Frank Brothers Solutions for ICSE Class 10 Chemistry Chapter 3 Study Of Acids Bases And Salts

Chapter 3. Study Of Acids, Bases And Salts

Solution 1:
Answer: An acid is defined as a compound which when dissolved in water produces hydronium ion (\( H_3O^+ \)), the only positively charged ion and a negative ion. At first the acid furnishes hydrogen ion (\( H^+ \)) in aqueous solution but this \( H^+ \) ion combines with a water molecule to form Hydronium ion.
For example: acetic acid, sulphuric acid, nitric acid.
\( CH_3COOH + H_2O \rightarrow H_3O^+ + CH_3COO^- \)
In simple words: An acid is a substance that makes water acidic by releasing special ions called hydronium ions when mixed with water.

📝 Teacher's Note: Start with familiar acids like lemon juice or vinegar to show students that acids are everywhere. Emphasize that acids need water to show their acidic properties - dry citric acid powder isn't acidic until dissolved.

🎯 Exam Tip: Always mention hydronium ion (\( H_3O^+ \)) formation, not just hydrogen ion, and give a balanced equation example for full marks.

Solution 2:
Answer:
1. (i) Hydrogen chloride HCl
(ii) Nitric acid \( HNO_3 \)
2. (i) Carbonic acid \( H_2CO_3 \)
(ii) Oxalic acid \( (COOH)_2 \)
3. (i) Sulphuric acid \( H_2SO_4 \)
(ii) Hydrogen chloride HCl
4. (i) Carbonic acid \( H_2CO_3 \)
(ii) Acetic acid
In simple words: These are common acids with their chemical names and formulas that students should memorize.

📝 Teacher's Note: Create flashcards with common acid names and formulas. Students often confuse hydrogen chloride (gas) with hydrochloric acid (aqueous solution).

🎯 Exam Tip: Write both the name and chemical formula for acids. Remember that carbonic acid is found in soft drinks and oxalic acid in spinach.

Solution 3:
Answer:
(a) A base is defined as a chemical compound which when reacts with hydronium ion or \( H^+ \) ions furnished by an acid to form salt and water only.
For example: CuO, \( Mg(OH)_2 \).
(b) An alkali is a base which is soluble in water but all bases are not water soluble.
For example Ferric hydroxide [\( Fe(OH)_3 \)] and cupric hydroxide [\( Cu(OH)_2 \)] are bases but these are not soluble in water but sodium hydroxide NaOH, calcium hydroxide \( Ca(OH)_2 \) are bases and are also soluble in water.
Hence it is rightly said that all alkalis are bases but all bases are not alkalis.
Concept Insight: An alkali is a basic hydroxide which when dissolved in water produces hydroxyl (\( OH^- \)) ions as the only negatively charged ions.
Example: \( NaOH(aq) \rightleftharpoons Na^+ + OH^- \)
In simple words: A base neutralizes acids, while an alkali is a special type of base that dissolves in water to make it basic.

📝 Teacher's Note: Use Venn diagrams to show that alkalis are a subset of bases. Demonstrate with calcium hydroxide (lime water) as a common alkali example.

🎯 Exam Tip: Remember the key difference: all alkalis are bases, but not all bases are alkalis. Alkalis must be water-soluble.

Solution 4:
Answer:
1. The pH of a solution is defined as the negative logarithm (base 10) of the hydronium ion concentration present in the solution.
pH = -\( log_{10} [H_3O^+] \)
2. The three applications of pH scale are:
○ It is used to determine the acidic or basic nature of the solution.
○ It is used to determine hydronium ion concentration present in the solution.
○ It is used to find out neutrality of the solution.
In simple words: pH is a number scale (0-14) that tells us how acidic or basic a solution is - like a thermometer for acidity.

📝 Teacher's Note: Use pH paper or digital pH meters with common household items like soap, lemon juice, and baking soda to make pH tangible for students.

🎯 Exam Tip: Always write the pH formula and mention that pH 7 is neutral, below 7 is acidic, and above 7 is basic.

Solution 5:
Answer:
(a) Indicators are organic compounds which when added in small amounts to a solution; indicate the nature (acidity or alkalinity) of the solution.
(b) Universal indicators are preferred to acid-base indicators because these give different colours in different pH ranges. A solution containing a drop of universal indicator is matched against a standard colour chart to find the pH of the solution.

In simple words: Indicators are like color-changing chemicals that tell us if something is acidic or basic by changing color, and universal indicator shows many colors for different pH levels.

📝 Teacher's Note: Demonstrate with red cabbage juice as a natural indicator. Students love seeing the dramatic color changes with different household substances.

🎯 Exam Tip: Remember that universal indicator gives a range of colors while simple indicators like litmus only give two colors (red/blue).

Solution 6:
Answer:
(a) The difference between an alkali and a base is:

(b)

In simple words: Alkalis are bases that dissolve in water, while metal hydroxides are compounds containing metals and OH groups that may or may not dissolve in water.

📝 Teacher's Note: Show students that calcium hydroxide (lime water) is both an alkali and a metal hydroxide, while copper oxide is a base but not an alkali.

🎯 Exam Tip: Draw clear tables showing differences. Remember: solubility in water is the key difference between alkalis and bases.

Solution 7:
Answer:
1. Base in solution furnishes the ions: Hydroxide ion/ oxide ion and a metallic ion.
2. A weak alkali furnishes the ions: Hydroxide ion and metallic ion and molecules of weak alkali.
3. An acid in a solution furnishes the ions: Hydronium / Hydrogen ion and a negative ion.
In simple words: Bases give OH⁻ ions, weak alkalis give some OH⁻ ions plus undissolved molecules, and acids give H⁺ ions when dissolved in water.

