RBSE Solutions Class 9 Science Chapter 4 Chemical Bond and Chemical Equation

Objective Type Questions

 

Question 1. Symbol of Na is:
(a) S
(b) Si
(c) Na
(d) Ni
Answer: (c) Na
In simple words: The chemical symbol for the element Sodium is Na.

🎯 Exam Tip: Memorizing the symbols of common elements is crucial for chemistry. A quick recall helps save time in exams.

 

Question 2. The formula of carbonate radical is:
(a) \( \text{CO}_2 \)
(b) \( \text{CO}_3^{2-} \)
(c) \( \text{CO}_3^{1-} \)
(d) \( \text{CO} \)
Answer: (b) \( \text{CO}_3^{2-} \)
In simple words: A carbonate radical is a group of atoms (carbon and oxygen) that has an overall electrical charge of 2- (two negative charges).

🎯 Exam Tip: Pay close attention to the charge and the subscript numbers in chemical formulas to correctly identify radicals.

 

Question 3. The chemical bond present in sodium chloride is:
(a) Ionic
(b) covalent
(c) Metallic
(d) Hydrogen
Answer: (a) Ionic
In simple words: In sodium chloride, electrons are transferred from sodium to chlorine, creating positive and negative ions that are strongly attracted to each other.

🎯 Exam Tip: Ionic bonds typically form between a metal and a non-metal due to electron transfer, while covalent bonds form between non-metals by sharing electrons.

 

Question 6. The inventor of the modern system of symbol for the elements is:
(a) Berzelius
(b) John Dalton
(c) Rutherford
(d) Neils Bohr
Answer: (a) Berzelius
In simple words: Berzelius was a scientist who created the system we use today where elements are represented by one or two letters.

🎯 Exam Tip: Knowing the key historical figures in chemistry helps understand the development of scientific concepts and notation.

 

Question 7. Which molecule in the following has covalent bond?
(a) \( \text{H}_2\text{O} \)
(b) \( \text{NaCl} \)
(c) \( \text{CaO} \)
(d) \( \text{CaCO}_3 \)
Answer: (a) \( \text{H}_2\text{O} \)
In simple words: Water \( (\text{H}_2\text{O}) \) has covalent bonds because hydrogen and oxygen atoms share electrons with each other.

🎯 Exam Tip: Covalent bonds occur when atoms share electrons, typically between non-metals, leading to molecular compounds like water.

 

Question 8. Fe is the symbol of:
(a) Iron
(b) Copper
(c) Gold
(d) Silver
Answer: (a) Iron
In simple words: The chemical symbol Fe stands for the element Iron.

🎯 Exam Tip: Many elemental symbols come from their Latin names, such as Fe from Ferrum for Iron.

Chemical Bond And Chemical Equation Very Short Answer Type Questions

 

Question 10. Define the ionic bond.
Answer: An ionic bond is a strong link formed between atoms when one atom completely gives away electrons to another. This typically happens between an electropositive element (like a metal) and an electronegative element (like a non-metal). The electropositive atom donates electrons from its outermost shell, becoming a positively charged ion, while the electronegative atom accepts these electrons, becoming a negatively charged ion. These oppositely charged ions then attract each other strongly, forming the ionic bond. This attraction holds the compound together very tightly.
In simple words: An ionic bond forms when atoms completely transfer electrons, creating positive and negative ions that attract each other very strongly.

🎯 Exam Tip: Remember that ionic bonds involve a complete transfer of electrons, leading to the formation of stable ions with opposite charges.

 

Question 11. Write the Latin name and symbol of potassium.
Answer: The Latin name for potassium is Kalium, and its chemical symbol is \( \text{K} \). This Latin origin is why potassium's symbol is K, not P.
In simple words: Potassium's Latin name is Kalium, and its symbol is \( \text{K} \).

🎯 Exam Tip: Many elements have symbols derived from their Latin or Greek names, which is why they don't always match their English names.

 

Question 12. Define molecular formula.
Answer: A molecular formula is a chemical formula that shows the exact number of atoms of each element present in one molecule of a substance. It provides a precise count of every atom. For example, the molecular formula for water is \( \text{H}_2\text{O} \), indicating two hydrogen atoms and one oxygen atom.
In simple words: A molecular formula tells you the exact number of each type of atom in one molecule of a substance.

🎯 Exam Tip: Differentiate molecular formula from empirical formula; molecular formula shows actual numbers, while empirical formula shows the simplest ratio.

 

Question 13. Write the formula of calcium carbonate.
Answer: The chemical formula for calcium carbonate is \( \text{CaCO}_3 \). This compound is a common substance found in rocks and shells.
In simple words: Calcium carbonate is written as \( \text{CaCO}_3 \).

🎯 Exam Tip: When writing formulas, always balance the valencies (charges) of the ions to ensure the compound is electrically neutral.

 

Question 14. Write the name of a negative acidic radical having tri-valency.
Answer: The name of a negative acidic radical having tri-valency is phosphate. Its chemical formula is \( \text{PO}_4^{3-} \). This radical is important in many biological processes.
In simple words: Phosphate is a negative radical with a valency of three.

🎯 Exam Tip: Remember common polyatomic ions and their charges, such as phosphate \( (\text{PO}_4^{3-}) \) and sulphate \( (\text{SO}_4^{2-}) \).

 

Question 16. Define valency. Give its one example.
Answer: Valency is defined as the combining capacity of an element, often expressed as the number of hydrogen atoms with which one atom of that element can combine directly or indirectly. For example, one atom of nitrogen combines with three atoms of hydrogen to form ammonia gas \( (\text{NH}_3) \). Therefore, the valency of nitrogen in ammonia is 3, showing its combining power.
In simple words: Valency tells us how many bonds an atom can form. For instance, nitrogen has a valency of 3 because it joins with three hydrogen atoms in ammonia.

