Acid, Base and Salt

What is the chemical composition of a soda-acid type fire extinguisher?

[amp_mcq option1=”Solution of sodium hydrogen carbonate and sulfuric acid” option2=”Solution of sodium carbonate and sulfuric acid” option3=”Solution of carbon dioxide and sulfuric acid” option4=”Solution of sodium chloride and sulfuric acid” correct=”option1″]

A soda-acid fire extinguisher contains a solution of sodium hydrogen carbonate (NaHCO₃), also known as sodium bicarbonate or baking soda, and a separate container of sulfuric acid (H₂SO₄). When the extinguisher is activated, the acid mixes with the sodium bicarbonate solution. A chemical reaction occurs:
$2NaHCO_3 (aq) + H_2SO_4 (aq) \rightarrow Na_2SO_4 (aq) + 2H_2O (l) + 2CO_2 (g)$
This reaction produces a large volume of carbon dioxide gas (CO₂), which is expelled under pressure. The CO₂ gas is denser than air and blankets the fire, displacing oxygen and suppressing the combustion.

– Soda-acid fire extinguishers utilize the reaction between sodium hydrogen carbonate and sulfuric acid.
– The extinguishing agent is primarily carbon dioxide gas produced by the reaction.

Soda-acid extinguishers are effective on Class A fires (ordinary combustibles like wood, paper, textiles). They are not suitable for Class B (flammable liquids) or Class C (electrical) fires, and can be dangerous to use on electrical fires due to the water content. Modern fire extinguishers use different agents like foam, dry chemical powder, or CO₂ itself.

Which of the following statements with regard to heating of lead nitrate powder over a flame are correct?

• Brown fumes of NO are released.
• Colourless O₂ gas is released.
• It is an example of oxidation reaction.
• It is an example of thermal decomposition used for the production of NO₂ gas.

Select the correct answer using the code given below.

[amp_mcq option1=”1 and 2″ option2=”2, 3 and 4″ option3=”1, 3 and 4″ option4=”2 and 4 only” correct=”option2″]

Heating lead(II) nitrate powder is a classic example of thermal decomposition. The reaction is: 2Pb(NO₃)₂(s) → 2PbO(s) + 4NO₂(g) + O₂(g).
Statement 1: Brown fumes of NO are released. Incorrect. The brown fumes are due to nitrogen dioxide (NO₂), not nitrogen monoxide (NO), which is colourless.
Statement 2: Colourless O₂ gas is released. Correct. Oxygen gas is a product of the decomposition and is colourless.
Statement 3: It is an example of oxidation reaction. Correct. While it’s a redox reaction involving both oxidation and reduction (Nitrogen is reduced from +5 to +4 in NO₂, and Oxygen is oxidized from -2 in NO₃⁻ to 0 in O₂), it is valid to say that it is *an example* of a reaction where oxidation occurs.
Statement 4: It is an example of thermal decomposition used for the production of NO₂ gas. Correct. Heating breaks down lead nitrate, and NO₂ is one of the products, making it a method for producing NO₂.
Statements 2, 3, and 4 are correct, while statement 1 is incorrect.

Thermal decomposition of lead nitrate produces solid lead oxide, brown nitrogen dioxide gas, and colourless oxygen gas. It is a redox reaction.

Metal nitrates decompose on heating, but the products vary depending on the reactivity of the metal. Nitrates of less reactive metals like lead and copper produce metal oxide, nitrogen dioxide, and oxygen. Nitrates of more reactive metals like sodium and potassium usually produce metal nitrite and oxygen.

Calcium oxide reacts vigorously with water to produce slaked lime whose solution is used for white-washing walls. This slaked lime reacts with component (A) in air to form a thin layer of component (B) to give a shiny finish. What are the components (A) and (B) ?

[amp_mcq option1=”A – O₂ ; B – CaCO₃” option2=”A – CO₂ ; B – Ca(OH)₂” option3=”A – O₂ ; B – Ca(OH)₂” option4=”A – CO₂ ; B – CaCO₃” correct=”option4″]

Calcium oxide (quicklime, CaO) reacts with water to form calcium hydroxide (slaked lime, Ca(OH)₂). When slaked lime solution is applied to walls during white-washing, it slowly reacts with carbon dioxide gas present in the air to form a thin, hard layer of calcium carbonate (CaCO₃).