📝 Teacher's Note: Use simple ionization equations to show how strong vs weak alkalis behave differently in solution. Ammonia solution is a good example of a weak alkali.

🎯 Exam Tip: Remember that weak alkalis only partially ionize, so they produce both ions and neutral molecules in solution.

Solution 8:
Answer:
Hydronium ion: Hydronium ion is \( H_3O^+ \). It is formed when a hydrogen ion \( H^+ \) released from an acid combines with a water molecule as:
\( H^+ + H_2O \rightarrow H_3O^+ \)
In hydronium ion, hydrogen ion is linked to water molecule via coordinate bond which is formed by the participation of lone pair of electrons present on oxygen atom. The structure of hydronium ion is as shown in the diagram.
In simple words: Hydronium ion is what forms when an acid releases hydrogen ions that stick to water molecules, creating a new charged particle.

📝 Teacher's Note: Draw the 3D structure showing the pyramidal shape of hydronium ion. Emphasize that free H⁺ ions don't really exist in water - they always attach to water molecules.

🎯 Exam Tip: Always draw the structure and mention coordinate bonding between H⁺ and the lone pair on oxygen for complete marks.

Solution 9:
Answer:
1. CaO
2. NaOH
3. CuO
4. Cu[(OH)2]
5. \( H_2CO_3 \)
6. Ferric hydroxide [Fe (OH)3].
7. CuO
8. \( NH_3 \)
In simple words: This is a list of different chemical compounds including bases, acids, and alkalis with their chemical formulas.

📝 Teacher's Note: Help students categorize these compounds - which are acids, which are bases, which are alkalis. This builds classification skills.

🎯 Exam Tip: Learn to recognize compound types from their formulas - hydroxides are usually bases, while compounds with H at the start are often acids.

Solution 10:
Answer: Anhydrous hydrogen chloride is not an acid but its aqueous solution is a strong acid because anhydrous means without water and we know that the property of acidity is shown by a substance only when it is dissolved in water or its aqueous solution is prepared.
In simple words: Dry hydrogen chloride gas isn't acidic - it only becomes acidic when mixed with water because acids need water to show their acidic properties.

📝 Teacher's Note: Demonstrate this concept by showing that dry HCl gas doesn't affect dry litmus paper, but HCl dissolved in water immediately turns blue litmus red.

🎯 Exam Tip: Remember the key point: acids only show acidic properties in the presence of water. Without water, they're just neutral compounds.

Solution 11:
Answer:
Hydronium ion: Hydronium ion is \( H_3O^+ \). It is formed when a hydrogen ion \( H^+ \) released from an acid combines with a water molecule as:
\( H^+ + H_2O \rightarrow H_3O^+ \)
In hydronium ion, hydrogen ion is linked to water molecule via coordinate bond which is formed by the participation of lone pair of electrons present on oxygen atom. The structure of hydronium ion is as shown in the diagram.
In simple words: Hydronium ion forms when acids dissolve in water and their hydrogen atoms join with water molecules to make a new charged particle.

📝 Teacher's Note: Emphasize that this is why we say acids produce H₃O⁺ ions rather than just H⁺ ions in modern chemistry - it's more accurate to reality.

🎯 Exam Tip: Always mention coordinate bonding and draw the structure when asked about hydronium ion formation for full marks.

Solution 12:
Answer: Strength of an acid measures the ease with which the acid can ionize to produce hydrogen or hydronium ions when dissolved in water. Those acids which can easily ionize to form hydrogen ions are called strong acids while those which can partially ionize to form hydrogen ions are called weak acids.
Strength of an acid depends upon many factors such as:
1. Molecular structure of the acid
2. The temperature
3. Properties of the solvent
In simple words: Acid strength tells us how easily an acid breaks apart in water to release hydrogen ions - strong acids break apart completely, weak acids only partially.

📝 Teacher's Note: Compare strong acids like HCl with weak acids like acetic acid using pH measurements. Show that concentration and strength are different concepts.

🎯 Exam Tip: Don't confuse concentration with strength - a dilute solution of a strong acid is still a strong acid, just less concentrated.

Solution 13:
Answer:
(i) Acid from a non metal:
Certain non metals like sulphur (S), phosphorous (P), can be oxidized by concentrated nitric acid to yield corresponding acid.
Non metal + Acid → Acid + Water + Oxide
S + 6\( HNO_3 \) → \( H_2SO_4 \) + 2\( H_2O \) + 6\( NO_2 \)
(ii) Base from a metal:
Active metals like K, Na, Ca react with water to produce water soluble bases. These bases are called alkalis.
Active metal + water → Base / Alkali + Hydrogen
2Na + 2\( H_2O \) → 2NaOH + \( H_2 \)
In simple words: Non-metals can be turned into acids using nitric acid, while active metals react with water to form bases called alkalis.

📝 Teacher's Note: Show the pattern - non-metals form acids, metals form bases. Use sodium reaction with water as a dramatic demonstration (with safety precautions).

🎯 Exam Tip: Write balanced equations for both reactions. Remember that only active metals react directly with water to form alkalis.

Solution 14:
Answer: Solution B with pH value 9 will give pink colour with phenolphthalein.
Concept Insight: Bases give pink colour with phenolphthalein because a base will abstract two protons from phenolphthalein and the resulting phenolphthalein ion provides pink colour to the solution.
In simple words: A solution with pH 9 is basic, and basic solutions turn phenolphthalein indicator pink while acidic solutions keep it colorless.