🎯 Exam Tip: Valency helps predict how atoms will combine to form compounds. For many elements, it relates to the number of electrons gained, lost, or shared to achieve a stable outer shell.

Chemical Bond And Chemical Equation Short Answer Type Questions

 

Question 17. Why is cation smaller than its corresponding atom?
Answer: A cation is smaller than its neutral parent atom for two main reasons. Firstly, when a metal atom forms a cation, it loses one or more electrons from its outermost shell. This loss of electrons means there are now fewer electrons than protons, increasing the net positive charge from the nucleus which pulls the remaining electrons closer. Secondly, sometimes the entire outermost electron shell is lost, leading to a significant decrease in the atomic radius. This removal of the outermost shell makes the cation much smaller.
In simple words: A cation is smaller because it loses outer electrons, making the remaining electrons pull closer to the nucleus. Sometimes, it even loses an entire electron shell.

🎯 Exam Tip: Remember that losing electrons (to form a cation) always reduces atomic size, while gaining electrons (to form an anion) always increases it.

 

Question 18. What is variable valency? Explain with an example,
Answer: Variable valency is when an element can show more than one combining capacity or valency. For instance, iron is a good example. Iron atoms have two electrons in their outermost shell (valence shell). If they lose these two electrons, their valency becomes 2, forming \( \text{Fe}^{2+} \) (ferrous) ions. However, under certain conditions, iron can also lose an additional electron from the shell just beneath the outermost one. In this case, it loses a total of three electrons, resulting in a valency of 3, forming \( \text{Fe}^{3+} \) (ferric) ions. The Latin name for iron is Ferrum. So, iron demonstrates both 2 and 3 valencies depending on the situation.
In simple words: Variable valency means an element can have different combining powers. For example, iron can lose two or three electrons, so it has valencies of 2 and 3.

🎯 Exam Tip: Transition metals frequently exhibit variable valency due to the involvement of inner d-shell electrons in bonding.

 

Question 19. What is a co-ordinate bond? Explain with an example.
Answer: A co-ordinate bond, also known as a dative covalent bond, is a type of covalent bond where both of the shared electrons in the bond come from only one of the two atoms. In a normal covalent bond, each atom contributes one electron to the shared pair, but in a co-ordinate bond, one atom donates a lone pair of electrons to be shared by both atoms. For example, in the formation of an ammonium ion \( (\text{NH}_4^{+}) \), the nitrogen atom in ammonia \( (\text{NH}_3) \) donates its lone pair of electrons to a hydrogen ion \( (\text{H}^{+}) \) to form a co-ordinate bond. This special sharing leads to a stable molecule.
In simple words: A co-ordinate bond is when one atom gives both electrons for a shared pair, instead of each atom giving one.

🎯 Exam Tip: Look for a lone pair of electrons on one atom and an empty orbital on another atom as indicators for the formation of a co-ordinate bond.

 

Question 20. Explain ionization energy.
Answer: Ionization energy is the minimum amount of energy needed to remove the most loosely held electron from an isolated gaseous atom or ion in its ground state. This process results in the formation of a positively charged ion (cation). Essentially, it's the energy required to overcome the attraction between the electron and the nucleus, setting the electron free. For example, removing the first electron is called the first ionization energy, the second electron requires the second ionization energy, and so on.
In simple words: Ionization energy is the energy needed to take away the outermost electron from an atom when it is a gas.

🎯 Exam Tip: Ionization energy trends across the periodic table are important: it generally increases across a period and decreases down a group.

 

Question 21. Explain the double bond and triple bond with examples.
Answer: A double bond forms when two atoms share two pairs of electrons, while a triple bond forms when they share three pairs of electrons. For example, two oxygen atoms \( (\text{O}) \) achieve stability by forming a double bond with each other. Each oxygen atom has six electrons in its outer shell. By sharing two pairs of electrons, both atoms fulfill their octet, creating molecular oxygen \( (\text{O}=\text{O}) \). Similarly, atoms like nitrogen form a triple bond. Each nitrogen atom has five electrons in its outer shell. By sharing three pairs of electrons, both nitrogen atoms become stable, resulting in the nitrogen molecule \( (\text{N}\equiv\text{N}) \). These bonds make the molecules very stable.
In simple words: A double bond means two pairs of electrons are shared (like in \( \text{O}=\text{O} \)), and a triple bond means three pairs are shared (like in \( \text{N}\equiv\text{N} \)).

🎯 Exam Tip: Remember that a single bond shares one pair, a double bond two pairs, and a triple bond three pairs of electrons, with increasing strength and decreasing bond length.

Chemical Bond And Chemical Equation Long Answer Type Questions

 

Question 22. Explain the differences between covalent compound and an ionic compound.
Answer: The differences between covalent and ionic compounds are as follows:

In simple words: Covalent compounds share electrons and are usually gases or liquids, don't conduct electricity, and react slowly. Ionic compounds transfer electrons to form ions, are usually solids, conduct electricity when dissolved, and react quickly.

🎯 Exam Tip: Focus on electron transfer vs. sharing, physical state, and electrical conductivity as key distinguishing features between ionic and covalent compounds.

 

Question 23. Write the molecular formula of the following:
(a) Sodium Carbonate
(b) Zinc Sulphide Ferric
(c) Aluminum Oxide
(d) Sulphate Magnesium
Answer:
(b) Zinc Sulphide: Symbol \( \text{Zn} \), Charges \( +2 \), \( \text{S} \), \( -2 \), Formula \( \text{ZnS} \)
(c) Aluminium Oxide: Symbol \( \text{Al} \), Charges \( +3 \), \( \text{O} \), \( -2 \), Formula \( \text{Al}_2\text{O}_3 \)
(d) Ferric Sulphate: Symbol \( \text{Fe} \), Charges \( +3 \), \( \text{SO}_4 \), \( -2 \), Formula \( \text{Fe}_2(\text{SO}_4)_3 \)
(e) Barium Chloride: Symbol \( \text{Ba} \), Charges \( +2 \), \( \text{Cl} \), \( -1 \), Formula \( \text{BaCl}_2 \)
(f) Magnesium Carbonate: Symbol \( \text{Mg} \), Charges \( +2 \), \( \text{CO}_3 \), \( -2 \), Formula \( \text{MgCO}_3 \)
In simple words: To find the formula, write the symbols and their charges, then criss-cross the numbers to balance them and form the compound.

