Chemistry preparation.
1. What is meant by the valency of an element?
Answer: B. Combining capacity of the element
Explanation:
Valency refers to the combining capacity of an element. It indicates the ability of an atom to combine with other atoms to form a chemical compound. The valency of an element is generally related to the number of electrons an atom loses, gains, or shares during chemical combination to attain a stable electronic configuration. For example, sodium has a valency of 1 because it loses one electron, while oxygen has a valency of 2 because it generally gains or shares two electrons. Thus, valency helps us understand how atoms combine with one another and is important in writing the chemical formulae of compounds.
Why the other options are incorrect:
- A. Atomic mass refers to the mass of an atom and does not indicate its combining capacity.
- C. Number of neutrons determines part of the atomic mass but does not directly represent valency.
- D. Number of shells indicates the energy levels occupied by electrons, not the combining capacity.
2. What is the usual valency of oxygen?
Answer: B. 2
Explanation:
The usual valency of oxygen is 2. An oxygen atom has six electrons in its outermost shell and needs two more electrons to achieve a stable electronic configuration. Therefore, oxygen generally gains two electrons or shares electrons with other atoms during chemical combination. Because of this, oxygen has a combining capacity of 2 and is said to have a valency of 2. This can be seen in common compounds such as water (H₂O), where one oxygen atom combines with two hydrogen atoms, and magnesium oxide (MgO), where magnesium combines with oxygen in a 1:1 ratio.
Why the other options are incorrect:
- A. 1 is not the usual valency of oxygen.
- C. 3 does not represent the usual combining capacity of oxygen.
- D. 4 is not the usual valency of oxygen in common compounds.
3. What is the usual valency of aluminium?
Answer: C. 3
Explanation:
The usual valency of aluminium is 3. Aluminium has three electrons in its outermost shell. During chemical combination, an aluminium atom generally loses these three electrons to attain a stable electronic configuration. Therefore, its combining capacity is 3, and it forms an Al³⁺ ion. This valency is reflected in compounds such as aluminium oxide (Al₂O₃), where two aluminium atoms combine with three oxygen atoms to balance their valencies. Hence, the usual valency of aluminium is 3.
Why the other options are incorrect:
- A. 1 does not represent the usual combining capacity of aluminium.
- B. 2 is not the usual valency of aluminium.
- D. 4 is not the usual valency of aluminium.
4. What is the valency of sodium in sodium chloride?
Answer: A. 1
Explanation:
The valency of sodium is 1. A sodium atom has one electron in its outermost shell and generally loses this electron during chemical combination to attain a stable electronic configuration. By losing one electron, sodium forms a positively charged Na⁺ ion. In sodium chloride (NaCl), one sodium atom combines with one chlorine atom because sodium has a valency of 1 and chlorine also has a valency of 1. Therefore, the combining capacity or valency of sodium in sodium chloride is 1.
Why the other options are incorrect:
- B. 2 is not the usual valency of sodium.
- C. 3 is not the usual valency of sodium.
- D. 4 is not the usual valency of sodium.
5. Which formula represents calcium chloride?
Answer: B. CaCl₂
Explanation:
The chemical formula of calcium chloride is CaCl₂. Calcium has a valency of 2, while chlorine has a valency of 1. To form a neutral compound, one calcium atom combines with two chlorine atoms. The valency of calcium is therefore balanced by two chlorine atoms, giving the formula CaCl₂. The subscript 2 after Cl shows that two chlorine atoms are present for every one calcium atom. Thus, calcium chloride is correctly represented by CaCl₂.
Why the other options are incorrect:
- A. CaCl does not correctly balance the valencies of calcium and chlorine.
- C. Ca₂Cl does not give the correct ratio of calcium to chlorine.
- D. Ca₂Cl₂ can be simplified to CaCl and therefore does not represent the correct formula of calcium chloride.
6. Which formula represents Aluminium oxide?
Answer: D. Al₂O₃
Explanation:
The chemical formula of aluminium oxide is Al₂O₃. Aluminium has a valency of 3, while oxygen has a valency of 2. To form a neutral compound, the valencies must be balanced. Two aluminium atoms provide a total valency of 6, while three oxygen atoms provide a total valency of 6. Therefore, the correct ratio of aluminium to oxygen is 2:3, giving the formula Al₂O₃. This is an important example of how the valencies of elements are used to determine the chemical formula of a compound.
Why the other options are incorrect:
- A. AlO does not balance the valencies of aluminium and oxygen.
- B. Al₂O does not give the correct ratio required for a neutral compound.
- C. AlO₂ also does not correctly balance the valencies of aluminium and oxygen.
