Physical and Chemical Changes, Solution

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

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

Ripening of banana
Souring of milk
Melting of ice
Passing CO₂ gas into water
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Melting of ice is a physical change because it involves a change of state from solid water (ice) to liquid water. The chemical composition of the substance remains H₂O throughout the process; only its physical form changes. Physical changes are typically reversible.
A physical change alters the form or appearance of a substance but not its chemical composition.
Ripening of banana and souring of milk are examples of chemical changes because they involve chemical reactions that result in the formation of new substances with different chemical properties (e.g., sugars converting to acids, proteins and sugars breaking down). Passing CO₂ gas into water leads to the formation of carbonic acid (H₂CO₃), which is also a chemical change. Chemical changes are often irreversible.

2. Which one of the following substances shows sublimation behaviour or p

Which one of the following substances shows sublimation behaviour or property at standard temperature and pressure ?

Solid carbon dioxide
Water
Liquid nitrogen
Iron
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The correct answer is Solid carbon dioxide.
Sublimation is a phase transition where a substance changes directly from a solid to a gas without passing through a liquid phase. The question asks which substance exhibits this behaviour at standard temperature and pressure (STP). STP is typically defined as 0°C (273.15 K) and 1 atmosphere (101.325 kPa).
Let’s examine the options:
A) Solid carbon dioxide (Dry ice): At atmospheric pressure (around 1 atm), solid CO₂ sublimes directly into gaseous CO₂ at -78.5°C. While this temperature is below the typical definition of standard temperature (0°C or 25°C), it readily sublimes at standard atmospheric pressure. Compared to other options, it’s the classic example of sublimation under common conditions. Its triple point is -56.6°C at 5.18 atm; below the triple point pressure, it only exists as solid and gas.
B) Water: At 1 atm, ice melts at 0°C and water boils at 100°C. Water exists as a liquid between 0°C and 100°C at 1 atm. Sublimation of ice can occur below 0°C, but water does not typically sublime from solid to gas at standard temperature (0°C or 25°C) and pressure (1 atm) as its primary transition.
C) Liquid nitrogen: Nitrogen boils at -196°C at 1 atm. It is a gas at standard temperature. It doesn’t sublime (solid to gas) at standard temperature and pressure; it transitions from liquid to gas at a much lower temperature.
D) Iron: Iron is a solid at standard temperature and pressure. It melts at 1538°C and boils at 2862°C at 1 atm. It does not sublime under standard conditions.
Solid carbon dioxide is the substance among the options that is well-known for undergoing sublimation at standard atmospheric pressure, even if the temperature is not strictly “standard room temperature”. Given the context of such questions, dry ice is the intended answer for a substance showing sublimation behaviour at standard pressure.
Other substances that sublime at standard atmospheric pressure include iodine, naphthalene (mothballs), camphor, and arsenic. The triple point of a substance is the temperature and pressure at which the three phases (solid, liquid, and gas) of that substance coexist in thermodynamic equilibrium. If the pressure is below the triple point pressure, heating the solid at that pressure will result in sublimation rather than melting. For CO₂, the triple point pressure (5.18 atm) is above standard atmospheric pressure (1 atm), which is why it sublimes instead of melting at 1 atm.

3. Dyes in black ink can be separated by :

Dyes in black ink can be separated by :

evaporation.
centrifugation.
sublimation.
chromatography.
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UPSC CISF-AC-EXE – 2022
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Black ink is typically a mixture of several coloured dyes. To separate the different components (dyes) of a mixture, various separation techniques can be used. The technique suitable for separating different coloured dyes in ink is chromatography.
Chromatography is a technique used to separate mixtures based on the differential distribution of components between a stationary phase and a mobile phase. For separating dyes in ink, paper chromatography is a common method. The paper acts as the stationary phase, and a solvent (like water or alcohol) acts as the mobile phase. As the solvent moves up the paper by capillary action, it carries the different dyes with it. Dyes that are more soluble in the solvent and have weaker interactions with the paper move faster and further up the paper, resulting in their separation into distinct spots at different heights.

Let’s consider the other options:
A) Evaporation separates a soluble solid from a liquid solvent by heating the solution to evaporate the solvent. This would only leave the mixture of dyes behind, not separate them.
B) Centrifugation separates components of a mixture based on their density by spinning at high speed. It is used for separating solids from liquids or liquids of different densities (e.g., separating cream from milk, separating blood cells from plasma). It is not suitable for separating dissolved dyes.
C) Sublimation is the process where a substance changes directly from a solid to a gas state upon heating (e.g., dry ice, iodine). It is used to separate sublimable solids from non-sublimable ones. Dyes in ink are not typically sublimable.

– Black ink is a mixture of different coloured dyes.
– Chromatography is a separation technique used to separate components of a mixture based on their different affinities for a stationary phase and a mobile phase.
– Paper chromatography is specifically effective for separating different dyes in ink.
Paper chromatography is a simple and effective type of chromatography often demonstrated in schools. Other types include thin-layer chromatography (TLC), column chromatography, gas chromatography (GC), and high-performance liquid chromatography (HPLC), all based on the same principle of differential partitioning between phases.