🎯 Exam Tip: Always write the cation first and the anion second. Use subscripts to denote the number of ions needed to achieve a neutral compound, dropping the charges.

 

Question 24. Balance the equations
(a) \( \text{KClO}_3 \rightarrow \text{KCl} + \text{O}_2 \uparrow \)
(b) \( \text{BaCl}_2 + \text{AgNO}_3 \rightarrow \text{AgCl} \downarrow + \text{Ba}(\text{NO}_3)_2 \)
(c) \( \text{Mg} + \text{HCl} \rightarrow \text{MgCl}_2 + \text{H}_2 \)
(d) \( \text{NaOH} + \text{Cl}_2 \rightarrow \text{NaCl} + \text{NaOCl} + \text{H}_2\text{O} \)
Answer:
(a) \( 2\text{KClO}_3 \rightarrow 2\text{KCl} + 3\text{O}_2 \uparrow \)
(b) \( \text{BaCl}_2 + 2\text{AgNO}_3 \rightarrow 2\text{AgCl} \downarrow + \text{Ba}(\text{NO}_3)_2 \)
(c) \( \text{Mg} + 2\text{HCl} \rightarrow \text{MgCl}_2 + \text{H}_2 \)
(d) \( 2\text{NaOH} + \text{Cl}_2 \rightarrow \text{NaCl} + \text{NaOCl} + \text{H}_2\text{O} \)
In simple words: To balance equations, make sure there is the same number of each type of atom on both sides of the arrow.

🎯 Exam Tip: Start balancing complex atoms first (like polyatomic ions or elements appearing in only one reactant and one product), then balance hydrogen and oxygen, and finish with simple elements.

 

Question 1. Which scientist gave the method to get gold from mercury?
(a) Nagarjuna
(b) Berzelius
(c) Dalton
(d) Rutherford
Answer: (d) Rutherford
In simple words: Rutherford was a scientist known for his work in nuclear physics and transmutation, which involves changing one element into another.

🎯 Exam Tip: Remember Rutherford's gold foil experiment, which led to the discovery of the atomic nucleus, a key step in understanding nuclear transformations.

 

Question 2. The radical of phosphate among the following radicals is.
(a) \( \text{PO}_4^{2-} \)
(b) \( \text{PO}_4^{3-} \)
(c) \( \text{PO}_2^4 \)
(d) \( \text{PO}_2^{2-} \)
Answer: (b) \( \text{PO}_4^{3-} \)
In simple words: The phosphate radical is a group of one phosphorus and four oxygen atoms that carries a 3- electrical charge.

🎯 Exam Tip: The charge of a polyatomic ion is crucial for writing correct chemical formulas for compounds containing it.

 

Question 3. The molecular formula of magnesium nitrate is-
(a) \( \text{Mg}(\text{NO}_3)_2 \)
(b) \( \text{MgNO}_3 \)
(c) \( \text{Mg}_2(\text{NO}_3) \)
(d) \( \text{Mg}(\text{NO}_3)_3 \)
Answer: (a) \( \text{Mg}(\text{NO}_3)_2 \)
In simple words: Magnesium nitrate has one magnesium atom and two nitrate groups to balance the charges.

🎯 Exam Tip: When writing formulas with polyatomic ions, use parentheses if you need more than one of that ion to balance charges.

 

Question 4. The group of elements showing variable valency is-
(a) Fe,Zn
(b) Mg,Cu
(c) Fe,Cu
(d) Na,K
Answer: (c) Fe,Cu
In simple words: Iron (Fe) and Copper (Cu) can both have more than one valency, meaning they can form different numbers of bonds.

🎯 Exam Tip: Transition metals like iron and copper are common examples of elements exhibiting variable valency because they can use d-shell electrons in bonding.

 

Question 6. An ionic bond is formed
(a) By giving and taking (exchange) of electrons
(b) By sharing of electrons
(c) By sharing of electrons on one side
(d) By the pairing of electrons
Answer: (a) By giving and taking (exchange) of electrons
In simple words: An ionic bond happens when one atom gives electrons away and another atom takes them, creating charged particles that stick together.

🎯 Exam Tip: Ionic bonds occur through the complete transfer of electrons, typically between a metal and a non-metal, leading to the formation of ions.

 

Question 7. The element which shows variable valency is-
(a) Calcium
(b) Zinc
(c) Magnesium
(d) Iron
Answer: (d) Iron
In simple words: Iron can have a valency of 2 or 3, depending on the chemical reaction, which is called variable valency.

🎯 Exam Tip: Elements from the transition metal block often show variable valency due to the availability of electrons in more than one energy shell for bonding.

 

Question 8. The person who first used the word valency was-
(a) Frankland
(b) Gaber
(c) Berzelius
(d) Priestley
Answer: (a) Frankland
In simple words: Edward Frankland was a chemist who first used the term "valency" to describe how elements combine.

🎯 Exam Tip: Historical context can sometimes provide clues for multiple-choice questions about scientific discoveries and terminology.

