Acid Bases and Salts Class 10 NCERT Solutions

Explanation: The student observes no color change on red litmus, suggesting the liquid neither reacts as an Acid nor a Base. Pure water or neutral solutions do not affect litmus indicators.

Red litmus turns blue in a basic solution, while Acids turn blue litmus red. No change means the liquid is chemically neutral, not acidic or basic. This indicates neutrality and absence of strong ions that can alter pH.

Like testing a solution with a pH indicator, no reaction confirms neutrality—similar to how distilled water doesn’t alter litmus paper.

Hint: Look for liquids that are neutral in nature.

The liquid is neutral, neither acidic nor basic, showing no reaction with red litmus paper.

Explanation: Compare the strength of Acids based on their tendency to donate protons (H⁺ ions) in water. Stronger Acids fully ionize, while weaker Acids partially ionize.

Sulfuric Acid (H₂SO₄) is a strong diprotic Acid, phosphorous Acid (H₃PO₃) is moderately weak, and acetic Acid (CH₃COOH) is a weak Organic acid. The correct decreasing order follows their relative ability to release H⁺ ions.

Like pouring different amounts of dye into water, stronger Acids release more H⁺, coloring the solution more intensely.

Hint: Compare a strong mineral acid, a moderate inorganic acid, and a weak organic acid.

Acid strength decreases in the order: H₂SO₄ > H₃PO₃ > CH₃COOH.

Explanation: Fresh milk is slightly acidic (pH ~6). When milk sours, lactic acid is produced due to bacterial fermentation, increasing H⁺ ion concentration.

This additional H⁺ lowers the pH, making the solution more acidic than 6. The souring process reflects the biochemical breakdown of lactose into lactic acid.

Like adding lemon juice to water, introducing Acids decreases pH, increasing acidity.

Hint: Sour taste in liquids usually corresponds to increased acidity.

Souring of milk decreases its pH below 6 due to lactic acid formation.

Explanation: Double Salts contain two different cations or anions crystallized together. Unlike complex Salts, double Salts separate into individual ions in solution.

Alum (K₂SO₄·Al₂(SO₄)₃·24H₂O) contains both potassium and aluminium ions along with sulfate ions, making it a classic example of a double Salt.

Like a recipe combining two ingredients into one Solid mixture, a double Salt contains multiple cations in a single crystal lattice.

Hint: Look for a hydrated Salt with two metallic ions.

K₂SO₄·Al₂(SO₄)₃·24H₂O is a double Salt combining potassium and aluminium sulfates.

Explanation: Groundwater in contact with air absorbs carbon dioxide (CO₂), which dissolves to form carbonic acid (H₂CO₃).

The carbonic acid partially dissociates into H⁺ ions, slightly lowering the pH of groundwater. Oxygen absorption does not significantly affect acidity.

Like carbonated water forming mild acid due to CO₂, exposed groundwater becomes slightly acidic.

Hint: Consider the effect of CO₂ from the Atmosphere on water.

Exposure to air allows CO₂ absorption, forming carbonic acid and decreasing pH slightly.

Explanation: Thermite reactions involve the reduction of metal oxides using a more reactive metal. Aluminium has a strong affinity for oxygen and serves as a reducing agent.

Aluminium reduces iron oxide to molten iron, releasing intense Heat for welding. Its Light weight is secondary; the key is its strong reactivity with oxygen.

Like magnesium igniting to produce bright Heat, aluminium in thermite releases energy for high-temperature welding.

Hint: Focus on the element’s ability to react with oxygen.

Aluminium is used because it has a strong affinity for oxygen, enabling efficient reduction in thermite reactions.

Explanation: Antacids neutralize excess stomach acid (HCl). Magnesium hydroxide is a mild Base that reacts with H⁺ ions to reduce acidity.

Other options may be Bases or Salts, but magnesium hydroxide is commonly used medicinally due to its effectiveness and low side effects.

Like sprinkling baking soda in acidic juice, antacids neutralize excess H⁺ to relieve discomfort.

Hint: Look for a mild, soluble hydroxide used in medicine.

Magnesium hydroxide neutralizes stomach acid and is widely used as an antacid.

Explanation: Degumming removes natural gums or pectins from fibers to make them suitable for papermaking. Caustic soda (NaOH) is commonly used to break down these gummy substances.