📝 Teacher's Note: Demonstrate with different pH solutions and phenolphthalein. Show that the color change is sharp at around pH 8.2-10.

🎯 Exam Tip: Remember that phenolphthalein is colorless in acidic solutions and pink in basic solutions. pH 9 is basic, so it will be pink.

Solution 15:
Answer: Two indicators for identification of acid are methyl red and Thymol blue.
In simple words: Methyl red and thymol blue are chemical indicators that change color when acids are present, helping us identify acidic solutions.

📝 Teacher's Note: Show the color changes of these indicators in acidic vs basic solutions. Students should know at least 3-4 common indicators and their color changes.

🎯 Exam Tip: Learn the color changes: methyl red is red in acidic solutions and yellow in basic solutions.

Solution 16:
Answer:
(i) Preparation from copper nitrate:
2Cu(\( NO_3 \))_2$ →^Δ 2CuO + 4\( NO_2 \) +\( O_2 \)
(ii) Preparation from copper carbonate:
CuCO_3$ → CuO +CO_2$ ↑ .
(iii) Preparation from copper sulphate:
CuSO_4$ + \( H_2O \) → CuO +\( H_2SO_4 \)
In simple words: Copper oxide can be made by heating copper nitrate, copper carbonate, or copper sulphate - each method breaks down the original compound to leave copper oxide.

📝 Teacher's Note: Demonstrate thermal decomposition with copper carbonate - students can see the color change and test for CO₂ gas with lime water.

🎯 Exam Tip: Write balanced equations and use the triangle (Δ) symbol to show heating is required for these decomposition reactions.

Solution 17:
Answer:
Preparation of acids by synthesis follows general reaction:
Hydrogen + Non metal → Acid
(i) \( H_2 \) + \( I_2 \) → 2HI
(ii) \( H_2 \) + \( Br_2 \) → 2HBr
(iii) \( H_2 \) + \( Cl_2 \) → 2HCl
(iv) \( H_2 \) + S → \( H_2S \)
In simple words: Acids can be made by directly combining hydrogen gas with non-metal elements - this is called synthesis reaction.

📝 Teacher's Note: Explain that these are direct synthesis reactions. Some require heat or catalysts. The hydrogen halides are particularly important examples.

🎯 Exam Tip: Remember the general pattern: H₂ + non-metal → acid. Write balanced equations and note that these are combination reactions.

Solution 18:
(a) Acid salt: The salt formed by the partial replacement of the replaceable hydrogen atoms of an acid molecule by a metallic or ammonium ion is called acid salts. For example \( \text{NaHSO}_4 \), \( \text{Na}_2\text{HPO}_4 \)
Normal salt: The salt formed by replacement of all the replaceable hydrogen atoms of an acid by metallic or ammonium ions is called a normal salt. For example \( \text{NaCl} \), \( \text{Na}_2\text{SO}_4 \)
(b) Orthophosphoric acid is \( \text{H}_3\text{PO}_4 \) i.e. a tribasic acid has three replaceable hydrogen atoms so it forms three series of salts, namely two acid salts and a normal salt, with \( \text{NaOH} \) as:
\( \text{NaOH} + \text{H}_3\text{PO}_4 \rightarrow \text{NaH}_2\text{PO}_4 + \text{H}_2\text{O} \)
Acid salt
(Monosodium hydrogen salt)
\( 2\text{NaOH} + \text{H}_3\text{PO}_4 \rightarrow \text{Na}_2\text{HPO}_4 + 2\text{H}_2\text{O} \)
Acid salt
(Disodium hydrogen phosphate)
\( 3\text{NaOH} + \text{H}_3\text{PO}_4 \rightarrow \text{Na}_3\text{PO}_4 + 3\text{H}_2\text{O} \)
Normal salt
(Sodium phosphate)
Answer: Acid salts are formed when only some hydrogen atoms in an acid are replaced by metal ions, while normal salts are formed when all hydrogen atoms are replaced. Phosphoric acid can form three different types of salts because it has three replaceable hydrogen atoms.

📝 Teacher's Note: Use the phosphoric acid example to show how tribasic acids can form multiple salts. Draw the stepwise replacement of hydrogen atoms to make it visual for students.

🎯 Exam Tip: Always mention the number of replaceable hydrogen atoms when explaining acid vs normal salts. Give clear examples with proper chemical formulas.

Solution 19:
(a) \( \text{MgCl}_2 + \text{Ca(OH)}_2 \rightarrow \text{Mg(OH)}_2 + \text{CaCl}_2 \)
\( \text{Mg(OH)}_2 + 2\text{CO}_2 \rightarrow \text{Mg(HCO}_3)_2 \)
\( \text{Mg(HCO}_3)_2 \rightarrow \text{MgCO}_3 + \text{CO}_2 + \text{H}_2\text{O} \)
(b) \( \text{Pb(NO}_3)_2 + \text{Na}_2\text{CO}_3 \rightarrow \text{PbCO}_3 + 2\text{NaNO}_3 \)
(c) \( \text{Na}_2\text{CO}_3 + \text{CO}_2 + \text{H}_2\text{O} \rightarrow 2\text{NaHCO}_3 \)
Answer: These are chemical reactions showing the formation of different compounds through precipitation, neutralization, and acid-base reactions.

📝 Teacher's Note: Emphasize balancing equations step by step. Show students how to identify reaction types - precipitation (b), decomposition (a), and synthesis (c).

🎯 Exam Tip: Balance equations carefully and identify the reaction type. Always check that atoms are conserved on both sides of the equation.

