Physical and Chemical Changes, Solution

81. Consider the following statements about mixture : A substance can be

Consider the following statements about mixture :

  • A substance can be separated into other kinds of matter by any physical process.
  • Dissolved sodium chloride can be separated from water by the physical process of evaporation.

Which of the statements given above is/are correct ?

[amp_mcq option1=”1 only” option2=”2 only” option3=”Both 1 and 2″ option4=”Neither 1 nor 2″ correct=”option2″]

This question was previously asked in
UPSC CDS-2 – 2019
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Statement 1 is incorrect. A substance (like an element or compound) cannot be separated into other kinds of matter by *any* physical process. Elements cannot be broken down by physical or chemical means. Compounds can be broken down into elements, but only by chemical processes (like electrolysis or decomposition), not physical processes. Mixtures, however, can be separated into their components by physical processes. The statement uses the word “substance” incorrectly in this context.
Statement 2 is correct. Dissolved sodium chloride (salt) in water forms a mixture. Water can be separated from the salt by evaporation, which is a physical process. The water turns into vapor and leaves the salt behind as a solid residue.
Physical processes are used to separate components of mixtures based on their physical properties. Chemical processes are required to break down compounds into simpler substances.
Other physical methods for separating mixtures include filtration, distillation, chromatography, decantation, and magnetism. The choice of method depends on the specific properties of the components in the mixture.

82. Desalination of seawater is done by using reverse osmosis. The pressur

Desalination of seawater is done by using reverse osmosis. The pressure applied to the solution is

[amp_mcq option1=”larger than osmotic pressure” option2=”smaller than osmotic pressure” option3=”equal to osmotic pressure” option4=”equal to atmospheric pressure” correct=”option1″]

This question was previously asked in
UPSC CDS-2 – 2017
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The correct answer is larger than osmotic pressure.
Osmosis is the spontaneous movement of solvent molecules through a semipermeable membrane from a region of lower solute concentration to a region of higher solute concentration. Osmotic pressure is the pressure that would have to be applied to a pure solvent to prevent it from passing into a given solution by osmosis, often used to express the concentration of the solution. Reverse osmosis is a process where a solvent is forced through a semipermeable membrane from a region of high solute concentration to a region of low solute concentration by applying a pressure greater than the osmotic pressure. This is the principle used in desalination to separate pure water from seawater.
In desalination using reverse osmosis, pressure is applied to the seawater (higher solute concentration) side of the membrane. This pressure overcomes the natural osmotic pressure and forces water molecules (solvent) through the membrane, leaving the salts behind. The membrane allows the passage of water molecules but not the dissolved salts.

83. An emulsion consists of

An emulsion consists of

[amp_mcq option1=”one liquid and one solid” option2=”one liquid and one gas” option3=”two liquids” option4=”two solids” correct=”option3″]

This question was previously asked in
UPSC CDS-2 – 2017
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An emulsion is a mixture of two liquids that are normally immiscible (unmixable or unblendable).
In an emulsion, one liquid is dispersed in the other in the form of tiny droplets.
Examples of emulsions include milk (fat dispersed in water) and vinaigrette (oil dispersed in vinegar). Emulsions are often stabilized by adding an emulsifying agent, which helps keep the two liquids dispersed and prevents separation.

84. Which one of the following is not an oxidation reaction?

Which one of the following is not an oxidation reaction?

[amp_mcq option1=”Rusting of iron” option2=”Opening of soda bottle” option3=”Rancidity” option4=”Combustion” correct=”option2″]

This question was previously asked in
UPSC CDS-1 – 2023
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The correct answer is B) Opening of soda bottle. This is a physical process involving the release of dissolved carbon dioxide gas due to reduced pressure upon opening the bottle. It does not involve a chemical reaction where a substance loses electrons or gains oxygen (oxidation).
– Oxidation is a chemical process involving the loss of electrons or an increase in oxidation state by a molecule, atom, or ion. It often involves reaction with oxygen.
– Rusting of iron is the oxidation of iron in the presence of oxygen and moisture to form iron oxides (rust).
– Rancidity is the oxidation or hydrolysis of fats and oils, leading to unpleasant smells and flavors.
– Combustion is a chemical process (often rapid) between a substance and an oxidant, usually oxygen, to produce heat and light. It is typically an oxidation-reduction (redox) reaction.
While opening a soda bottle involves a release of energy (due to decreasing potential energy of the compressed gas), the fundamental process is a physical change of state and solubility due to pressure change, not a chemical oxidation reaction like rusting, rancidity, or combustion.