The initial reaction is CaO(s) + H₂O(l) → Ca(OH)₂(aq). This slaked lime solution is used for white-washing. The slaked lime then reacts with atmospheric carbon dioxide: Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l). Component (A) from the air is carbon dioxide (CO₂), and the component (B) formed as a thin layer on the wall is calcium carbonate (CaCO₃). This layer of calcium carbonate gives the walls a smooth, shiny finish after a few days as the reaction completes and the water evaporates.

This process is a form of carbonation. The calcium carbonate layer is chemically similar to limestone and marble, providing a durable and aesthetically pleasing surface. This reaction also helps to ‘set’ the white-wash onto the wall surface.

Which one of the following sodium compounds is used for softening hard water ?

[amp_mcq option1=”Na₂CO₃” option2=”NaHCO₃” option3=”NaOH” option4=”Na₂SO₄” correct=”option1″]

Hard water contains dissolved minerals, primarily calcium and magnesium ions. Sodium carbonate (Na₂CO₃), also known as washing soda, is commonly used to soften hard water by removing these ions.

When sodium carbonate is added to hard water, the calcium and magnesium ions react with the carbonate ions to form insoluble calcium carbonate (CaCO₃) and magnesium carbonate (MgCO₃). These precipitates settle down or can be filtered out, effectively reducing the concentration of hardening ions in the water. The overall reaction with calcium ions is: Ca²⁺(aq) + Na₂CO₃(aq) → CaCO₃(s) + 2Na⁺(aq). The resulting sodium ions (Na⁺) do not contribute to water hardness.

Other methods for water softening include using ion-exchange resins, which swap calcium and magnesium ions for sodium ions, or using chelating agents that bind the metal ions. While boiling can remove temporary hardness (caused by bicarbonates), it does not remove permanent hardness (caused by sulfates and chlorides), which sodium carbonate can address.

Which one of the following is used in soda-acid fire extinguishers ?

[amp_mcq option1=”Sodium chloride” option2=”Sodium hydrogen carbonate” option3=”Calcium hydroxide” option4=”Acetic acid” correct=”option2″]

Soda-acid fire extinguishers utilize a chemical reaction to produce carbon dioxide gas, which is used to extinguish fires. The primary components involved in this reaction are an acid (commonly sulfuric acid) and sodium hydrogen carbonate (NaHCO₃), also known as sodium bicarbonate or baking soda.

When the fire extinguisher is activated, the acid mixes with the sodium hydrogen carbonate solution. The reaction is: 2NaHCO₃(aq) + H₂SO₄(aq) → Na₂SO₄(aq) + 2H₂O(l) + 2CO₂(g). The rapid production of carbon dioxide gas builds up pressure, expelling the mixture out of the nozzle. The CO₂ gas also helps smother the fire by displacing oxygen, and the water cools the burning material.

Soda-acid fire extinguishers are effective against Class A fires (ordinary combustibles like wood, paper, and cloth) but are not suitable for Class B (flammable liquids), Class C (electrical equipment), or Class D (combustible metals) fires. Using them on electrical fires is particularly dangerous due to the presence of water.

The number of water molecules associated with copper sulphate molecule to form crystals is

[amp_mcq option1=”2″ option2=”4″ option3=”5″ option4=”6″ correct=”option3″]

Copper sulphate commonly exists as a hydrate with the chemical formula CuSO₄ · 5H₂O. This form is copper(II) sulfate pentahydrate, which is bright blue. The notation ‘· 5H₂O’ indicates that there are 5 water molecules associated with each unit of copper sulphate in the crystal lattice. These water molecules are called water of crystallization.

– Crystalline copper sulphate is a hydrate.
– The formula of common copper sulphate crystals is CuSO₄ · 5H₂O.
– This means 5 water molecules are associated with each CuSO₄ unit.

Anhydrous copper sulphate (CuSO₄), which has no water of crystallization, is white. When water is added to anhydrous copper sulphate, it forms the blue pentahydrate, and this reaction is exothermic. This property makes anhydrous copper sulphate useful as a test for the presence of water.

Match List-I with List-II and select the correct answer using the code given below the Lists :

List-I (Name)	List-II (Formula)
A. Bleaching powder	1. NaHCO3
B. Baking soda	2. Na2CO3 · 10H2O
C. Washing soda	3. Ca(OH)2
D. Slaked lime	4. CaOCl2

Code :

[amp_mcq option1=”A-4, B-1, C-2, D-3″ option2=”A-4, B-2, C-1, D-3″ option3=”A-3, B-2, C-1, D-4″ option4=”A-3, B-1, C-2, D-4″ correct=”option1″]

Matching the names with their correct chemical formulas:
– Bleaching powder is Calcium oxychloride, formula CaOCl₂. (A-4)
– Baking soda is Sodium bicarbonate or Sodium hydrogen carbonate, formula NaHCO₃. (B-1)
– Washing soda is Sodium carbonate decahydrate, formula Na₂CO₃ · 10H₂O. (C-2)
– Slaked lime is Calcium hydroxide, formula Ca(OH)₂. (D-3)
The correct matching is A-4, B-1, C-2, D-3.