 

Question 9. The molecular formula of a metallic chloride is \( \text{MCl} \). The molecular formula of its sulphate will be-
(a) \( \text{MSO}_4 \)
(b) \( \text{M}(\text{SO}_4)_3 \)
(c) \( \text{M}_2\text{SO}_4 \)
(d) \( \text{M}_2(\text{SO}_4)_2 \)
Answer: (c) \( \text{M}_2\text{SO}_4 \)
In simple words: If the metal (M) forms \( \text{MCl} \), its valency is 1. Since sulfate \( (\text{SO}_4) \) has a valency of 2, two metal atoms are needed to balance one sulfate, giving \( \text{M}_2\text{SO}_4 \).

🎯 Exam Tip: To determine the formula of a compound with a polyatomic ion, first find the valency of the metal from its simple compound, then balance it with the charge of the polyatomic ion.

Chemical Bond And Chemical Equation Very Short Answer Type Questions

 

Question 1. Name the elements having the following symbols: Au, Pb, Sn, Mn, Sr, and Si.
Answer:
Symbol: \( \text{Au} \) , Name: Gold
Symbol: \( \text{Pb} \) , Name: Lead
Symbol: \( \text{Sn} \) , Name: Tin
Symbol: \( \text{Mn} \) , Name: Manganese
Symbol: \( \text{Sr} \) , Name: Strontium
Symbol: \( \text{Si} \) , Name: Silicon
All these symbols are derived from the Latin or English names of the elements.
In simple words: The symbols represent Gold, Lead, Tin, Manganese, Strontium, and Silicon.

🎯 Exam Tip: Practice linking element symbols to their names, especially those derived from Latin, as this is a fundamental skill in chemistry.

 

Question 2. Give one example each of a strong electrolyte and a weak electrolyte.
Answer: An example of a strong electrolyte is sodium chloride \( (\text{NaCl}) \), which completely dissociates into ions in water. An example of a weak electrolyte is acetic acid \( (\text{CH}_3\text{COOH}) \), which only partially ionizes in water. Strong electrolytes conduct electricity very well, while weak ones conduct poorly.
In simple words: Sodium chloride \( (\text{NaCl}) \) is a strong electrolyte, and acetic acid \( (\text{CH}_3\text{COOH}) \) is a weak electrolyte.

🎯 Exam Tip: Strong electrolytes are usually soluble ionic compounds or strong acids/bases, while weak electrolytes are typically weak acids or bases.

 

Question 3. Give one difference between a molecule and a radical.
Answer: The main difference is that a molecule is an electrically neutral group of two or more atoms held together by chemical bonds, meaning it has no net electrical charge. In contrast, a radical (or ion) is an atom or a group of atoms that carries either a positive or a negative electrical charge. For example, \( \text{H}_2\text{O} \) is a molecule, but \( \text{OH}^- \) is a radical.
In simple words: A molecule has no electrical charge, but a radical always carries a positive or negative charge.

🎯 Exam Tip: Remember that the presence of a charge is the defining characteristic of an ion or radical, distinguishing it from a neutral molecule.

 

Question 4. Name any four elements which have variable valency.
Answer: Four elements that exhibit variable valency are: 1. Mercury, 2. Iron, 3. Copper, 4. Lead. These elements, mostly transition metals, can form different types of ions depending on the chemical environment.
In simple words: Mercury, Iron, Copper, and Lead can all have different valencies.

🎯 Exam Tip: Elements with variable valency are common among transition metals and can form compounds with different properties depending on their oxidation state.

 

Question 6. Define a chemical equation.
Answer: A chemical equation is a symbolic representation of an actual chemical reaction. It uses chemical formulas for reactants and products, along with symbols to indicate the direction of the reaction and physical states. It’s a concise way to describe chemical changes. For example, \( 2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O} \) represents the formation of water from hydrogen and oxygen.
In simple words: A chemical equation uses symbols and formulas to show what happens in a chemical reaction.

🎯 Exam Tip: Ensure your chemical equations are balanced to follow the law of conservation of mass, meaning atoms are neither created nor destroyed.

 

Question 7. What do you mean by a chemical formula?
Answer: A chemical formula is a concise way to show the composition of a chemical compound or molecule using chemical symbols and numerical subscripts. It represents a group of symbols of elements that combine to form one molecule of a substance. For instance, the formula of water, \( \text{H}_2\text{O} \), tells us that one molecule of water contains two hydrogen atoms and one oxygen atom.
In simple words: A chemical formula uses letters and small numbers to show what elements are in a substance and how many atoms of each element there are.

🎯 Exam Tip: Always write the symbol of the element followed by a subscript number to show how many atoms of that element are present in the molecule.

 

Question 8. What do you understand by molecular formula? Give one example.
Answer: A molecular formula is a type of chemical formula that shows the exact number of atoms of each element present in one molecule of a substance. It provides a complete picture of the molecule's composition. For example, the molecular formula of ammonia gas is \( \text{NH}_3 \). This formula indicates that one molecule of ammonia consists of one atom of nitrogen and three atoms of hydrogen. This helps distinguish it from other nitrogen-hydrogen compounds.
In simple words: A molecular formula tells you the exact count of each type of atom in a molecule. Like \( \text{NH}_3 \) shows one nitrogen and three hydrogen atoms.

🎯 Exam Tip: Remember that molecular formulas represent the actual composition, unlike empirical formulas, which show the simplest whole-number ratio of atoms.

 

Question 9. Write the formula of magnesium nitride.
Answer: To write the formula for magnesium nitride, we consider the valencies. Magnesium \( (\text{Mg}) \) has a valency of \( +2 \), and nitride \( (\text{N}) \) has a valency of \( -3 \). By criss-crossing these valencies to balance the charges, we get the formula \( \text{Mg}_3\text{N}_2 \). This means three magnesium atoms combine with two nitrogen atoms.
In simple words: The formula for magnesium nitride is \( \text{Mg}_3\text{N}_2 \), balancing magnesium's 2+ charge with nitrogen's 3- charge.