7. Which formula represents magnesium oxide?
Answer: A. MgO
Explanation:
The chemical formula of magnesium oxide is MgO. Magnesium has a valency of 2, while oxygen also has a valency of 2. Since their valencies are equal, one magnesium atom combines with one oxygen atom to form a neutral compound. Therefore, the ratio of magnesium to oxygen is 1:1, giving the formula MgO. This is a simple example of using the valencies of two elements to determine the correct chemical formula.
Why the other options are incorrect:
- B. Mg₂O does not correctly balance the valencies of magnesium and oxygen.
- C. MgO₂ gives an incorrect ratio of magnesium to oxygen.
- D. Mg₂O₃ does not represent the correct valency ratio for magnesium oxide.
8. Which formula represents sodium sulphide?
Answer: B. Na₂S
Explanation:
The chemical formula of sodium sulphide is Na₂S. Sodium has a valency of 1, while sulphur has a valency of 2 when it forms sulphide. Therefore, two sodium atoms are required to combine with one sulphur atom so that their valencies are balanced. The two sodium atoms contribute a total valency of 2, which balances the valency of one sulphur atom. Hence, the correct formula is Na₂S.
Why the other options are incorrect:
- A. NaS does not balance the valencies of sodium and sulphur.
- C. NaS₂ gives an incorrect ratio of sodium to sulphur.
- D. Na₂S₂ does not represent the simplest whole-number ratio of the ions.
9. A positively charged ion is called a
Answer: B. Cation
Explanation:
A positively charged ion is called a cation. An atom becomes a cation when it loses one or more electrons. Since electrons carry negative charges, losing electrons leaves the atom with more positive charge than negative charge. For example, a sodium atom loses one electron and forms a Na⁺ ion, which is a cation. Metals commonly form cations by losing electrons during chemical combination. Therefore, the correct term for a positively charged ion is cation.
Why the other options are incorrect:
- A. Anion refers to a negatively charged ion.
- C. Molecule is a group of atoms chemically bonded together and is not a term for a positively charged ion.
- D. Isotope refers to atoms of the same element having the same number of protons but different numbers of neutrons.
10. A negatively charged ion is called an
Answer: C. Anion
Explanation:
A negatively charged ion is called an anion. An atom becomes an anion when it gains one or more electrons. Since electrons have a negative charge, gaining electrons gives the atom an overall negative charge. For example, a chlorine atom gains one electron to form a Cl⁻ ion, which is an anion. Non-metal elements commonly form anions by gaining electrons during chemical combination. Therefore, the correct term for a negatively charged ion is anion.
Why the other options are incorrect:
- A. Atom refers to the basic unit of an element and is not specifically a negatively charged ion.
- B. Cation refers to a positively charged ion.
- D. Isotope refers to atoms of the same element with different numbers of neutrons.
11. Which of the following is a cation?
Answer: C. Na⁺
Explanation:
A cation is a positively charged ion. It is formed when an atom loses one or more electrons. Among the given options, Na⁺ carries a positive charge, so it is a cation. Sodium forms Na⁺ by losing one electron from its outermost shell and thereby attains a more stable electronic configuration. In contrast, Cl⁻, O²⁻, and S²⁻ carry negative charges and are therefore anions. Hence, Na⁺ is the correct answer.
Why the other options are incorrect:
- A. Cl⁻ is negatively charged and is an anion.
- B. O²⁻ is negatively charged and is an anion.
- D. S²⁻ is negatively charged and is an anion.
12. Which of the following is an anion?
Answer: D. Cl⁻
Explanation:
An anion is a negatively charged ion. It is generally formed when an atom gains one or more electrons. Among the given options, Cl⁻ has a negative charge and is therefore an anion. A chlorine atom gains one electron to form the chloride ion, Cl⁻, with a stable electronic configuration. The other options, Na⁺, Ca²⁺, and Al³⁺, carry positive charges and are cations. Therefore, Cl⁻ is the correct answer.
Why the other options are incorrect:
- A. Na⁺ is positively charged and is a cation.
- B. Ca²⁺ is positively charged and is a cation.
- C. Al³⁺ is positively charged and is a cation.
13. Which statement is correct about an ionic compound?
Answer: B. It consists of oppositely charged ions
Explanation:
An ionic compound consists of positively and negatively charged ions held together by electrostatic forces of attraction. Ionic compounds are generally formed when a metal transfers one or more electrons to a non-metal. The metal forms a positively charged cation, while the non-metal forms a negatively charged anion. For example, sodium chloride (NaCl) contains Na⁺ and Cl⁻ ions, which are held together by strong electrostatic attraction. Therefore, the correct statement is that an ionic compound consists of oppositely charged ions.