Solution 20:
1. Efflorescence: It is the phenomenon by which hydrated salts on exposure to dry air, lose their water of crystallization and crumble to powder.
2. Hygroscopy: It is the phenomenon by which substances absorb moisture from air, but only sufficiently so as to become wet.
3. Water of crystallization: It is the fixed amount of water that is present in a crystal as an integral part of its constitution. Hydrated salts are salts having water of crystallisation.
Answer: These are three different phenomena related to water behavior in crystals - losing water (efflorescence), absorbing moisture (hygroscopy), and containing fixed water molecules (water of crystallization).

📝 Teacher's Note: Use everyday examples like washing soda losing water when left open (efflorescence) or salt becoming wet in humid weather (hygroscopy) to make concepts relatable.

🎯 Exam Tip: Remember the key difference - efflorescence loses water, hygroscopy gains water, and water of crystallization is fixed water in the crystal structure.

Solution 21:
Deliquescence is the phenomenon by which certain salts absorb moisture from air, lose their water of crystallization and dissolve in it to form a saturated solution. The substances which exhibit deliquescence are called deliquescent. For example Caustic soda \( \text{NaOH} \), Caustic potash \( \text{KOH} \).
Answer: Deliquescence is when substances absorb so much moisture from air that they completely dissolve and form a liquid solution. This is more extreme than hygroscopy.

📝 Teacher's Note: Demonstrate with a small piece of NaOH left in open air - students will see it becoming a liquid puddle over time. This visual makes the concept unforgettable.

🎯 Exam Tip: Distinguish deliquescence from hygroscopy - deliquescent substances form solutions, hygroscopic substances just become wet but remain solid.

Solution 22:
Salt hydrolysis: The reaction between a salt and water to produce an acid or a base is called salt hydrolysis or hydrolysis. By hydrolysis the salt, we can find out its parent acid and parent base.
For example: \( \text{NaCl} + \text{H}_2\text{O} \rightarrow \text{Na}^+ + \text{Cl}^- \)
Salts which are acidic: i. Ammonium sulphate
ii. Ammonium chloride.
Salts which are basic: i. Potassium carbonate
ii. Sodium acetate.
Salts which are neutral: i. Ammonium acetate
ii. Ammonium carbonate.
Answer: Salt hydrolysis is the reaction of salts with water to form acids or bases. Different salts can be acidic, basic, or neutral depending on their parent acid and base strength.

📝 Teacher's Note: Teach the rule: strong acid + weak base = acidic salt; weak acid + strong base = basic salt; strong acid + strong base = neutral salt.

🎯 Exam Tip: Remember to identify the parent acid and base of each salt to predict whether the salt will be acidic, basic, or neutral in solution.

Solution 23:

i. Preparation of Zinc sulphate by displacement method:
\( \text{Zn} + \text{H}_2\text{SO}_4 \rightarrow \text{ZnSO}_4 + \text{H}_2 \)
ii. Preparation of Ferrous sulphide by synthesis method:
\( \text{Fe} + \text{S} \rightarrow \text{FeS} \)
iii. Preparation of Barium sulphate by precipitation method:
\( \text{BaCl}_2 + \text{H}_2\text{SO}_4 \rightarrow \text{BaSO}_4 + 2\text{HCl} \)
iv. Preparation of Sodium sulphate by neutralization method:
\( 2\text{NaOH} + \text{H}_2\text{SO}_4 \rightarrow \text{Na}_2\text{SO}_4 + 2\text{H}_2\text{O} \)
Answer: Different salts are prepared by different methods - displacement (metal + acid), synthesis (direct combination), precipitation (double decomposition), and neutralization (acid + base).

📝 Teacher's Note: Connect each method to its principle - displacement uses reactivity series, synthesis combines elements, precipitation forms insoluble products, neutralization balances acids and bases.

🎯 Exam Tip: Match the salt type to the preparation method - insoluble salts by precipitation, soluble salts by neutralization, and reactive metals by displacement.

Solution 24:
1. Common salt gets wet during rainy season because the commercially available salt contains impurities, like magnesium chloride, which are deliquescent substances. These absorb moisture from atmosphere and make the table salt wet.
2. (i) \( \text{Na}_2\text{CO}_3 \cdot 10\text{H}_2\text{O} \) = Washing soda
(ii) \( \text{MgSO}_4 \cdot 7\text{H}_2\text{O} \) = Epsom salt
(iii) \( \text{CuSO}_4 \cdot 5\text{H}_2\text{O} \) = Blue vitriol
(iv) \( \text{ZnSO}_4 \cdot 7\text{H}_2\text{O} \) = White vitriol
Answer: Common salt becomes wet due to deliquescent impurities like magnesium chloride. The given compounds are hydrated salts with their common names and water of crystallization.

📝 Teacher's Note: Show students how pure NaCl doesn't become wet, but commercial salt does. This demonstrates the effect of impurities in real-world applications.

🎯 Exam Tip: Remember that deliquescent impurities in salt cause it to absorb moisture. Learn the common names of hydrated salts as they frequently appear in exams.

Solution 1996-1:
1. pH of a solution having pH 7 can be increased by adding a base to it such as \( \text{NaOH} \).
2. pH can be decreased by adding an acid such as \( \text{HCl} \) to it.
If a solution changes colour of litmus from red to blue, it shows that its pH is above 7.
Answer: pH can be increased by adding bases and decreased by adding acids. Red litmus turning blue indicates a basic solution with pH above 7.

📝 Teacher's Note: Use universal indicator to show color changes at different pH values. This visual demonstration helps students understand pH scale better.

