Physical and Chemical Changes, Solution

61. 20 g of common salt is dissolved in 180 g of water. What is the mass p

20 g of common salt is dissolved in 180 g of water. What is the mass percentage of the salt in the solution?

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5%
9%
10%
15%
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UPSC NDA-1 – 2017
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The correct value for the mass percentage of the salt in the solution is 10%. Mass percentage is a way to express the concentration of a solution.
The mass percentage of a solute in a solution is calculated as (Mass of solute / Mass of solution) * 100%.
In this case, the mass of the solute (common salt) is 20 g, and the mass of the solvent (water) is 180 g.
The mass of the solution is the sum of the mass of the solute and the mass of the solvent: Mass of solution = 20 g + 180 g = 200 g.
Mass percentage of salt = (20 g / 200 g) * 100%.
Mass percentage = (1/10) * 100% = 10%. This means that 10% of the total mass of the solution is due to the common salt. This concentration unit is often used in chemistry and everyday life, for example, in expressing the concentration of solutions like saline or sugar syrups.

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62. A homogeneous mixture contains two liquids. How are they separated?

A homogeneous mixture contains two liquids. How are they separated?

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By filtration
By evaporation
By distillation
By condensation
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The correct method for separating a homogeneous mixture containing two liquids is distillation. A homogeneous mixture of two liquids is typically a solution, where the liquids are miscible.
Distillation is a separation technique used to separate components of a liquid mixture by selective boiling and condensation. If the two liquids in the homogeneous mixture have different boiling points, one liquid will vaporize more readily than the other when heated. The vapor is then condensed back into a liquid and collected, achieving separation.
Filtration is used to separate insoluble solids from liquids. Evaporation separates a dissolved solid from a volatile solvent, or separates liquids if one is much more volatile and the other is the desired component (or discarded). Condensation is the phase change from gas to liquid and is a step within the distillation process, not a standalone separation method for a liquid mixture.

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63. Glass is a

Glass is a

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liquid
colloid
non-crystalline amorphous solid
crystalline solid
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Glass is a non-crystalline amorphous solid. Unlike crystalline solids which have a highly ordered, repeating atomic structure, glass has a disordered atomic structure similar to a liquid, but it is rigid and retains its shape like a solid over geological timescales. It is sometimes described as a supercooled liquid because it flows extremely slowly, although this is a simplified view; a more accurate description is that it exists in a glass state, which is a rigid, disordered state.
Glass lacks the long-range crystalline order found in crystalline solids and is considered an amorphous solid.
Common glass (soda-lime glass) is primarily composed of silicon dioxide (silica, SiO2), with additives like sodium carbonate (soda ash) and calcium carbonate (lime) to lower its melting point and improve workability.

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64. The temperature at which a solid melts to become a liquid at the atmos

The temperature at which a solid melts to become a liquid at the atmospheric pressure is called its melting point. The melting point of a solid is an indication of

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strength of the intermolecular forces of attraction
strength of the intermolecular forces of repulsion
molecular mass
molecular size
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The correct answer is A) strength of the intermolecular forces of attraction.
The melting point of a solid is the temperature at which it transitions from the solid state to the liquid state. In the solid state, particles are held together by intermolecular forces of attraction (or interatomic/interionic forces in the case of network solids or ionic compounds). Melting requires supplying enough thermal energy to overcome these forces, allowing the particles to move more freely as a liquid. A higher melting point indicates that stronger attractive forces exist between the particles in the solid, requiring more energy to break them apart.
Molecular mass and size can influence the strength of intermolecular forces (e.g., larger molecules can have stronger van der Waals forces), but the melting point is a direct indicator of the strength of these forces within the specific solid structure. Intermolecular repulsive forces also exist, but melting is primarily about overcoming the attractive forces that maintain the rigid structure of the solid.

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65. Which one of the following is a reduction reaction?

Which one of the following is a reduction reaction?

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2 Mg (s) + O₂ (g) → 2 MgO (s)
S (s) + O₂ (g) → SO₂ (g)
2 HgO (s) $xrightarrow{heat}$ 2 Hg (l) + O₂ (g)
Mg (s) + S (s) → MgS (s)
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The correct answer is C) 2 HgO (s) $\xrightarrow{heat}$ 2 Hg (l) + O₂ (g).
Reduction is a chemical process that involves the gain of electrons, a decrease in oxidation state, gain of hydrogen, or loss of oxygen. In option C, Mercury (Hg) in mercuric oxide (HgO) has an oxidation state of +2. In the product, elemental mercury (Hg) has an oxidation state of 0. The oxidation state of Hg decreases from +2 to 0, which signifies a reduction. Also, the metal oxide is losing oxygen to form the metal, which is a characteristic of reduction processes.
Let’s look at the other options:
A) 2 Mg (s) + O₂ (g) → 2 MgO (s): Mg is oxidized (0 to +2), O₂ is reduced (0 to -2).
B) S (s) + O₂ (g) → SO₂ (g): S is oxidized (0 to +4), O₂ is reduced (0 to -2).
D) Mg (s) + S (s) → MgS (s): Mg is oxidized (0 to +2), S is reduced (0 to -2).
All listed reactions are redox reactions involving both oxidation and reduction. However, option C specifically shows the reduction of a metal cation (Hg²⁺) to its elemental form (Hg⁰) by the loss of oxygen from its compound, making it a clear example of a reduction process relative to the metal. The question asks which *one* is *a* reduction reaction, and while all involve reduction, C best exemplifies the reduction of the stated element (Hg).