– Bleaching powder: CaOCl₂
– Baking soda: NaHCO₃
– Washing soda: Na₂CO₃ · 10H₂O
– Slaked lime: Ca(OH)₂

Quicklime is Calcium oxide (CaO). When quicklime reacts with water, it forms slaked lime (Calcium hydroxide, Ca(OH)₂). Bleaching powder is manufactured by passing chlorine gas over dry slaked lime. Baking soda is used in baking as a leavening agent. Washing soda is used in laundry detergents and for softening water.

When air is blown from mouth into a test tube containing limewater, the limewater turns milky. This is due to the presence of

[amp_mcq option1=”water vapour” option2=”oxygen” option3=”carbon dioxide” option4=”carbon monoxide” correct=”option3″]

Limewater is a solution of calcium hydroxide (Ca(OH)₂). When carbon dioxide (CO₂) is passed through limewater, it reacts to form insoluble calcium carbonate (CaCO₃), which precipitates out of the solution, causing it to turn milky or cloudy. Human exhaled air contains a higher concentration of carbon dioxide compared to inhaled air.

– Limewater (Calcium hydroxide solution) is a test for carbon dioxide.
– The reaction is: Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l).
– Calcium carbonate (CaCO₃) is insoluble, causing the milky appearance.
– Human breath contains significant amounts of carbon dioxide.

The reaction is reversible. If excess carbon dioxide is passed through the milky solution, the calcium carbonate can react further with water and carbon dioxide to form soluble calcium bicarbonate (Ca(HCO₃)₂), causing the milkiness to disappear.
CaCO₃(s) + H₂O(l) + CO₂(g) → Ca(HCO₃)₂(aq)

Match List-I with List-II and select the correct answer using the code given below the Lists :

List-I (Compound)	List-II (Use)
A. Boric acid	1. Antiseptic
B. Citric acid	2. Food preservative
C. Magnesium hydroxide	3. Antacid
D. Acetic acid	4. Pickle

Code :

[amp_mcq option1=”A-1, B-2, C-3, D-4″ option2=”A-1, B-3, C-2, D-4″ option3=”A-4, B-3, C-2, D-1″ option4=”A-4, B-2, C-3, D-1″ correct=”option1″]

The matching is as follows:
– A. Boric acid: Known for its mild antiseptic properties, often used in eyewashes and topical applications.
– B. Citric acid: A common food additive used as a preservative due to its acidic nature and antioxidant properties.
– C. Magnesium hydroxide: A component of antacids (like Milk of Magnesia) used to neutralize excess stomach acid.
– D. Acetic acid: The main component of vinegar, commonly used in pickling as both a flavouring agent and preservative.
Thus, the correct mapping is A-1, B-2, C-3, D-4.

– Boric acid acts as an antiseptic.
– Citric acid functions as a food preservative.
– Magnesium hydroxide is used as an antacid.
– Acetic acid is used in pickles.

Many substances have multiple uses. For instance, citric acid is also used in cleaning products and cosmetics. Magnesium hydroxide is also used as a laxative at higher doses.

A solution having pH equal to zero is known as

[amp_mcq option1=”highly alkaline solution” option2=”highly acidic solution” option3=”weakly acidic solution” option4=”neutral solution” correct=”option2″]

The pH scale measures the acidity or alkalinity of a solution. A pH value of 7 indicates a neutral solution. Values less than 7 indicate acidic solutions, with decreasing pH corresponding to increasing acidity. Values greater than 7 indicate alkaline (basic) solutions, with increasing pH corresponding to increasing alkalinity. A pH of zero represents a high concentration of hydrogen ions, characteristic of a very strong acidic solution.

– pH is defined as the negative logarithm of the hydrogen ion concentration ([H+]).
– pH < 7 is acidic, pH > 7 is alkaline, pH = 7 is neutral.
– A lower pH indicates higher acidity.

While the standard pH scale usually ranges from 0 to 14, highly concentrated strong acids can have negative pH values, and highly concentrated strong bases can have pH values greater than 14. A pH of 0 is within the range of very acidic solutions.