Other Acids or bleaching agents may treat fibers but do not remove gums effectively. The alkaline nature of NaOH saponifies and dissolves pectins, improving fiber quality.

Like washing sticky residue off cloth using soap, caustic soda cleans the fiber surface.

Hint: Look for a strong alkaline chemical that removes natural gums.

Caustic soda is used to degum fibers during paper production.

Explanation: Dolomite (CaMg(CO₃)₂) is used to neutralize acidic soils. It increases soil pH by reacting with H⁺ ions, improving nutrient availability for plants.

It also supplies calcium and magnesium, essential for plant growth. Acidic soils limit crop yield, so dolomite acts as a soil conditioner.

Like adding baking soda to acidic water to neutralize it, dolomite raises soil pH.

Hint: Consider how farmers reduce soil acidity.

Dolomite powder is added to increase soil pH and reduce acidity.

Explanation: Salts of weak Acids and strong Bases produce basic solutions due to hydrolysis. Sodium acetate (CH₃COONa) reacts with water, generating OH⁻ ions.

Other Salts like NaCl or CuSO₄ do not hydrolyze to form OH⁻ ions, so the solution remains neutral or acidic.

Like dissolving baking soda in water to make it slightly basic, sodium acetate produces OH⁻ ions.

Hint: Identify salts derived from a weak acid and strong Base.

Sodium acetate produces a basic solution due to hydrolysis in water.

Explanation: Base strength depends on the degree of dissociation in water and OH⁻ availability. Ammonium hydroxide is weak, aluminium hydroxide is amphoteric, magnesium hydroxide is moderately strong, and sodium hydroxide is very strong.

Thus, increasing basicity order is NH₄OH < Al(OH)₃ < Mg(OH)₂ < NaOH. Like comparing the strength of soap solutions, stronger Bases release more OH⁻ ions.

Hint: Consider solubility and OH⁻ release in water.

Increasing basicity: Ammonium hydroxide < Aluminium hydroxide < Magnesium hydroxide < Sodium hydroxide. [/explain] Show Answer

Which of the following is not a typical characteristic of salts?

(a) They have a structured lattice arrangement

(b) They possess low melting points but high boiling points

(c) They are brittle

(d) They can conduct Electricity in molten form or when dissolved in water

[explain]Explanation: Salts are ionic compounds forming crystalline lattices, brittle, with high melting points, and conduct Electricity in molten or aqueous state. Low melting points are not characteristic; most salts have high melting points.

Like sugar crystals versus metal wires, salts are brittle and not flexible.

Hint: Compare expected melting points with real ionic Solids.

Low melting points are not a typical property of salts.

Explanation: At very low HCl concentration (10⁻⁸ M), H⁺ from water autoionization (~10⁻⁷ M) significantly contributes to pH. Total [H⁺] ≈ 1.1 × 10⁻⁷ M, so pH slightly less than 7, not exactly 7.

This shows that extremely dilute strong Acids can be affected by water’s own H⁺.

Like measuring a few drops of acid in a bucket of water, the pH remains near neutral.

Hint: Consider autoionization of water in very dilute solutions.

The pH is slightly less than 7 due to the combined effect of water and dilute HCl.

Explanation: The stomach secretes hydrochloric acid (HCl) to maintain low pH (~1–2), activate pepsinogen to pepsin, and kill bacteria.

Other Acids like acetic or citric acid are not naturally secreted in the stomach.

Like lemon juice aiding Digestion externally, HCl does it internally.

Hint: Strong mineral acid present in gastric juice.

The human stomach secretes hydrochloric acid to aid Digestion.

Explanation: Litmus is extracted from lichens, a symbiotic association of fungi and algae. It contains dye compounds that change color depending on pH.

Earthworms, ants, and algae alone do not produce litmus.

Like extracting color from beetroot for pH testing, lichens provide natural pH indicators.

Hint: It is a natural, plant-like organism.

Litmus is obtained from lichens and used as a pH indicator.

Explanation: Bases are corrosive and react with Metals or plastics; glass is inert and resists corrosion. Storing strong Bases in glass prevents container degradation and ensures chemical stability.

Like storing acids in glass bottles, glass is safe for corrosive solutions.

Hint: Consider the material’s resistance to chemical attack.

Glass containers prevent corrosion and safely store bases.

