Acid, Base and Salt

1. A solution has a pH value around 13.0. The solution may be called

A solution has a pH value around 13.0. The solution may be called

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a strong acid
a strong base
neutral
a weak acid
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UPSC CISF-AC-EXE – 2024
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The pH scale measures the acidity or basicity of an aqueous solution. The scale ranges typically from 0 to 14. Solutions with a pH less than 7 are acidic, a pH of 7 is neutral, and solutions with a pH greater than 7 are basic (alkaline). A pH value around 13.0 is significantly greater than 7 and is close to the maximum value on the scale, indicating a highly basic solution.
A pH value of 13.0 indicates a strong base because it is high on the pH scale (greater than 7).
Strong acids have very low pH values (close to 0 or 1), weak acids have pH values between 1 and 7, neutral solutions have a pH of 7, weak bases have pH values between 7 and 14, and strong bases have very high pH values (close to 13 or 14). Examples of strong bases include sodium hydroxide (NaOH) and potassium hydroxide (KOH).

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2. The chemical formula of washing soda is

The chemical formula of washing soda is

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NaHCO₃
Na₂CO₃
Na₂CO₃·10H₂O
NaOCl
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UPSC CISF-AC-EXE – 2024
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Washing soda is the common name for hydrated sodium carbonate. Its chemical formula is Na₂CO₃·10H₂O, which represents sodium carbonate decahydrate. This compound is widely used as a cleaning agent and in various industrial processes.
Washing soda is chemically known as sodium carbonate decahydrate (Na₂CO₃·10H₂O).
Option A, NaHCO₃, is the chemical formula for baking soda (sodium bicarbonate). Option B, Na₂CO₃, is anhydrous sodium carbonate, also known as soda ash. Option D, NaOCl, is the chemical formula for sodium hypochlorite, the main ingredient in many household bleaches.

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3. When electricity is passed through brine, it produces

When electricity is passed through brine, it produces

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NaCl
NaOH
KCl
KOH
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UPSC CISF-AC-EXE – 2021
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The electrolysis of brine (a concentrated aqueous solution of sodium chloride, NaCl) is an industrial process that produces sodium hydroxide (NaOH), chlorine gas (Cl₂), and hydrogen gas (H₂). The overall reaction is 2NaCl (aq) + 2H₂O (l) → 2NaOH (aq) + Cl₂ (g) + H₂ (g).
Brine is a solution of NaCl in water.
Electrolysis is the process of using electricity to drive a non-spontaneous chemical reaction.
The products are sodium hydroxide, chlorine, and hydrogen.
This process is known as the chlor-alkali process and is a major industrial method for producing these three important chemicals. NaOH is a strong base used in many industries, while Cl₂ is used for disinfection and manufacturing, and H₂ is used as a fuel or in chemical synthesis.

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4. pH of blood is

pH of blood is

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7·0
6·5
7·15
7·40
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The pH of healthy human blood is maintained within a narrow range of 7.35 to 7.45. This slightly alkaline range is crucial for proper metabolic function. Option D, 7.40, falls within this normal range and is often cited as an average value.
pH measures the acidity or alkalinity of a solution.
A pH of 7 is neutral. Below 7 is acidic, and above 7 is alkaline (basic).
Maintaining blood pH homeostasis is vital for physiological processes.
Deviations from the normal blood pH range can lead to serious health conditions such as acidosis (pH below 7.35) or alkalosis (pH above 7.45). The body has buffer systems to help maintain this narrow pH range.

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5. Which one of the following aqueous solutions will be neutral ?

Which one of the following aqueous solutions will be neutral ?

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NH₄Cl
NaCl
KCN
NaHSO₄
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UPSC CISF-AC-EXE – 2020
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An aqueous solution is neutral if the salt dissolved in water is formed from the reaction of a strong acid and a strong base.
A) NH₄Cl is formed from a weak base (NH₄OH) and a strong acid (HCl). The solution will be acidic due to the hydrolysis of NH₄⁺ ions.
B) NaCl is formed from a strong base (NaOH) and a strong acid (HCl). The solution will be neutral as neither Na⁺ nor Cl⁻ ions undergo significant hydrolysis.
C) KCN is formed from a strong base (KOH) and a weak acid (HCN). The solution will be basic due to the hydrolysis of CN⁻ ions.
D) NaHSO₄ is sodium bisulfate. It is formed from a strong base (NaOH) and sulfuric acid (H₂SO₄), which is a strong acid. However, HSO₄⁻ is the conjugate base of the strong acid H₂SO₄, but it is also an acid itself that can donate a proton (HSO₄⁻ ⇌ H⁺ + SO₄²⁻). This makes the solution acidic.
Therefore, only NaCl forms a neutral aqueous solution.
The neutrality of an aqueous salt solution depends on the strength of the parent acid and base from which the salt is derived. Salts of strong acid and strong base yield neutral solutions. Salts of strong acid and weak base yield acidic solutions. Salts of weak acid and strong base yield basic solutions. Salts of weak acid and weak base yield solutions whose pH depends on the relative strengths of the weak acid and weak base.
Hydrolysis is the reaction of an ion with water. Cations of weak bases undergo acidic hydrolysis, releasing H⁺ ions. Anions of weak acids undergo basic hydrolysis, releasing OH⁻ ions. Cations of strong bases and anions of strong acids do not undergo significant hydrolysis.

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6. An acid solution of pH equal to three is gradually diluted with water.