85. Consider the following statements regarding burning of magnesium ribbo

Consider the following statements regarding burning of magnesium ribbon in air :

  • 1. White powder of MgO is formed.
  • 2. It is an example of combustion reaction.
  • 3. Heat and light are produced.

Which of the statements given above are correct?

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

This question was previously asked in
UPSC CDS-1 – 2023
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When magnesium ribbon is burned in air, it reacts vigorously with oxygen to form magnesium oxide (MgO). The balanced chemical equation is 2Mg(s) + O₂(g) → 2MgO(s). This reaction produces a bright white light and considerable heat.
Statement 1: White powder of MgO is formed. Correct, magnesium oxide is a white solid.
Statement 2: It is an example of combustion reaction. Correct, combustion is a rapid reaction between a substance and an oxidant, usually oxygen, to produce heat and light.
Statement 3: Heat and light are produced. Correct, the burning of magnesium is highly exothermic and produces a characteristic bright white flame.
All three statements are correct.
Burning of magnesium is a combustion reaction producing magnesium oxide, heat, and light.
Magnesium also reacts with nitrogen in the air during burning, forming a small amount of magnesium nitride (Mg₃N₂), but the primary product with oxygen is MgO. Magnesium is used in fireworks and flares due to the intense light it produces when burning.

86. Which one of the following is not an example of an oxidation reaction

Which one of the following is not an example of an oxidation reaction ?

[amp_mcq option1=”The taste of butter changes if left for a longer period” option2=”A white substance is formed when an aqueous solution of barium chloride is mixed with sodium sulphate solution” option3=”A reddish-brown powder gets coated on articles made of iron” option4=”Wine gets sour with time” correct=”option2″]

This question was previously asked in
UPSC CDS-1 – 2022
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The reaction where a white substance (BaSO4) is formed when barium chloride solution is mixed with sodium sulphate solution is a double displacement reaction (precipitation). In this type of reaction, ions are exchanged between two compounds, but there is typically no change in the oxidation states of the elements involved, meaning it is not an oxidation-reduction (redox) reaction.
Oxidation is a chemical process involving the loss of electrons, gain of oxygen, or loss of hydrogen. Redox reactions involve both oxidation and reduction. Precipitation reactions, where insoluble salts form from the mixing of solutions, are generally non-redox reactions if the oxidation states of the ions remain unchanged.
A) Rancidity of butter is due to oxidation of fats. C) Rusting of iron involves the oxidation of iron. D) Wine turning sour (acetification) is the oxidation of ethanol to acetic acid. These are all examples of oxidation reactions (specifically, redox processes). The reaction BaCl2(aq) + Na2SO4(aq) -> BaSO4(s) + 2NaCl(aq) is a pure double displacement (precipitation) reaction without electron transfer between reacting species.

87. The number of moles of oxygen gas used in the complete combustion of 1

The number of moles of oxygen gas used in the complete combustion of 1 mole of glucose is :

[amp_mcq option1=”1″ option2=”3″ option3=”6″ option4=”12″ correct=”option3″]

This question was previously asked in
UPSC CDS-1 – 2022
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The balanced chemical equation for the complete combustion of glucose (C6H12O6) shows that 1 mole of glucose reacts with 6 moles of oxygen gas (O2).
Complete combustion of a hydrocarbon or carbohydrate involves reaction with sufficient oxygen to produce carbon dioxide (CO2) and water (H2O). Balancing the chemical equation ensures that the number of atoms of each element is the same on both the reactant and product sides, representing the stoichiometric ratios.
The balanced combustion reaction for glucose is: C6H12O6(s) + 6O2(g) -> 6CO2(g) + 6H2O(l). From this equation, the molar ratio of glucose to oxygen is 1:6. Therefore, 1 mole of glucose requires 6 moles of oxygen gas for complete combustion.

88. A solution contains 20 g of solute in 180 g of solvent. If the solvent

A solution contains 20 g of solute in 180 g of solvent. If the solvent is water, what is the concentration of the solution in terms of mass percentage ?

[amp_mcq option1=”11·1%” option2=”22·2%” option3=”10%” option4=”20%” correct=”option3″]

This question was previously asked in
UPSC CDS-1 – 2021
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The concentration of a solution in terms of mass percentage is calculated as (Mass of solute / Mass of solution) × 100%.
– Mass of solute = 20 g
– Mass of solvent = 180 g
– Mass of solution = Mass of solute + Mass of solvent = 20 g + 180 g = 200 g
– Mass percentage = (20 g / 200 g) × 100% = (1/10) × 100% = 10%.
– Mass percentage is a common way to express concentration and is useful when dealing with mixtures of solids or liquids.
– Other concentration units include molarity, molality, volume percentage, etc.