Explanation: Calamine solution contains zinc hydroxide (Zn(OH)₂), often mixed with ferric oxide (ZnO + Fe₂O₃), used as a mild antiseptic and soothing lotion.

Other options like sodium hydrogen carbonate or magnesium hydroxide are not used in calamine.

Like adding soothing powder to water, Zn(OH)₂ forms the active solution.

Hint: Zinc compound with mild antiseptic property.

Calamine solution contains zinc hydroxide as the active ingredient.

Explanation: The chlor-alkali process electrolyzes aqueous NaCl to produce NaOH, Cl₂, and H₂. The correct reaction uses NaCl(aq) + H₂O(l), producing NaOH(aq) + Cl₂(g) + H₂(g).

This process is industrially important for producing chlorine and caustic soda.

Like splitting water into H₂ and O₂, electrolysis splits saltwater into useful products.

Hint: Focus on aqueous NaCl and products of electrolysis.

2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g)

Explanation: Vinegar (acetic acid, CH₃COOH) reacts with sodium bicarbonate (NaHCO₃) to produce carbon dioxide (CO₂), water, and sodium acetate.

CO₂ is a gas that escapes as bubbles, causing fizzing. Other options like H₂ or O₂ are not produced in this acid-Base reaction.

Like soda fizzing when opened, CO₂ forms as a visible effervescence during the reaction.

Hint: Look for the gas responsible for bubbling in vinegar-baking soda reaction.

Carbon dioxide is released during the reaction between vinegar and baking soda.

Explanation: Dilute H₂SO₄ is safer and promotes better crystal formation. Concentrated acid is highly corrosive and may hinder uniform crystallization.

Using dilute acid also reduces the risk of side reactions or decomposition of copper salts.

Like adding a small amount of sugar to water gradually for clear crystals, dilute acid improves crystallization.

Hint: Consider safety and crystal quality in acid use.

Dilute H₂SO₄ is preferred for safe and effective formation of copper sulphate crystals.

Explanation: Calcium chloride (CaCl₂) is a desiccant that absorbs moisture. HCl gas is hygroscopic and reacts with water, so passing through CaCl₂ removes moisture.

This ensures dry HCl gas for subsequent reactions or collection.

Like using silica gel to keep electronics dry, CaCl₂ prevents HCl gas from being wet.

Hint: Think of a drying agent for gases.

The guard tube with CaCl₂ removes moisture from hydrogen chloride gas.

Explanation: Al₂(SO₄)₃ in water undergoes hydrolysis producing Al³⁺ ions and H⁺ ions. This makes the solution acidic.

It is not basic or amphoteric in aqueous solution; pH is below 7.

Like dissolving alum in water for water purification, it slightly acidifies the solution.

Hint: Consider hydrolysis of metal cations in water.

Aluminium sulphate solution is acidic due to hydrolysis of Al³⁺ ions.

Explanation: Bleaching powder (CaOCl₂) reacts with dilute acids to release nascent oxygen (O), which acts as a strong oxidizing agent.

This property is used in disinfection and bleaching. Other options like acids or bases are not formed significantly.

Like oxygen released from hydrogen peroxide cleans stains, nascent oxygen from bleaching powder oxidizes impurities.

Hint: Look for the reactive oxygen species produced.

Nascent oxygen is released when bleaching powder reacts with dilute acid.

Explanation: In Chemistry, ‘p’ in pH denotes ‘potenz’ (German for power), representing the negative logarithm of H⁺ ion concentration.

pH = –log[H⁺]; it is not related to pressure, physical change, or precipitation.

Like using a logarithmic scale to simplify large numbers, pH expresses acidity compactly.

Hint: Focus on the logarithmic notation of H⁺ concentration.

‘p’ in pH stands for potenz, indicating –log[H⁺].

Explanation: Boric acid (B(OH)₃) is a weak Lewis acid, not a Base. KOH, Ca(OH)₂, and NH₄OH release OH⁻ ions and behave as bases.

Boric acid reacts differently in water and does not increase OH⁻ concentration.

Like vinegar being acidic instead of basic, B(OH)₃ behaves oppositely to typical bases.

Hint: Identify the compound that doesn’t provide OH⁻ in water.

B(OH)₃ does not act as a Base.

Explanation: Ammonium chloride (NH₄Cl) results from neutralization of a strong acid (HCl) and a weak base (NH₄OH).