🎯 Exam Tip: When forming compounds, ensure the total positive charge from the cations equals the total negative charge from the anions to make the compound electrically neutral.

 

Question 10. Which of the following is trivalent? Sodium, magnesium, and aluminum.
Answer: Among sodium, magnesium, and aluminum, **aluminum** is trivalent. Sodium \( (\text{Na}) \) has a valency of 1, and magnesium \( (\text{Mg}) \) has a valency of 2. Aluminum \( (\text{Al}) \) has a valency of 3, meaning it can form three chemical bonds. This is due to its electron configuration.
In simple words: Aluminum is trivalent because it has a valency of 3, while sodium has 1 and magnesium has 2.

🎯 Exam Tip: Valency can often be determined by an element's group number in the periodic table (for main group elements) or by its ability to gain or lose electrons to achieve a stable octet.

 

Question 11. Write the symbols for the elements copper, lead, gold, mercury.
Answer: The symbols for the given elements are: Copper - \( \text{Cu} \) (from Latin name cuperum), Lead - \( \text{Pb} \) (from Latin name plumbum), Gold - \( \text{Au} \) (from Latin name aurum), Mercury - \( \text{Hg} \) (from Latin name hydrargyrum). These elements have ancient Latin names from which their symbols are derived, rather than their English names.
In simple words: The symbols are Copper \( (\text{Cu}) \), Lead \( (\text{Pb}) \), Gold \( (\text{Au}) \), and Mercury \( (\text{Hg}) \).

🎯 Exam Tip: Familiarize yourself with elements whose symbols do not directly correspond to their English names, as these are common points of confusion.

Chemical Bond And Chemical Equation Short Answer Type Questions

 

Question 1. Write the cations and anions present (if any) in the following compounds:
(a) \( \text{CH}_3\text{COONa} \)
(b) \( \text{NaCl} \)
(c) \( \text{H}_2 \)
(d) \( \text{NH}_4\text{NO}_3 \)
Answer:
(a) \( \text{CH}_3\text{COONa} \): Cation- \( \text{Na}^+ \), Anion- \( \text{CH}_3\text{COO}^- \)
(b) \( \text{NaCl} \): Cation- \( \text{Na}^+ \), Anion- \( \text{Cl}^- \)
(c) \( \text{H}_2 \): Cation - None, Anion - None (Hydrogen gas is a molecule, not an ionic compound)
(d) \( \text{NH}_4\text{NO}_3 \): Cation- \( \text{NH}_4^+ \), Anion- \( \text{NO}_3^- \)
In simple words: We find the positive (cation) and negative (anion) parts of each compound. For molecules like \( \text{H}_2 \), there are no separate ions.

🎯 Exam Tip: Identify ionic compounds by the presence of a metal and a non-metal or polyatomic ions. Covalent molecules typically do not form cations and anions in solution.

 

Question 2. The formula of copper sulphate is \( \text{CuSO}_4 \). What is the valency of copper, if the valency of \( \text{SO}_4 \) is 2?
Answer: The formula for copper sulphate is \( \text{CuSO}_4 \). We are given that the valency of the sulphate radical \( (\text{SO}_4) \) is 2, specifically \( \text{SO}_4^{2-} \). For the compound to be electrically neutral, the positive charge from the copper ion must balance the negative charge from the sulphate ion. Therefore, if \( \text{SO}_4 \) has a \( 2- \) charge, copper \( (\text{Cu}) \) must have a \( 2+ \) charge. This means the valency of copper in this compound is 2. The compound contains an equal number of copper and sulfate ions.
In simple words: Since the sulphate group has a valency of 2, copper must also have a valency of 2 to balance the charges in \( \text{CuSO}_4 \).

🎯 Exam Tip: In a neutral ionic compound, the total positive valency must always equal the total negative valency. Use this principle to find unknown valencies.

 

Question 3. Give the formulae of the compounds formed from the following sets of elements:
(1) Calcium and fluorine
(2) Hydrogen and sulphur
(3) Nitrogen and hydrogen
(4) Carbon and chlorine
(5) Sodium and oxygen
(6) Carbon and oxygen.
Answer:
(1) Calcium and fluorine: Symbol \( \text{Ca} \), Charges \( +2 \), \( \text{F} \), \( -1 \), Formula \( \text{CaF}_2 \)
(2) Hydrogen and sulphur: Symbol \( \text{H} \), Charges \( +1 \), \( \text{S} \), \( -2 \), Formula \( \text{H}_2\text{S} \)
(3) Nitrogen and hydrogen: Symbol \( \text{N} \), Charges \( +3 \), \( \text{H} \), \( -1 \), Formula \( \text{NH}_3 \)
(4) Carbon and chlorine: Symbol \( \text{C} \), Charges \( +4 \), \( \text{Cl} \), \( -1 \), Formula \( \text{CCl}_4 \)
(5) Sodium and oxygen: Symbol \( \text{Na} \), Charges \( +1 \), \( \text{O} \), \( -2 \), Formula \( \text{Na}_2\text{O} \)
(6) Carbon and oxygen: Symbol \( \text{C} \), Charges \( +2 \), \( \text{O} \), \( -1 \), Formula \( \text{CO}_2 \)
In simple words: Combine the elements using their valencies, criss-crossing the numbers to form neutral compounds. For example, Calcium is 2+ and Fluorine is 1-, so you need two Fluorine atoms for one Calcium atom.

🎯 Exam Tip: The "criss-cross" method is a helpful visual aid for writing chemical formulas, but always simplify the subscripts to the lowest whole-number ratio.

 

Question 5. Find out the valency of the atoms represented by the figure (i) and (ii).
Answer:
(1) The electronic configuration for figure (i) is 2, 8, 8. This means the atom has a full outermost shell, so its valency is 0, indicating it is stable and does not tend to gain or lose electrons.
(2) The electronic configuration for figure (ii) is 2, 8, 7. This atom needs one more electron to complete its outermost shell and achieve stability, so its valency is 1.
In simple words: For the first atom, electrons are in shells of 2, 8, then 8, making it stable (valency 0). For the second atom, electrons are 2, 8, then 7, meaning it needs one more electron to be stable (valency 1).