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66. Which one of the following is not a chemical change?

Which one of the following is not a chemical change?

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Ripening of fruits
Curdling of milk
Freezing of water
Digestion of food
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A chemical change involves a change in the chemical composition of a substance, resulting in the formation of new substances with different properties. A physical change alters the form or appearance of a substance but not its chemical identity.
A) Ripening of fruits: Involves complex chemical reactions that change color, sweetness, texture, and aroma (e.g., conversion of starch to sugars, breakdown of pigments). This is a chemical change.
B) Curdling of milk: Usually caused by the action of acids or enzymes on milk proteins (casein), causing them to coagulate and separate from the liquid whey. This is a chemical change.
C) Freezing of water: Water changes from liquid state to solid state (ice). The chemical composition remains H₂O. This is a change of state, which is a physical change.
D) Digestion of food: Involves the breakdown of complex food molecules (carbohydrates, proteins, fats) into simpler molecules by enzymes. These are chemical reactions. This is a chemical change.
Therefore, freezing of water is not a chemical change; it is a physical change.
Chemical changes result in new substances; physical changes do not change the chemical composition.
Changes of state (melting, freezing, boiling, condensation, sublimation) are common examples of physical changes. Burning, rusting, cooking, and decomposition are examples of chemical changes.

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67. In the reaction ZnO + C → Zn + CO, ‘C’ acts as

In the reaction ZnO + C → Zn + CO, ‘C’ acts as

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an acid
a base
an oxidising agent
a reducing agent
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In the reaction ZnO + C → Zn + CO, zinc oxide (ZnO) loses oxygen to become zinc (Zn), undergoing reduction. Carbon (C) gains oxygen to become carbon monoxide (CO), undergoing oxidation.
A reducing agent is a substance that causes another substance to be reduced, while it itself is oxidized. In this reaction, carbon (C) removes oxygen from ZnO, thus causing the reduction of ZnO. Carbon itself is oxidized in the process. Therefore, carbon acts as a reducing agent.
Conversely, an oxidising agent is a substance that causes another substance to be oxidized, while it itself is reduced. ZnO acts as the oxidising agent in this reaction because it provides oxygen for the oxidation of carbon and is itself reduced. This is a common method for the extraction of metals from their oxides using carbon as a reducing agent (smelting).

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68. Which one of the following is an example of physical change?

Which one of the following is an example of physical change?

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Flowering of plants
Clotting of blood
Cooking of food
Melting of ice
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The correct answer is Melting of ice.
A physical change alters the form or appearance of a substance but not its chemical composition. It is often reversible. A chemical change results in the formation of new substances with different chemical properties.
Melting of ice is a change of state from solid water (ice) to liquid water. The chemical formula remains H₂O. This process is reversible (freezing). Flowering, clotting of blood, and cooking of food involve chemical reactions and the formation of new substances.

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69. Which one of the following mixtures will not have uniform composition?

Which one of the following mixtures will not have uniform composition?

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Oil in water
Sulphur in carbon disulphide
Alcohol in water
Sugar in water
This question was previously asked in
UPSC Geoscientist – 2022
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The correct answer is Oil in water.
A mixture with uniform composition throughout is called a homogeneous mixture, while a mixture with non-uniform composition is called a heterogeneous mixture. Solutions (like sugar in water, alcohol in water, sulphur in carbon disulphide) are homogeneous mixtures.
Oil and water are immiscible liquids, meaning they do not dissolve in each other. When mixed, they form distinct layers, resulting in a heterogeneous mixture with non-uniform composition.

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70. Tincture of iodine is:

Tincture of iodine is:

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a solution of iodine in alcohol only.
a solution of iodine in water only.
a solution of iodine in alcohol and water.
iodine.
This question was previously asked in
UPSC Geoscientist – 2021
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Tincture of iodine is a solution of iodine in a solvent that includes alcohol (typically ethanol) and water. It is commonly used as an antiseptic. Sometimes, potassium iodide or sodium iodide is also added to increase the solubility of iodine in the solvent.
Tincture of iodine is a solution of iodine, commonly dissolved in a mixture of alcohol (ethanol) and water.
The term “tincture” generally refers to a medicine made by dissolving a drug in alcohol. While historically some tinctures were purely alcoholic, modern tinctures, especially antiseptic ones like iodine, often use a mixture of alcohol and water.

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