Why the other options are incorrect:
- A. It contains only neutral atoms is incorrect because ionic compounds contain charged ions.
- C. It contains only non-metal atoms is incorrect because ionic compounds commonly contain both metals and non-metals.
- D. It cannot contain a metal is incorrect because metals commonly form the positive ions in ionic compounds.
14. Which formula represents calcium oxide?
Answer: A. CaO
Explanation:
The chemical formula of calcium oxide is CaO. Calcium has a valency of 2, while oxygen also has a valency of 2. Since the valencies are equal, one calcium atom combines with one oxygen atom to form a neutral compound. Therefore, the ratio of calcium to oxygen is 1:1, giving the formula CaO. This illustrates that when the valencies of the combining elements are equal, the simplest whole-number ratio of their atoms may be 1:1.
Why the other options are incorrect:
- B. Ca₂O does not correctly balance the valencies of calcium and oxygen.
- C. CaO₂ gives an incorrect ratio for calcium oxide.
- D. Ca₂O₃ does not represent the correct valency ratio of calcium and oxygen.
15. Which formula represents aluminium chloride?
Answer: C. AlCl₃
Explanation:
The chemical formula of aluminium chloride is AlCl₃. Aluminium has a valency of 3, while chlorine has a valency of 1. Therefore, one aluminium atom combines with three chlorine atoms so that the total positive and negative valencies are balanced. One aluminium atom contributes a valency of 3, while three chlorine atoms contribute a total valency of 3. Hence, the correct formula is AlCl₃. This is another example of using the valencies of the elements to determine the chemical formula of an ionic compound.
Why the other options are incorrect:
- A. AlCl does not balance the valencies of aluminium and chlorine.
- B. AlCl₂ does not provide the correct ratio of aluminium to chlorine.
- D. Al₂Cl also does not balance the valencies correctly.
16. According to the law of conservation of mass, in a chemical reaction
Answer: C. Total mass remains constant
Explanation:
The law of conservation of mass states that mass can neither be created nor destroyed during a chemical reaction. In a chemical reaction, the substances present before the reaction, called reactants, are converted into new substances called products. Although the properties of the substances may change, the total mass remains unchanged. This is because the atoms involved in a chemical reaction are only rearranged to form new substances; they are not created or destroyed. Therefore, in a closed system, the total mass of the reactants is equal to the total mass of the products.
Why the other options are incorrect:
- A. Mass is created is incorrect because mass is not created during a chemical reaction.
- B. Mass is destroyed is incorrect because mass is not destroyed during a chemical reaction.
- D. Mass always increases is incorrect because the total mass remains constant in a closed system.
17. Who proposed the law of conservation of mass?
Answer: B. Antoine Lavoisier
Explanation:
The law of conservation of mass was proposed by the French chemist Antoine Lavoisier. Through careful experiments, Lavoisier showed that the total mass of substances remains unchanged during a chemical reaction when the reaction is carried out in a closed system. His experiments provided strong evidence that matter is neither created nor destroyed during ordinary chemical reactions. Lavoisier made important contributions to the development of modern chemistry and is widely regarded as one of the founders of modern chemistry. Therefore, Antoine Lavoisier is the correct answer.
Why the other options are incorrect:
- A. John Dalton is mainly associated with the development of atomic theory.
- C. J. J. Thomson is known for the discovery of the electron.
- D. Ernest Rutherford is associated with the discovery of the atomic nucleus and important work on atomic structure.
18. The law of constant proportions states that a pure compound always contains its elements in
Answer: B. Fixed proportions by mass
Explanation:
The law of constant proportions, also known as the law of definite proportions, states that a pure compound always contains the same elements combined in the same fixed proportion by mass, regardless of its source or method of preparation. For example, pure water always contains hydrogen and oxygen in a fixed mass ratio of 1:8. This means that the composition of a pure compound remains definite and does not change from one sample to another. Therefore, the correct answer is fixed proportions by mass.
Why the other options are incorrect:
- A. Variable proportions by mass is incorrect because the elements in a pure compound are present in a definite ratio.
- C. Equal proportions by volume is incorrect because the law refers specifically to mass proportions.
- D. Random proportions by mass is incorrect because the elements combine in a fixed and definite mass ratio.