Weak base + strong acid → acidic Salt. Other combinations produce neutral or different salts.

Like combining lemon juice with baking soda, the reaction depends on relative strength of acid and base.

Hint: Identify acid-base strengths.

NH₄Cl forms from a strong acid and a weak base.

Explanation: Baking soda is sodium bicarbonate (NaHCO₃). It is used for baking, cleaning, and neutralizing acids.

Other options like sodium carbonate or hydroxide have different chemical formulas and properties.

Like using baking powder in cakes, NaHCO₃ releases CO₂ for leavening.

Hint: Identify the common name of NaHCO₃.

Baking soda is sodium bicarbonate.

Explanation: Pure water undergoes autoionization, producing [H⁺] = 10⁻⁷ M, giving pH = 7. Neutral pH indicates neither acidic nor basic.

Like neutral milk or distilled water, pH is exactly 7.

Hint: Consider autoionization of water.

Pure water has a pH of 7.

Explanation: Brine is concentrated aqueous sodium chloride (NaCl) solution. It is widely used in Food preservation and chemical industries.

Other compounds listed are not referred to as brine.

Like seawater, brine is salty water with dissolved NaCl.

Hint: Look for common Salt in water.

Brine is a solution of sodium chloride.

Explanation: Temporary hardness is caused by soluble hydrogen carbonates of calcium and magnesium. Upon boiling, these decompose, removing hardness.

Other salts like chlorides or sulfates cause permanent hardness.

Like adding vinegar to hard water, temporary hardness can be removed by heating.

Hint: Focus on bicarbonate salts.

Temporary hardness arises due to hydrogen carbonates of Ca²⁺ and Mg²⁺.

Explanation: Sodium hydroxide (NaOH) is a strong base, dissociating completely to release OH⁻ ions, making the solution basic and conductive.

Acids or neutral compounds do not provide OH⁻ ions for basicity.

Like saltwater conducting Electricity, NaOH solution conducts due to mobile ions.

Hint: Look for a strong hydroxide.

NaOH forms a basic, electrically conductive solution.

Explanation: Sodium carbonate (Na₂CO₃·10H₂O) is washing soda. It removes hardness in water and acts as a cleaning agent.

Other compounds like sodium bicarbonate or calcium carbonate have different uses.

Like soap softening water, washing soda improves cleaning efficiency.

Hint: Look for hydrated sodium carbonate.

Washing soda is sodium carbonate.

Explanation: Ammonium hydroxide (NH₄OH) is partially ionized in water, releasing few OH⁻ ions compared to strong bases like NaOH.

Low degree of ionization makes it weak despite being chemically a hydroxide.

Like weak tea versus strong tea, the concentration of OH⁻ is low.

Hint: Consider degree of ionization in water.

Ammonium hydroxide is weak because it is only partially ionized.

Explanation: Lime water (Ca(OH)₂) reacts with CO₂ to form calcium carbonate (CaCO₃), making it milky. Excess CO₂ further reacts to form soluble calcium bicarbonate, making the solution clear again.

Like cloud forming in clear water due to suspended particles, initial turbidity is due to CaCO₃ precipitation.

Hint: Focus on CO₂ interaction with Ca(OH)₂.

Lime water turns milky initially due to CaCO₃ formation.

Explanation: HCl gas is acidic and reacts with wet litmus paper to turn blue to red. Dry litmus paper does not react because H⁺ ions need water to migrate.

Like using a wet indicator to detect acidity, moisture is required for color change.

Hint: Consider solubility and ion mobility.

HCl affects wet litmus paper, turning blue to red.

Explanation: NaCl is neutral; it does not change pH. Thus, the strip remains green, indicating neutrality.

Other colors would indicate acidic or basic changes, which don’t occur here.

Like adding table Salt to water, the solution remains neutral.

Hint: Sodium chloride is a neutral salt.

The pH strip remains green after adding common salt.

Explanation: Plaster of Paris (CaSO₄·½H₂O) reacts with water to form gypsum (CaSO₄·2H₂O), a process called setting. The reaction involves crystallization, not gas release.

Like cement hardening when mixed with water, calcium sulfate hydrates and solidifies.

Hint: Focus on reaction with water leading to crystallization.

Plaster of Paris sets due to reaction with water forming gypsum crystals.