Chemical Reactions and Equations MCQs

Answer: B) Yellow ppt of lead iodide and potassium nitrate will be produced

Explanation: When lead nitrate (Pb(NO₃)₂) reacts with potassium iodide (KI), a double displacement reaction takes place. The chemical equation for the reaction is:

Pb(NO₃)₂ + 2KI → PbI₂ + 2KNO₃

• In this reaction, the lead ion (Pb²⁺) from lead nitrate combines with the iodide ion (I^-) from potassium iodide to form yellow lead iodide (PbI₂) precipitate, which is insoluble in water. 
• The potassium ion (K⁺) from potassium iodide combines with the nitrate ion (NO₃^-) from lead nitrate to form potassium nitrate (KNO₃), which remains in the solution.
• Therefore, the correct answer is B) Yellow ppt of lead iodide and potassium nitrate will be produced.

Answer: D. Decomposition reaction

Explanation: In black and white photography, silver halide crystals are used as the light-sensitive material in the film. 

• These crystals undergo a decomposition reaction when exposed to light, leading to the formation of metallic silver. 
• The unexposed areas of the film remain as silver halide crystals, while the exposed areas are converted into metallic silver, resulting in the formation of an image. 
• Therefore, the correct answer is a decomposition reaction. 
• The other reactions listed do not play a significant role in the process of black and white photography.

Answer: C) MgO

Explanation: Magnesium oxide is an ionic compound that is formed by the transfer of electrons between magnesium and oxygen atoms. The chemical formula of magnesium oxide is MgO, which indicates that it contains one magnesium ion (Mg²⁺) and one oxide ion (O^2-). The chemical formula is written in such a way that the total charge of the compound is neutral.

• Option A, MgOH, is not the correct formula for magnesium oxide. This formula suggests that the compound contains a hydroxide ion (OH^-) in addition to the magnesium ion and oxide ion, which is not the case.
• Option B, MgO₂, is also not the correct formula for magnesium oxide. This formula suggests that the compound contains two oxygen atoms for every magnesium atom, which is incorrect.
• Option D, Mg₂O, is the formula for another ionic compound that is formed by the combination of magnesium and oxygen atoms. 
• However, the correct formula for magnesium oxide is MgO, as it contains one magnesium ion and one oxide ion.

Therefore, the correct answer is C) MgO.

Answer: B) 2:1

Explanation: The chemical formula for water is H₂O, which means that each molecule of water contains two atoms of hydrogen and one atom of oxygen.

• The ratio of the volume of hydrogen to oxygen in water can be determined using the molar volumes of the gases at standard temperature and pressure (STP), which is 0 degrees Celsius and 1 atmosphere of pressure. 
• At STP, one mole of any gas occupies a volume of 22.4 liters.
• Using this information, we can calculate the ratio of the volumes of hydrogen to oxygen in the water as follows - 
• The molar mass of hydrogen is 1.008 g/mol, which means that one mole of hydrogen gas weighs 1.008 grams and occupies a volume of 22.4 litres at STP.
• The molar mass of oxygen is 16.00 g/mol, which means that one mole of oxygen gas weighs 16.00 grams and occupies a volume of 22.4 litres at STP.

Since there are two hydrogen atoms and one oxygen atom in each water molecule, the ratio of the volumes of hydrogen to oxygen in water is (2 x 22.4 L):(1 x 22.4 L), which simplifies to 2:1.

Therefore, the correct answer is B) 2:1, which means that for every two volumes of hydrogen in water, there is one volume of oxygen.

Answer: c) 2Al + 3Cl₂ → 2AlCl₃

Explanation: The correct chemical equation for the formation of aluminium chloride from aluminium and chlorine is 2Al + 3Cl₂ → 2AlCl₃. 

• In this reaction, two atoms of aluminium combine with three molecules of chlorine gas to form two molecules of aluminium chloride. 
• This equation is balanced, as the number of atoms of each element is equal on both sides of the reaction. 
• Other options are incorrect because the product is not balanced, and the number of atoms of chlorine is not equal on both sides of the equation. 