🎯 Exam Tip: Remember pH 7 is neutral, above 7 is basic, below 7 is acidic. Bases increase pH, acids decrease pH.

Solution 1996-2:
The solution that liberates carbon dioxide from sodium carbonate has pH below 7 and hence acidic in nature. Since acids react with carbonates to liberate carbon dioxide gas as:
Carbonate + dil. Acid → Salt + Water + Carbon dioxide
For example: \( \text{NaHCO}_3 + \text{HCl} \rightarrow \text{NaCl} + \text{H}_2\text{O} + \text{CO}_2 \)
Answer: A solution that liberates carbon dioxide from sodium carbonate must be acidic because only acids react with carbonates to produce CO₂ gas.

📝 Teacher's Note: Demonstrate this with baking soda and vinegar - the fizzing shows CO₂ liberation. This confirms the acidic nature of vinegar.

🎯 Exam Tip: CO₂ liberation from carbonates is a confirmatory test for acids. Remember the general reaction pattern.

Solution 1996-3:
1. Zinc sulphate = Zinc and dilute sulphuric acid
2. Copper sulphate = Copper oxide and dilute sulphuric acid
3. Sodium sulphate = Sodium carbonate solution and dilute sulphuric acid
4. Lead sulphate = Lead carbonate and dilute sulphuric acid
Answer: These sulphate salts can be prepared by reacting the appropriate metal, metal oxide, or metal carbonate with dilute sulphuric acid.

📝 Teacher's Note: Explain why different starting materials are used - reactive metals react directly, less reactive metals need their oxides, and very unreactive metals use carbonates.

🎯 Exam Tip: Choose the starting material based on metal reactivity - active metals react directly with acid, less active ones need oxide or carbonate forms.

Solution 1997-1:
The term acid salt means the salt formed by partial replacement of the hydrogens present in the acid by metallic or ammonium ions.
For example: \( \text{NaHCO}_3 \)
Answer: An acid salt is formed when only some of the replaceable hydrogen atoms in an acid are replaced by metal ions, leaving some hydrogen atoms still attached.

📝 Teacher's Note: Compare NaHCO₃ (acid salt) with Na₂CO₃ (normal salt) to show the difference in hydrogen replacement.

🎯 Exam Tip: Look for hydrogen still present in the salt formula - if H is there along with metal ions, it's an acid salt.

Solution 1997-2:
1. pH scale is used to express the acidic or basic nature of solution.
2. pH of pure water is 7 since it does not have any impurities.
3. (a) A soluble oxide of A will have pH less than the pH of pure water i.e. below 7.
(b) A solution of 'B' will have more pH than pure water i.e. above 7.
Answer: pH scale measures acidity/basicity. Pure water has pH 7. Metal oxides (like A) give basic solutions with pH above 7, while non-metal oxides give acidic solutions with pH below 7.

📝 Teacher's Note: Clarify that A likely refers to a non-metal oxide (acidic) and B to a metal oxide (basic) based on the pH values given.

🎯 Exam Tip: Metal oxides are basic (pH > 7), non-metal oxides are acidic (pH < 7), pure water is neutral (pH = 7).

Solution 1997-3:
1. Water of crystallization: It is the fixed amount of water that is present in a crystal as an integral pat of its constitution. Compounds having water of crystallization are called hydrous salts.
For example: Sodium carbonate \( \text{Na}_2\text{CO}_3 \) has 10 molecules of water present as water of crystallization \( \text{Na}_2\text{CO}_3 \cdot 10\text{H}_2\text{O} \)
2. Anhydrous: Hydrous salt on heating lose their water of crystallization, such salts are then called anhydrous.
For example: \( \text{Na}_2\text{CO}_3 \cdot 10\text{H}_2\text{O} \) on losing 10 molecules of water forms \( \text{Na}_2\text{CO}_3 \).
Answer: Water of crystallization is fixed water molecules in crystal structure. When this water is removed by heating, the salt becomes anhydrous.

📝 Teacher's Note: Heat some copper sulphate crystals to show the color change from blue to white as water of crystallization is lost - this makes the concept tangible.

🎯 Exam Tip: Hydrous salts contain water of crystallization (shown as ·nH₂O), anhydrous salts have this water removed by heating.

Solution 1997-4:
\( \text{MnO}_2 + 4\text{HCl} \) (conc) → \( \text{MnCl}_2 + \text{Cl}_2 + 2\text{H}_2\text{O} \).
Chlorine thus obtained is dried by passing through concentrated Sulphuric acid. This dried chlorine is passed over heated Iron to get anhydrous Iron (III) chloride.
\( 2\text{Fe} \) (heated) + \( 3\text{Cl}_2 \) (dry) → \( 2\text{FeCl}_3 \).
Answer: Anhydrous iron(III) chloride is prepared by passing dry chlorine gas over heated iron metal. The chlorine is first generated from MnO₂ and concentrated HCl, then dried.

📝 Teacher's Note: Emphasize why the chlorine must be dried - any moisture would form hydrated FeCl₃ instead of anhydrous FeCl₃.

🎯 Exam Tip: Two-step process: first generate and dry chlorine gas, then pass it over heated iron. Both steps are essential for anhydrous product.

Solution 1998-1:
1. Water of cystallization.
2. White.
3. Efflorescence.
4. Sodium chloride.
Answer: These appear to be answers to specific questions about chemical properties - water of crystallization, white color, efflorescence phenomenon, and sodium chloride salt.

📝 Teacher's Note: These seem like fill-in-the-blank answers. Ensure students understand each concept rather than just memorizing terms.