An acid solution of pH equal to three is gradually diluted with water. As a result of this dilution, the pH of the solution will

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reach near to zero.
reach near to fourteen.
reach near to seven.
remain unchanged.
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UPSC CISF-AC-EXE – 2018
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An acid solution with pH = 3 has a relatively high concentration of H+ ions. Diluting this solution with pure water (which has a neutral pH of 7) reduces the concentration of the acid, and thus the concentration of H+ ions from the acid. The total concentration of H+ ions approaches the concentration of H+ ions in pure water (which is $10^{-7}$ M at 25°C, corresponding to pH 7). As the concentration of H+ decreases, the pH value (which is -log[H+]) increases. The pH will move from 3 towards 7, but it will not exceed 7 when diluting with pure water.
Diluting an acidic solution with neutral water increases its pH, moving it towards 7.
Similarly, diluting a basic solution with neutral water decreases its pH, moving it towards 7. The pH of a solution being diluted with pure water will approach the pH of pure water (7), but will not cross the neutrality point unless the original solution was extremely concentrated or involved complex acid-base equilibria not typical for this type of question.

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7. An aqueous solution of a salt is known as brine. The salt is :

An aqueous solution of a salt is known as brine. The salt is :

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Sodium chloride
Potassium chloride
Calcium chloride
Sodium nitrate
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UPSC CAPF – 2023
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Brine is the term used for a highly concentrated aqueous solution of sodium chloride (NaCl). While other salt solutions exist, “brine” specifically refers to sodium chloride solution, often much more concentrated than seawater. Seawater itself is a form of brine, but artificial brines are typically prepared by dissolving large amounts of NaCl in water for industrial or culinary purposes.
Brine is a concentrated aqueous solution of sodium chloride (common salt).
Brine has various uses, including food preservation, as a heat-transfer fluid, for de-icing roads, and in industrial processes like chlor-alkali production.

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8. Which of the following makes bread soft and spongy when baking soda is

Which of the following makes bread soft and spongy when baking soda is added?

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Sodium salt of acid
NaHCO₃
CO₂
H₂O
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The correct answer is C, CO₂.
Baking soda is sodium bicarbonate (NaHCO₃). When used in baking, it acts as a leavening agent.
In the presence of heat or an acidic ingredient (like those often found in dough or baking powder), baking soda undergoes a chemical reaction that produces carbon dioxide (CO₂) gas.
The main reactions are:
1. Decomposition by heat (slow without acid): 2NaHCO₃(s) → Na₂CO₃(s) + H₂O(g) + CO₂(g)
2. Reaction with an acid (e.g., citric acid, tartaric acid, or acids in buttermilk/yogurt): NaHCO₃(s) + H⁺(aq) → Na⁺(aq) + H₂O(l) + CO₂(g)
The carbon dioxide gas gets trapped within the gluten matrix of the dough, causing it to expand and rise. This creates small air pockets, which give the bread or cake a light, fluffy, soft, and spongy texture.
While NaHCO₃ is the source of the leavening action, it is the carbon dioxide gas produced from its reaction or decomposition that physically causes the dough to rise and become soft and spongy. Sodium carbonate (Na₂CO₃), formed from heat decomposition, can leave a slightly soapy taste if not neutralized by an acid.

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9. Which one of the following oxides reacts with both acid and base?

Which one of the following oxides reacts with both acid and base?

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Aluminium oxide
Calcium oxide
Sodium oxide
Potassium oxide
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UPSC CAPF – 2022
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The correct answer is A, Aluminium oxide.
Oxides are classified as acidic, basic, neutral, or amphoteric based on their reaction with acids and bases.
Amphoteric oxides are oxides that exhibit both acidic and basic properties, meaning they can react with both acids and bases to form salts and water.
Aluminium oxide (Al₂O₃) is a common example of an amphoteric oxide.
It reacts with acids: Al₂O₃(s) + 6HCl(aq) → 2AlCl₃(aq) + 3H₂O(l)
It reacts with bases: Al₂O₃(s) + 2NaOH(aq) + 3H₂O(l) → 2Na[Al(OH)₄](aq) (Sodium tetrahydroxoaluminate, also written as 2NaAlO₂ + 4H₂O or similar)
Calcium oxide (CaO), Sodium oxide (Na₂O), and Potassium oxide (K₂O) are typically basic oxides, reacting with acids but not bases (except perhaps very strong, concentrated bases in specific conditions not characteristic of amphoterism).
Other examples of amphoteric oxides include Zinc oxide (ZnO) and Lead(II) oxide (PbO). The amphoteric nature of oxides often relates to the position of the metal in the periodic table; elements like Aluminium, Zinc, and Lead are on the border between metals and nonmetals.

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10. A milkman adds a small amount of baking soda to milk. This makes the p

A milkman adds a small amount of baking soda to milk. This makes the pH of the milk

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strongly alkaline
weakly alkaline
strongly acidic
weakly acidic
This question was previously asked in
UPSC CAPF – 2021
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The correct answer is B) weakly alkaline.
Baking soda is sodium bicarbonate (NaHCO₃), which is a weak base. Milk is slightly acidic or close to neutral, typically having a pH between 6.7 and 6.9. Adding a small amount of a weak base like baking soda to milk will neutralize some of the natural acidity and shift the pH towards the alkaline side. A small amount of a weak base will make the solution weakly alkaline, not strongly alkaline or acidic. Milkmen traditionally added baking soda to milk to prevent it from souring quickly, as it neutralizes the lactic acid produced by bacteria, which causes souring and lowers the pH.
The pH scale measures acidity and alkalinity. A pH of 7 is neutral. A pH less than 7 is acidic, and a pH greater than 7 is alkaline (basic). Adding a base increases the pH.

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