🎯 Exam Tip: Valency is the combining capacity of an element. It is determined by the number of electrons an atom needs to gain, lose, or share to achieve a stable electron configuration, usually 8 electrons in its outermost shell (octet rule).

 

Question 6. The valency of Cl is 1 and not 7. Give reason.
Answer: The electronic configuration of Chlorine (Cl) is 2, 8, 7, because its atomic number is 17. Chlorine needs just 1 electron to complete its outermost shell and become stable (achieve an octet). While it could theoretically lose 7 electrons to reach stability, gaining one electron is much easier and requires significantly less energy. This is why chlorine's valency is 1, as it prefers to gain one electron rather than lose seven. Its ability to form bonds depends on this tendency.
In simple words: Chlorine has 7 electrons in its outer shell. It is much easier for it to gain 1 electron to become stable than to lose all 7 electrons. So, its valency is 1.

🎯 Exam Tip: Elements tend to achieve a stable electron configuration, usually an octet (8 electrons) in their outermost shell, by gaining, losing, or sharing the fewest possible electrons. This principle dictates their valency.

 

Question 7. Write the chemical formulae of the compounds formed by the following ions:
(1) \( \text{Mg}^{2+} \) and \( \text{S}^{2-} \)
(2) \( \text{Cu}^{2+} \) and \( \text{OH}^- \)
(3) \( \text{Al}^{3+} \) and \( \text{Br}^- \)
Name the compounds formed in each case.
Answer:
(1) Magnesium Sulphide

(2) Copper(II) Hydroxide

(3) Aluminium Bromide

In simple words: To find the chemical formula, you "criss-cross" the numerical values of the charges of each ion. If the charges are the same, they cancel out. If a group of atoms (like OH) has a charge, put it in brackets if you need more than one of that group.

🎯 Exam Tip: Remember to use parentheses for polyatomic ions (like hydroxide, sulfate, nitrate) when their subscript in the formula is greater than one. This ensures the correct number of the entire ion is represented.

 

Question 8. Why are ionic compounds hard?
Answer: Ionic compounds are hard because they have very strong electrostatic forces, which are the powerful attractions between oppositely charged ions. These forces bind the ions tightly together in a regular, repeating arrangement called a crystal lattice. Since these ions are held so strongly in fixed positions and cannot be easily moved apart, ionic compounds require a significant amount of energy to break them, making them hard. This strong bonding explains their rigidity.
In simple words: Ionic compounds are hard because positive and negative ions are held very strongly together by electric forces, forming a solid structure that is difficult to break apart.

🎯 Exam Tip: The hardness of ionic compounds is a direct result of the strong electrostatic attraction between their constituent ions, which forms a rigid crystal structure and requires a lot of energy to overcome.

 

Question 9. Why are ionic compounds brittle?
Answer: Ionic compounds are brittle because when an external force is applied, the layers of ions can shift slightly. This shift brings ions with the same charge next to each other in a straight line. When like charges come close, they strongly repel each other. This strong repulsion causes the layers to break apart, making the ionic compound brittle and prone to shattering. Despite their hardness, they cannot withstand sharp impacts or deformation easily.
In simple words: Ionic compounds break easily because when hit, layers of ions shift. This causes same-charged ions to line up and push each other away strongly, making the material split.

🎯 Exam Tip: Remember that while strong electrostatic forces make ionic compounds hard, the specific arrangement of ions can lead to brittleness if layers are displaced and repulsion between like charges occurs.

 

Question 10. What is an acidic radical?
Answer: Acidic radicals are negatively charged ions that are obtained from acids, bases, or salts when these substances ionise (break apart into ions) in their aqueous solution. For example, when sodium chloride dissolves, the chloride ion (\( \text{Cl}^- \)) formed is an acidic radical. These radicals are called acidic because they often represent the non-hydrogen part of an acid. Some common examples include hydroxide (\( \text{OH}^- \)), nitrate (\( \text{NO}_3^- \)), oxide (\( \text{O}^{2-} \)), carbonate (\( \text{CO}_3^{2-} \)), sulphate (\( \text{SO}_4^{2-} \)), and phosphate (\( \text{PO}_4^{3-} \)).
In simple words: An acidic radical is a negatively charged ion, usually formed when an acid or a salt breaks apart in water. It helps form the acidic part of a compound.

🎯 Exam Tip: Acidic radicals are always anions (negatively charged ions) and are typically formed when an acid loses its hydrogen ions or when salts dissociate in solution.

 

Question 11. Why is anion bigger than its corresponding atom?
Answer: An anion is formed when a neutral atom gains one or more electrons. When an atom gains electrons, the number of electrons increases, but the number of protons in the nucleus remains the same. This increase in negative charge leads to greater repulsion between the electrons in the electron cloud, causing them to spread out. Additionally, the constant positive pull from the nucleus is now distributed among more electrons, making the attraction to each individual electron weaker. Both these factors contribute to the anion having a larger radius than its parent atom.
In simple words: An anion is bigger than its original atom because it gains extra electrons. These new electrons push each other away, and the nucleus can't pull them in as tightly, causing the electron cloud to expand and making the ion larger.

🎯 Exam Tip: Remember, gaining electrons (forming an anion) increases electron-electron repulsion and effectively decreases the nuclear charge's pull on each electron, leading to a larger atomic radius. Losing electrons (forming a cation) has the opposite effect.