Answer: C) Reactants – sodium and chlorine; products – sodium chloride

Explanation: In the given question, it is stated that sodium and chlorine are reacted to form table salt, which is another name for sodium chloride. Therefore, the reactants are sodium and chlorine, and the product is sodium chloride. 

• Option (A) incorrectly states that sodium chloride is a reactant, which is not true. 
• Option (B) also incorrectly includes sodium chloride as a reactant. 
• Option (D) has sodium and sodium chloride as reactants, which is incorrect. 
• Option (C) correctly represents the reactants and products of the reaction. Therefore, option (c) is the correct answer.

Answer: D) KMnO₄ is an oxidising agent, it oxidises FeSO₄.

Explanation: In this experiment, we are observing a redox reaction, where reduction and oxidation are occurring simultaneously. In the acidified permanganate solution, KMnO₄ acts as an oxidising agent, i.e., it accepts electrons and is reduced itself. On the other hand, FeSO₄ is a reducing agent, i.e., it donates electrons and is oxidised itself.

• When a dilute ferrous sulphate solution is gradually added to the acidified permanganate solution, the KMnO₄ is reduced to Mn²⁺ ions, which are colourless, and the light purple colour of the solution fades and finally disappears. 
• At the same time, FeSO₄ is oxidised to Fe³⁺ ions. The overall reaction can be represented as:

5FeSO₄ + 2KMnO₄ + 8H₂SO₄ → 5Fe₂(SO₄)₃ + K₂SO₄ + 2MnSO₄ + 8H₂O

• Therefore, the correct explanation for the observation is that KMnO₄ is an oxidising agent, and it oxidises FeSO₄.

Answer: C) Ag₂S

Explanation: When Ag is exposed to air, it gets tarnished due to the formation of a black coating of Ag₂S, which is also known as tarnish. Ag₂S is formed due to the reaction of silver with hydrogen sulfide (H₂S) present in the atmosphere. The reaction between Ag and H₂S can be represented as follows:

2Ag + H₂S + O₂ → Ag₂S + H₂O

• Therefore, option C) AgS is the correct answer to the given question. 
• Option A) AgCl is incorrect as the reaction of silver with chloride ions (Cl^-) does not result in the formation of a black coating. AgCl is a white-coloured compound that forms when silver is exposed to chloride ions. 
• Option B) Ag₂O is also incorrect as the black coating formed on silver is not Ag₂O. Ag₂O is a brown colored compound that forms when silver is exposed to oxygen. 
• Option D) AgSO₄ is also incorrect as it is a white-coloured compound that forms when silver is exposed to sulfuric acid or sulfur dioxide.

Answer: A) Antioxidant

Explanation: When fats and oils are exposed to air, they undergo oxidation and become rancid. To prevent this oxidation, antioxidants are added to food containing fats and oils. These antioxidants react with oxygen and prevent fats and oils from getting oxidized. 

Therefore, option A) Antioxidant is the correct answer.

• Option B) Emulsifier is a substance added to food to help mix water-based and oil-based ingredients together, for example, mayonnaise.
• Option C) Flavoring agent is a substance added to food to provide flavour, for example, salt, herbs, and spices.
• Option D) Sweetener is a substance added to food to make it sweet, for example, sugar, honey, and artificial sweeteners.

Answer: Option A) Rancidity.

Explanation: Aerial oxidation of fats and oils in foods is a chemical process that leads to the breakdown of these substances. This process produces unpleasant odours and flavours in foods, making them unfit for consumption. The condition produced by this process is called rancidity. This process is caused by exposure to air, heat, and light, and can be prevented by storing foods in airtight containers, away from heat and light.

• Fermentation is a process in which microorganisms like yeast and bacteria are used to break down complex substances like carbohydrates and sugars into simpler ones, producing energy in the process. 
• Dehydration is a process of removing water from foods to increase their shelf life. 
• Hydrolysis is a chemical process that involves the addition of water to break down a substance into its component parts.