🎯 Exam Tip: Know the definitions and examples of water of crystallization, efflorescence, and common salts like sodium chloride.

Solution 1998-2:
Those acids which ionize partially in aqueous solution and thus they contain ions as well as molecules of the acid. Organic acid such as \( \text{CH}_3\text{COOH} \), is a weak acid.
Answer: Weak acids ionize only partially in water, so the solution contains both ions and molecules. Acetic acid (CH₃COOH) is a common example of a weak acid.

📝 Teacher's Note: Contrast with strong acids like HCl that ionize completely. Use conductivity tests to show the difference in ion concentration.

🎯 Exam Tip: Weak acids partially ionize (incomplete dissociation), strong acids completely ionize. Remember organic acids are typically weak.

Solution 1998-3:
(a) Solution P with pH 13 will liberate ammonia from ammonium sulphate on heating:
Ammonium salt + alkali → Salt + Water + Ammonia.
(b) Solution R with pH 2 is a strong acid.
(c) Solution Q with pH 6 contains molecules as well as ions.
Answer: pH 13 indicates a strong alkali that liberates ammonia from ammonium salts, pH 2 indicates a strong acid, and pH 6 indicates a weak acid with partial ionization.

📝 Teacher's Note: Use the pH scale to explain why pH 13 is strongly basic (liberates NH₃), pH 2 is strongly acidic, and pH 6 is weakly acidic.

🎯 Exam Tip: pH 13-14 = strong base, pH 0-2 = strong acid, pH 3-6 = weak acid. Ammonia liberation confirms presence of strong alkali.

Solution 1998-4:
The name and formula of the acid salt which gives sodium ions and sulphate ions in solution is Sodium hydrogen sulphate \( \text{NaHSO}_4 \).
Answer: Sodium hydrogen sulphate (NaHSO₄) is the acid salt that produces both sodium ions and sulphate ions when dissolved in water.

📝 Teacher's Note: Show how NaHSO₄ dissociates: NaHSO₄ → Na⁺ + HSO₄⁻, and HSO₄⁻ can further release H⁺ and SO₄²⁻ ions.

🎯 Exam Tip: Acid salts contain hydrogen that can be released as H⁺ ions. NaHSO₄ is a common example that gives both Na⁺ and SO₄²⁻ ions.

Solution 1999-1:
(a) (i) Acid: An acid is a compound which when dissolved in water produces hydronium ion (\( \text{H}_3\text{O}^+ \)), the only positively charged ion and a negative ion.
(ii) pH scale: The scale over which a range of pH values from acidic to basic are arranged is called a pH scale.
(iii) Neutralisation: It is the process by which \( \text{H}^- \) ions of an acid react completely with water the [\( \text{OH}^- \)] ions of a base to give salt and water only.
For example \( 2\text{NaOH} + \text{H}_2\text{SO}_4 \rightarrow \text{Na}_2\text{SO}_4 + 2\text{H}_2\text{O} \)
(b) (i) Treatment of lead dioxide with 4 molecules of hydrochloric acid gives lead(II) chloride as well as chlorine gas
(ii) Steps required for this reaction are:
\( \text{PbO}_2 \rightarrow \text{PbO} + \text{O} \)
\( \text{PbO} + 2\text{HCl} \rightarrow \text{PbCl}_2 + \text{H}_2\text{O} \)
\( 2\text{HCl} + \text{O} \rightarrow \text{H}_2\text{O} + \text{Cl}_2 \)
\( \text{PbO}_2 + 4\text{HCl} \rightarrow \text{PbCl}_2 + 2\text{H}_2\text{O} + \text{Cl}_2 \)
(iii) Iron reacts with dilute sulphuric acid as:
\( \text{Fe} + \text{H}_2\text{SO}_4 \rightarrow \text{FeSO}_4 + \text{H}_2 \)
(iv) Since the value of pH is less than 7 so the solution will be acidic.
Answer: These definitions and reactions cover basic acid-base concepts, redox reactions of lead dioxide, metal-acid reactions, and pH interpretation.

📝 Teacher's Note: Break down the PbO₂ reaction step-by-step to show both oxidation (Cl⁻ to Cl₂) and reduction (Pb⁴⁺ to Pb²⁺) occurring simultaneously.

🎯 Exam Tip: Learn the definitions precisely. For redox reactions, identify what gets oxidized and reduced. pH below 7 always means acidic.

Solution 2000-1:

Answer: Different salts require different preparation methods based on their solubility and reactivity of their constituent ions.

📝 Teacher's Note: Connect each method to the salt's properties - soluble salts by titration/neutralization, insoluble salts by precipitation, reactive metals directly with acids.

🎯 Exam Tip: Match the preparation method to salt properties - insoluble salts by precipitation, soluble salts by titration, and consider metal reactivity for direct methods.