Write the Formulae of the Following

Answer:
(i) Ammonium Sulphate: \( \text{(NH}_4\text{)}_2\text{SO}_4 \)
(ii) Aluminium Carbonate: \( \text{Al}_2\text{(CO}_3\text{)}_3 \)
(iii) Stannous Sulphite: \( \text{SnSO}_3 \) (Stannous indicates Tin with a +2 charge)
In simple words: For ammonium sulphate, ammonium has a +1 charge and sulphate a -2 charge, so you need two ammonium ions. For aluminium carbonate, aluminium is +3 and carbonate is -2, so you need two aluminium and three carbonate ions. For stannous sulphite, both tin (stannous) and sulphite have a +2 and -2 charge respectively, so they combine in a 1:1 ratio.

🎯 Exam Tip: When writing formulas for compounds with polyatomic ions, remember to use parentheses around the ion if there is more than one of them needed to balance the charges.

Chemical Bond and Chemical Equation Long Answer Type Questions

 

Question 1. (A) Write the number of atoms present in
(a) \( \text{SO}_2 \) molecule
(b) \( \text{SO}_4^{2-} \)
(B) Write down the chemical formulae of
(a) Magnesium nitride
(b) Calcium nitrate
(c) Sodium nitrite
Answer:
(A)
(a) Three (1 Sulfur + 2 Oxygen atoms) in a \( \text{SO}_2 \) molecule. Sulfur dioxide is a common air pollutant.
(b) Five (1 Sulfur + 4 Oxygen atoms) in the \( \text{SO}_4^{2-} \) ion. The charge of the ion does not change the number of atoms.
(B)
(a) Magnesium Nitride

(b) Calcium Nitrate

(c) Sodium Nitrite

In simple words: To find the number of atoms, count each element in the formula. For chemical formulae, match the positive and negative charges so the total charge is zero, often by using the criss-cross method for valencies.

🎯 Exam Tip: Always remember that the subscript outside the parentheses of a polyatomic ion applies to all atoms within that ion. For example, in \( \text{Ca(NO}_3\text{)}_2 \), there are two nitrogen atoms and six oxygen atoms.

 

Question 2. What is a chemical equation? Mention the characteristics and limitations of a chemical equation.
Answer: A chemical equation is a symbolic way to show an actual chemical reaction. It uses chemical formulas for the reactants (starting materials) and products (substances formed), connected by an arrow indicating the direction of the reaction. For example, \( \text{N}_2 + 3\text{H}_2 \rightarrow 2\text{NH}_3 \) shows nitrogen and hydrogen reacting to form ammonia, where the coefficients show the number of molecules. This simple representation helps chemists understand reactions quickly.
Characteristics of a Chemical Equation:
1. It shows the reactants and products involved in a reaction.
2. It tells us about the number of molecules (or moles) of each reactant and product that take part.
3. It gives information about the relative weights of reactants and products based on their molar masses.
4. For gaseous reactants and products, it shows their relative volumes according to Avogadro's law.
Limitations of a Chemical Equation:
1. It does not always clearly show the physical state of reactants or products (solid, liquid, or gas) unless special symbols are added.
2. It does not always tell the specific conditions needed for the reaction to occur, such as required temperature, pressure, or if a catalyst is necessary.
3. It does not indicate how fast or slow the reaction will happen.
4. It does not explicitly show if heat is given off (exothermic) or taken in (endothermic) during the reaction.
5. It does not tell us the concentration of the reactants used.
6. It does not explain the detailed step-by-step process or mechanism of the reaction.
7. It does not show if the reaction can proceed in both directions (reversible) or only one way (irreversible).
In simple words: A chemical equation uses symbols to show a chemical change, telling us what reacts and what is formed. However, it usually doesn't tell us about the reaction speed, exact conditions, or how much energy is involved.

🎯 Exam Tip: When describing characteristics and limitations, focus on what a chemical equation *reveals* (composition, stoichiometry) versus what it *doesn't tell* (kinetics, thermodynamics, precise conditions) about a chemical reaction.

 

Question 3. What is the meaning of the chemical formula of a compound? What information do we get from the formula? Write your answer by giving a suitable example.
Answer: A chemical formula is a concise way to represent the composition of a compound using elemental symbols and numerical subscripts. It specifically indicates the types of elements present and the exact number of atoms of each element that combine to form one molecule or formula unit of that compound. For example, the chemical formula for water is \( \text{H}_2\text{O} \), which precisely means one molecule of water contains two hydrogen atoms and one oxygen atom. This structural detail is crucial for understanding chemical reactions.
Information obtained from a chemical formula:
1. The formula represents one molecule (or formula unit) of the substance. For instance, \( \text{NaCl} \) represents one formula unit of sodium chloride.
2. It clearly names all the constituent elements present in the molecule. For example, in \( \text{Na}_2\text{SO}_4 \), the elements are sodium (Na), sulphur (S), and oxygen (O).
3. It explicitly shows the number of atoms of each element in one molecule. For example, \( \text{Na}_2\text{SO}_4 \) indicates there are two sodium atoms, one sulphur atom, and four oxygen atoms.
4. If the atomic weights are known, the molecular weight or molar mass of the substance can be calculated. For example, using atomic masses of Na=23, S=32, and O=16, the molecular weight of \( \text{Na}_2\text{SO}_4 \) is calculated as \( (23 \times 2) + 32 + (16 \times 4) = 46 + 32 + 64 = 142 \).
5. A formula can also show the simplest ratio of elements by weight in the compound. For \( \text{Na}_2\text{SO}_4 \), the ratio of Na:S:O by weight is \( (23 \times 2) : 32 : (16 \times 4) = 46 : 32 : 64 \), which simplifies to 23:16:8.
In simple words: A chemical formula is like a short code for a compound. It tells us what elements are in it and how many atoms of each element are joined together to make one unit. It also helps us find the total weight of one unit of that compound.

🎯 Exam Tip: Understand that a chemical formula provides both qualitative (what elements are present) and quantitative (how many atoms of each) information about a compound, making it a powerful tool in chemistry.