19. Water always contains hydrogen and oxygen in a fixed ratio by mass. This illustrates the
Answer: B. Law of constant proportions
Explanation:
The law of constant proportions, also known as the law of definite proportions, states that a pure chemical compound always contains the same elements combined in a fixed proportion by mass. Water is a compound made up of hydrogen and oxygen, and these elements are always present in the same mass ratio of 1:8, regardless of where the water is obtained or how it is prepared. This fixed composition shows that the elements in a pure compound do not combine randomly or in changing proportions. Therefore, the example of water containing hydrogen and oxygen in a fixed ratio by mass illustrates the law of constant proportions.
Why the other options are incorrect:
- A. Law of conservation of mass states that mass is neither created nor destroyed during a chemical reaction.
- C. Law of multiple proportions deals with the different simple whole-number ratios in which two elements can combine to form more than one compound.
- D. Law of reciprocal proportions describes the relationship between the combining masses of elements when they combine separately with a third element.
20. Which statement best describes chemical combination?
Answer: B. Elements combine chemically in definite proportions to form compounds
Explanation:
Chemical combination is the process in which two or more elements combine chemically in definite proportions to form a new substance called a compound. During this process, the atoms of the elements undergo chemical changes and form new chemical bonds, so the resulting compound has properties different from those of the elements from which it is formed. For example, hydrogen and oxygen chemically combine in a definite proportion to form water. The elements do not simply retain their original identities as they would in a physical mixture. Therefore, the statement that elements combine chemically in definite proportions to form compounds best describes chemical combination.
Why the other options are incorrect:
- A. Elements combine physically without changing their identity describes a physical mixture rather than chemical combination.
- C. Elements always combine in equal quantities is incorrect because elements combine in specific proportions, which are not necessarily equal.
- D. Compounds are formed only by mixing metals is incorrect because compounds can be formed from metals, non-metals, or both.
Quick Revision For This Test Questions
Before attempting the test, revise these important concepts related to Valency, Chemical Formulae, Ions, Chemical Combination and Chemical Laws.
🔹 Valency
- Valency is the combining capacity of an element.
- It is related to the number of electrons an atom loses, gains, or shares during chemical combination.
- The usual valency of oxygen is 2.
- The usual valency of aluminium is 3.
- The valency of sodium is 1.
🔹 Common Chemical Formulae
- Magnesium oxide → MgO
- Calcium oxide → CaO
- Calcium chloride → CaCl₂
- Sodium sulphide → Na₂S
- Aluminium chloride → AlCl₃
- Aluminium oxide → Al₂O₃
The chemical formula of an ionic compound is determined by balancing the valencies of the ions so that the compound is electrically neutral.
🔹 Cations and Anions
- A positively charged ion is called a cation.
- A negatively charged ion is called an anion.
- Cations are generally formed when atoms lose electrons.
- Anions are generally formed when atoms gain electrons.
- Na⁺, Ca²⁺ and Al³⁺ are examples of cations.
- Cl⁻, O²⁻ and S²⁻ are examples of anions.
🔹 Ionic Compounds
- An ionic compound consists of oppositely charged ions.
- Positive and negative ions are held together by electrostatic forces of attraction.
- Ionic compounds are commonly formed through the transfer of electrons between a metal and a non-metal.
🔹 Law of Conservation of Mass
- The law of conservation of mass was proposed by Antoine Lavoisier.
- It states that mass can neither be created nor destroyed during a chemical reaction.
- In a closed system, the total mass of reactants equals the total mass of products.
- In an ordinary chemical reaction, atoms are rearranged, rather than created or destroyed.
🔹 Law of Constant Proportions
- The law of constant proportions is also called the law of definite proportions.
- It states that a pure compound always contains the same elements in a fixed proportion by mass.
- For example, water always contains hydrogen and oxygen in the mass ratio 1:8.
🔹 Chemical Combination
- Chemical combination occurs when two or more elements combine chemically in definite proportions to form a compound.
- A compound has properties that are different from those of the elements from which it is formed.
- Chemical combination is different from a physical mixture, where substances retain their individual identities.
Important Facts to Remember
Valency → Combining capacity
Oxygen → 2
Aluminium → 3
Sodium → 1
Positive ion → Cation
Negative ion → Anion
Lavoisier → Conservation of mass
Constant proportions → Fixed proportion by mass
Na⁺ → Cation
Cl⁻ → Anion
MgO → Magnesium oxide
CaO → Calcium oxide
CaCl₂ → Calcium chloride
Na₂S → Sodium sulphide
AlCl₃ → Aluminium chloride
Al₂O₃ → Aluminium oxide
Further Reference Material for Valency, Formulae & Chemical Combination
For further reference, you can read the relevant NCERT Science textbook material to strengthen your understanding of the concepts covered in this test.