89. For a certain reaction, $\Delta G^0 = -45$ kJ/mol and $\Delta H^0 = -9

For a certain reaction, $\Delta G^0 = -45$ kJ/mol and $\Delta H^0 = -90$ kJ/mol at 0 °C. What is the minimum temperature at which the reaction will become spontaneous, assuming that $\Delta H^0$ and $\Delta S^0$ are independent of temperature?

[amp_mcq option1=”273 K” option2=”298 K” option3=”546 K” option4=”596 K” correct=”option3″]

This question was previously asked in
UPSC CDS-1 – 2019
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The spontaneity of a reaction is determined by the change in Gibbs Free Energy, $\Delta G$. A reaction is spontaneous if $\Delta G < 0$. The relationship between $\Delta G$, $\Delta H$, and $\Delta S$ is given by $\Delta G = \Delta H - T\Delta S$. We are given $\Delta G^0 = -45$ kJ/mol and $\Delta H^0 = -90$ kJ/mol at 0 °C (273 K). Assuming $\Delta H^0$ and $\Delta S^0$ are independent of temperature, we can calculate $\Delta S^0$ at 273 K: $\Delta G^0_{273} = \Delta H^0 - (273 \text{ K})\Delta S^0$ $-45 \text{ kJ/mol} = -90 \text{ kJ/mol} - 273\Delta S^0$ $273\Delta S^0 = -90 + 45 = -45 \text{ kJ/mol}$ $\Delta S^0 = \frac{-45}{273} \text{ kJ/(mol·K)}$ The reaction is spontaneous when $\Delta G < 0$: $\Delta H^0 - T\Delta S^0 < 0$ $-90 \text{ kJ/mol} - T \left(\frac{-45}{273} \text{ kJ/(mol·K)}\right) < 0$ $-90 + T \left(\frac{45}{273}\right) < 0$ $T \left(\frac{45}{273}\right) < 90$ $T < 90 \times \frac{273}{45}$ $T < 2 \times 273$ $T < 546 \text{ K}$ So, the reaction is spontaneous at temperatures below 546 K. The temperature at which $\Delta G$ becomes zero (equilibrium) is $T = 546$ K. The question asks for the "minimum temperature at which the reaction will become spontaneous". While the phrasing is awkward for a reaction that is spontaneous below a certain temperature, it likely refers to the boundary temperature (546 K) where spontaneity starts or ends depending on the temperature direction, or potentially the lowest temperature among options where it is spontaneous. However, the calculation directly yields 546 K as the key temperature threshold.
– Spontaneity requires $\Delta G < 0$. - $\Delta G = \Delta H - T\Delta S$. - Calculate $\Delta S$ from the given data at 273 K. - Find the temperature range where $\Delta G < 0$. - The transition temperature where $\Delta G = 0$ is $T_{eq} = \Delta H / \Delta S$.
For reactions with $\Delta H < 0$ and $\Delta S < 0$, the reaction is spontaneous at low temperatures ($T < T_{eq}$) and non-spontaneous at high temperatures ($T > T_{eq}$). The boundary temperature is $T_{eq}$.

90. Very small insoluble particles in a liquid may be separated from it by

Very small insoluble particles in a liquid may be separated from it by using

[amp_mcq option1=”crystallization” option2=”fractional distillation” option3=”centrifugation” option4=”decantation” correct=”option3″]

This question was previously asked in
UPSC CDS-1 – 2019
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To separate very small insoluble particles from a liquid, techniques that enhance sedimentation or filtration are required. Centrifugation (C) is a process that uses centrifugal force to accelerate the sedimentation of particles suspended in a liquid. This is particularly effective for separating very fine or colloidal particles that would settle very slowly or not at all under gravity alone. Crystallization (A) is for dissolved substances. Fractional distillation (B) is for separating miscible liquids with different boiling points. Decantation (D) involves carefully pouring off the liquid after the solid has settled, which is less effective for very small particles that remain suspended.
– Centrifugation uses centrifugal force to separate solids from liquids based on density.
– It is effective for separating very small or finely suspended insoluble particles.
Centrifugation is widely used in laboratories, industry, and medical settings (e.g., separating blood components). Other methods for separating very fine insoluble particles include filtration, but centrifugation is often faster for small particles in suspension.
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