 

Question 4. Explain symbols and chemical formulae. Differentiate between empirical formula and molecular formula with examples.
Answer:
Symbols: A chemical symbol is a shorthand representation used for a chemical element. In 1813, Jöns Jacob Berzelius proposed a system where symbols are derived from the letters of the element's name.
1. Generally, the first letter of the English name of the element is used as its symbol, and it is always capitalized. For example, Hydrogen (H), Oxygen (O), Nitrogen (N), Carbon (C), Phosphorus (P), and Sulphur (S).
2. If there is more than one element that shares the first letter, a second, often a prominent, letter from the name is added in lowercase. For example, Calcium (Ca), Chlorine (Cl), Copper (Cu).
3. In some cases, the symbols are derived from the Latin names of the elements to avoid confusion or due to historical naming. For example, Potassium (Kalium - K), Iron (Ferrum - Fe), Copper (Cuprum - Cu), Silver (Argentum - Ag), Mercury (Hydrargyrum - Hg), Lead (Plumbum - Pb), Gold (Aurum - Au), Tin (Stannum - Sn).

Significance of a Symbol:
1. It is the shortest way to write the name of an element.
2. It represents one atom of the element. For example, 'O' means one atom of oxygen.
3. It also represents the atomic weight of that element. For example, 'H', 'C', and 'S' represent atomic weights of hydrogen (1), carbon (12), and sulfur (32), respectively.
Chemical Formulae: A chemical formula (as explained in Question 3) represents one molecule of a substance, indicating the types and exact numbers of atoms of elements present.
Difference between Empirical Formula and Molecular Formula:
- Empirical Formula: This formula shows the simplest whole-number ratio of atoms of different elements in a compound. It is like the most reduced form of a chemical formula. For example, for acetic acid, the molecular formula is \( \text{C}_2\text{H}_4\text{O}_2 \), but its empirical formula is \( \text{CH}_2\text{O} \), meaning the ratio of Carbon, Hydrogen, and Oxygen atoms is 1:2:1.
- Molecular Formula: This formula shows the actual number of atoms of each element present in one molecule of a compound. It provides the exact composition. For example, the molecular formula of acetic acid is \( \text{C}_2\text{H}_4\text{O}_2 \), clearly indicating it contains 2 carbon atoms, 4 hydrogen atoms, and 2 oxygen atoms. The molecular formula is always an integer multiple of the empirical formula.
In simple words: Symbols are short names for elements. Chemical formulas show all atoms in a molecule. An empirical formula shows the simplest ratio of atoms in a compound, while a molecular formula shows the exact number of each atom.

🎯 Exam Tip: For symbols, remember the rules for single-letter, two-letter, and Latin-derived symbols. For formulae, understand that the molecular formula is the 'real' formula, and the empirical formula is its simplest, reduced form, crucial for understanding composition.

 

Question 5. Balance the following chemical equations:
(1) \( \text{NaOH} + \text{Cl}_2 \rightarrow \text{NaCl} + \text{H}_2\text{O} + \text{NaOCl} \)
(2) \( \text{NaOH} + \text{Cl}_2 \rightarrow \text{NaClO}_3 + \text{NaCl} + \text{H}_2\text{O} \)
(3) \( \text{Cu} + \text{HNO}_3 \rightarrow \text{Cu(NO}_3\text{)}_2 + \text{NO}_2 + \text{H}_2\text{O} \)
(4) \( \text{Cu} + \text{HNO}_3 \rightarrow \text{Cu(NO}_3\text{)}_2 + \text{NO} + \text{H}_2\text{O} \)
(5) \( \text{Zn} + \text{HNO}_3 \rightarrow \text{Zn(NO}_3\text{)}_2 + \text{H}_2\text{O} + \text{N}_2\text{O} \)
(6) \( \text{KMnO}_4 + \text{SO}_2 + \text{H}_2\text{O} \rightarrow \text{K}_2\text{SO}_4 + \text{MnSO}_4 + \text{H}_2\text{SO}_4 \)
Answer:
The balanced chemical equations are:
(1) \( 2\text{NaOH} + \text{Cl}_2 \rightarrow \text{NaCl} + \text{H}_2\text{O} + \text{NaOCl} \)
(2) \( 6\text{NaOH} + 3\text{Cl}_2 \rightarrow \text{NaClO}_3 + 5\text{NaCl} + 3\text{H}_2\text{O} \). This reaction is important in industrial processes for bleach production.
(3) \( \text{Cu} + 4\text{HNO}_3 \rightarrow \text{Cu(NO}_3\text{)}_2 + 2\text{NO}_2 + 2\text{H}_2\text{O} \)
(4) \( 3\text{Cu} + 8\text{HNO}_3 \rightarrow 3\text{Cu(NO}_3\text{)}_2 + 2\text{NO} + 4\text{H}_2\text{O} \)
(5) \( 4\text{Zn} + 10\text{HNO}_3 \rightarrow 4\text{Zn(NO}_3\text{)}_2 + \text{N}_2\text{O} + 5\text{H}_2\text{O} \)
(6) \( 2\text{KMnO}_4 + 5\text{SO}_2 + 2\text{H}_2\text{O} \rightarrow \text{K}_2\text{SO}_4 + 2\text{MnSO}_4 + 2\text{H}_2\text{SO}_4 \)
In simple words: Balancing a chemical equation means making sure that the number of atoms for each element is exactly the same on both sides of the reaction arrow (reactants and products). You adjust the numbers placed in front of the chemical formulas (called coefficients) until all atoms match up, showing mass conservation.

🎯 Exam Tip: Always use a systematic approach, such as the hit-and-trial method or the algebraic method, to balance equations. Start by balancing atoms other than hydrogen and oxygen, then balance oxygen, and finally hydrogen.