Solution 2001-1:
Answer:
(a) (i) Preparation of sodium sulphate:
Sodium carbonate + dilute sulphuric acid → Sodium sulphate + water + carbon dioxide.
\( \text{Na}_2\text{CO}_3 + \text{H}_2\text{SO}_4 \rightarrow \text{Na}_2\text{SO}_4 + \text{H}_2\text{O} + \text{CO}_2 \)

(ii) Preparation of Copper sulphate:
Copper carbonate + dilute sulphuric acid → Copper sulphate + water + carbon dioxide.
\( \text{CuCO}_3 + \text{H}_2\text{SO}_4 \rightarrow \text{CuSO}_4 + \text{H}_2\text{O} + \text{CO}_2 \)

(iii) Preparation of iron (II) sulphate:
Iron + dilute sulphuric acid → Iron (II) sulphate + Hydrogen
\( \text{Fe} + \text{H}_2\text{SO}_4 \rightarrow \text{FeSO}_4 + \text{H}_2 \)

(iv) Preparation of Zinc carbonate:
\( \text{Zn} + \text{H}_2\text{SO}_4 \text{ (dil)} \rightarrow \text{ZnSO}_4 + \text{H}_2 \)
\( \text{ZnSO}_4 + \text{Na}_2\text{CO}_3 \rightarrow \text{ZnCO}_3 + \text{Na}_2\text{SO}_4 \)

(b)(i) \( \text{NaHSO}_4 \)
(ii) \( \text{AgCl} \)
(iii) \( \text{CuSO}_4.5\text{H}_2\text{O} \)
(iv) \( \text{CuCO}_3 \)
(v) \( \text{Pb(NO}_3)_2 \)
In simple words: These are chemical reactions where acids react with metals or carbonates to form salts. The first part shows how to make common salts in the lab, and the second part gives chemical formulas for various salts.

📝 Teacher's Note: Demonstrate these salt preparations practically in the lab to help students understand the fizzing action when acids react with carbonates and the displacement reactions with metals.

🎯 Exam Tip: Always balance chemical equations and include state symbols when asked. Remember that dilute acids react differently than concentrated acids.

Solution 2002-1:
Answer:
(a) (i) \( \text{Fe} + \text{dilute HCl} \rightarrow \text{FeCl}_2 + \text{H}_2 \)
(ii) \( \text{2Fe} + \text{3Cl}_2 \rightarrow \text{2FeCl}_3 \)
(iii) \( \text{Fe} + \text{dilute H}_2\text{SO}_4 \rightarrow \text{FeSO}_4 + \text{H}_2 \)
(iv) \( \text{Fe} + \text{S} \rightarrow \text{FeS} \)

(b) (i) \( \text{2NaOH} + \text{dilute H}_2\text{SO}_4 \rightarrow \text{Na}_2\text{SO}_4 + \text{2H}_2\text{O} \)
(ii) \( \text{Pb(OH)}_2 + \text{dilute H}_2\text{SO}_4 \rightarrow \text{PbSO}_4 + \text{2H}_2\text{O} \)
In simple words: Iron metal can react with different substances to form various compounds. Part (a) shows iron forming different salts, while part (b) shows how bases react with acids to form salts and water.

📝 Teacher's Note: Emphasize that iron forms different oxidation states (+2 and +3) depending on the reaction conditions. Show students the color differences between iron(II) and iron(III) compounds.

🎯 Exam Tip: Pay attention to the oxidation states of iron in different compounds - FeCl₂ has iron in +2 state, while FeCl₃ has iron in +3 state.

Solution 2003-1:
Answer:
1. Hydronium, positive.
2. Acid, metal.
In simple words: When acids dissolve in water, they form hydronium ions which carry a positive charge. Acids can react with metals to produce hydrogen gas.

📝 Teacher's Note: Use simple demonstrations like testing pH with universal indicator to show how acids form hydronium ions in water.

🎯 Exam Tip: Remember that hydronium ion (H₃O⁺) is always positively charged, and acid-metal reactions always produce hydrogen gas.

Solution 2003-2:
Answer:
(i) \( \text{Fe} + \text{dilute H}_2\text{SO}_4 \rightarrow \text{FeSO}_4 + \text{H}_2 \)
(ii) \( \text{Cu} + \text{conc. H}_2\text{SO}_4 \rightarrow \text{CuSO}_4 + \text{2H}_2\text{O} + \text{SO}_2 \)
(iii) \( \text{Pb(NO}_3)_2 + \text{dilute H}_2\text{SO}_4 \rightarrow \text{PbSO}_4 + \text{2HNO}_3 \)
(iv) \( \text{Na}_2\text{CO}_3 + \text{dilute H}_2\text{SO}_4 \rightarrow \text{Na}_2\text{SO}_4 + \text{2H}_2\text{O} + \text{CO}_2 \)
In simple words: Different substances react with sulphuric acid in different ways. Some metals like iron react with dilute acid to give hydrogen, while copper needs concentrated acid and produces different gases.

📝 Teacher's Note: Point out that concentrated sulphuric acid behaves differently from dilute acid - it acts as an oxidizing agent and doesn't produce hydrogen gas with metals.

🎯 Exam Tip: Always check if the question mentions dilute or concentrated acid as this affects the products formed in the reaction.

Solution 2004-1:
Answer:
Methods for preparation:
1. Preparation of copper(II) chloride.
Action of an acid on an oxide or carbonate
2. Preparation of iron(III) chloride.
Direct combination
3. Preparation of iron (II) chloride.
Action of an acid on a metal
4. Preparation of lead (ii) chloride
Precipitation (double decomposition)
5. Preparation of sodium chloride
Neutralization of an alkali by an acid.
In simple words: There are five main ways to prepare different types of salts in the laboratory, each method suitable for making specific salts depending on the starting materials available.

📝 Teacher's Note: Create a chart showing which method works best for which type of salt preparation. Students often get confused about which method to use for insoluble salts.

🎯 Exam Tip: For precipitation method, remember it's used for insoluble salts like lead chloride. For neutralization, it's used when you have an acid and a base.

Solution 2005-1:
Answer:
(a) Lead carbonate is first treated with dilute \( \text{HNO}_3 \).
(b) \( \text{PbCO}_3 + \text{2HNO}_3 \rightarrow \text{Pb(NO}_3)_2 + \text{H}_2\text{O} + \text{CO}_2 \)
(c) Lead carbonate and lead sulphate both are insoluble in water. So when dilute \( \text{H}_2\text{SO}_4 \) is added to lead carbonate, lead sulphate formed deposits on the remaining lead carbonate thereby stopping the further reaction.
In simple words: Lead carbonate doesn't react properly with sulphuric acid because the lead sulphate formed covers the unreacted carbonate like a protective coating, stopping the reaction from continuing.

📝 Teacher's Note: Demonstrate this concept using the analogy of painting a wall - once you paint over something, you can't access what's underneath.

🎯 Exam Tip: When explaining why certain reactions don't go to completion, always mention if a protective layer or coating is formed that prevents further reaction.

Solution 2005-2:
Answer:
Positive, hydroxyl, Salt, Neutralization.
In simple words: These are key terms related to acids, bases and salts - positive refers to the charge on hydronium ions, hydroxyl refers to OH⁻ ions in bases.

📝 Teacher's Note: Use fill-in-the-blank exercises regularly to help students remember these fundamental terms and their relationships.

🎯 Exam Tip: Learn the key terms by heart - neutralization always produces salt and water when an acid reacts with a base.

Solution 2005-3:
Answer:
When neutral litmus solution is added to sodium hydrogen carbonate solution, litmus solution turns red
In simple words: Sodium hydrogen carbonate is acidic in nature, so it turns blue litmus paper red, showing its acidic properties.

📝 Teacher's Note: This is a common misconception - students think all sodium compounds are basic, but sodium hydrogen carbonate is actually acidic.

🎯 Exam Tip: Remember that sodium hydrogen carbonate (NaHCO₃) is acidic despite having sodium in it - it turns litmus red.

Solution 2006-1:
Answer:
1. From pink to colourless.
2. From orange to pink.
3. From colourless to red.
In simple words: These are color changes that happen when indicators are added to different solutions to test if they are acidic or basic.

📝 Teacher's Note: Demonstrate these color changes practically with phenolphthalein, methyl orange, and litmus to help students remember the changes.

🎯 Exam Tip: Learn the color changes of common indicators by heart - phenolphthalein goes from pink (basic) to colorless (acidic).

Solution 2007-1:
Answer:
1. Hydronium
2. Hydroxide
3. Salt
4. Water
5. Hydrogen
In simple words: These are the main products formed in various acid-base reactions and related chemical processes.

📝 Teacher's Note: Connect these terms to specific reactions - hydronium forms when acids dissolve in water, hydroxide is present in bases.

🎯 Exam Tip: Remember that acid-base neutralization always produces salt and water, while acid-metal reactions produce salt and hydrogen gas.

Solution 2007-2:
Answer:

In simple words: Different types of salts are formed based on how completely the hydrogen in acids is replaced - acid salts have some hydrogen left, while normal salts have all hydrogen replaced.

📝 Teacher's Note: Use examples of each type of salt and show students how to identify them by their chemical formulas and properties.

🎯 Exam Tip: Learn to identify salt types by their formulas - acid salts contain hydrogen (like NaHCO₃), while normal salts don't (like Na₂CO₃).

Solution 2008-1:
Answer:
1. Complex salt.
2. Alkali.
In simple words: These are classifications of chemical compounds - complex salts contain complex ions, and alkalis are water-soluble bases.

📝 Teacher's Note: Show students examples of complex salts and explain how they differ from simple salts in their structure and properties.

🎯 Exam Tip: Remember that all alkalis are bases, but not all bases are alkalis - alkalis must be soluble in water.

Solution 2009-1:
Answer:
Acidified potassium dichromate paper
In simple words: This is a special test paper used to detect certain gases or substances that can change its color from orange to green.

📝 Teacher's Note: Demonstrate how this paper changes color when exposed to reducing agents like sulphur dioxide gas.

🎯 Exam Tip: Acidified potassium dichromate paper is used to test for reducing agents - it changes from orange to green when positive.

Solution 2009-2:
Answer:
1. Solution B.
2. Solution A.
3. Solution B
4. Solution of ammonium hydroxide NH₄OH is a weak alkali.
In simple words: Different solutions have different properties - some are stronger acids or bases than others, and ammonium hydroxide is a weak base.

📝 Teacher's Note: Compare strong and weak acids/bases using conductivity tests and pH measurements to show the difference clearly.

🎯 Exam Tip: Remember that ammonium hydroxide is always described as a weak alkali in chemistry exams.

Solution 2009-3:
Answer:
(a) (i) Substance B is a dehydrating agent.
(ii) B prevents the entry of moisture inside the apparatus because it absorbs moisture.
(iii) Iron (III) chloride is stored in a closed container because it is a deliquescent compound.
(iv) \( \text{2Fe} + \text{3Cl}_2 \rightarrow \text{2FeCl}_3 \)

(b) \( \text{PbCO}_3 + \text{2HNO}_3 \rightarrow \text{Pb(NO}_3)_2 + \text{H}_2\text{O} + \text{CO}_2 \)
\( \text{Pb(NO}_3)_2 + \text{H}_2\text{SO}_4 \rightarrow \text{PbSO}_4 + \text{2HNO}_3 \)
In simple words: Some substances absorb water from air (deliquescent), so they need to be stored in closed containers. Dehydrating agents remove moisture to keep reactions dry.

📝 Teacher's Note: Show students examples of deliquescent substances like iron(III) chloride and how they absorb moisture from the air to become wet.

🎯 Exam Tip: Deliquescent compounds always need to be stored in airtight containers to prevent them from absorbing atmospheric moisture.