4. Conversion of limestone to quick lime is an example of

Conversion of limestone to quick lime is an example of

decomposition reaction.
substitution reaction.
combination reaction.
double decomposition reaction.
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Limestone is primarily calcium carbonate (CaCO₃). Quick lime is calcium oxide (CaO). The conversion of limestone to quick lime involves heating calcium carbonate to a high temperature, which causes it to break down into calcium oxide and carbon dioxide gas: CaCO₃ (s) → CaO (s) + CO₂ (g). This reaction where a single compound breaks down into two or more simpler substances is called a decomposition reaction.
Decomposition reaction: A single reactant breaks down into simpler products.
Combination reaction: Two or more reactants combine to form a single product.
Substitution reaction: An atom or group is replaced by another atom or group.
Double decomposition reaction: Two compounds exchange ions or groups to form two new compounds.
This process, the thermal decomposition of calcium carbonate, is known as calcination or lime burning and is carried out in a kiln.

5. Which one of the following statements about air is correct ?

Which one of the following statements about air is correct ?

Air is a compound.
Air is a mixture.
Air is an element.
Air always has more oxygen than nitrogen.
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The correct answer is Air is a mixture.
Air is composed of various gases such as nitrogen, oxygen, argon, carbon dioxide, and trace amounts of other gases, along with water vapour and particulate matter. These components are not chemically combined in fixed proportions but are simply mixed together. The composition of air can vary depending on factors like location and altitude. This characteristic defines it as a mixture.
A compound is formed when two or more elements are chemically bonded in a fixed ratio (e.g., water H₂O). An element is a pure substance consisting only of atoms that all have the same numbers of protons (e.g., Oxygen O₂). Air is primarily about 78% nitrogen and 21% oxygen by volume, so nitrogen is present in a greater proportion than oxygen under normal conditions.

6. Water can dissolve more substances than any other liquid because

Water can dissolve more substances than any other liquid because

it is dipolar in nature
it is a good conductor of heat
it has high value of specific heat
it is an oxide of hydrogen
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Water is often called the “universal solvent” because it can dissolve a wide variety of substances. This ability is primarily due to its dipolar nature. The water molecule (H₂O) has a bent shape with the oxygen atom having a partial negative charge and the two hydrogen atoms having partial positive charges. This creates a polar molecule with distinct positive and negative ends. These charged ends can attract and interact with the charged or partially charged parts of other molecules, effectively surrounding and separating them (solvation), thus dissolving them.
– Water is a polar molecule due to unequal sharing of electrons between oxygen and hydrogen atoms.
– The polarity creates partial positive and negative charges on the molecule (dipole).
– The dipolar nature allows water to form hydrogen bonds and to effectively hydrate and dissolve other polar and ionic substances.
While water’s high specific heat and other properties are important for various natural processes, they are not the reason for its dissolving capacity. Being an oxide of hydrogen describes its chemical composition but not the mechanism of solvation. Water’s polarity is key to its role as a solvent in biological systems and the environment.

7. Which of the following is/are the example/examples of chemical change?

Which of the following is/are the example/examples of chemical change?

  • 1. Crystallization of sodium chloride
  • 2. Melting of ice
  • 3. Souring of milk

Select the correct answer using the code given below.

1 and 2 only
3 only
1, 2 and 3
None
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UPSC IAS – 2014
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The correct answer is B) 3 only. A chemical change results in the formation of new substances with different properties, while a physical change only alters the form or appearance of a substance, but not its chemical composition.
1. Crystallization of sodium chloride: This is a process where dissolved salt precipitates out of a solution as crystals. It is a physical change because the chemical composition (NaCl) remains the same.
2. Melting of ice: This is a change of state from solid water (ice) to liquid water. It is a physical change because the substance is still water (H₂O), just in a different form.
3. Souring of milk: This occurs when bacteria convert lactose (milk sugar) into lactic acid. This process creates new chemical substances (lactic acid) and changes the milk’s properties (taste, smell, texture). This is a chemical change.
Indicators of a chemical change often include the formation of a gas, a change in color, a change in temperature, the formation of a precipitate, or the production of light or sound. Souring of milk is a form of fermentation, a biochemical process leading to chemical transformation.

8. Which one among the following is an example of endothermic process ?

Which one among the following is an example of endothermic process ?

Combustion of carbon
Mixing acid and alkali
Photosynthesis
Respiration
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The correct option is C. Photosynthesis is an example of an endothermic process because it requires energy input (light energy) from the surroundings to proceed.
– An endothermic process absorbs heat or energy from the surroundings. The temperature of the surroundings typically decreases.
– An exothermic process releases heat or energy into the surroundings. The temperature of the surroundings typically increases.
– Photosynthesis converts light energy into chemical energy stored in glucose: $6\text{CO}_2 + 6\text{H}_2\text{O} + \text{Light Energy} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$.
Combustion of carbon, mixing acid and alkali (neutralization), and respiration are all exothermic processes, releasing energy.

9. Water is a good solvent for dissolving ionic compounds because

Water is a good solvent for dissolving ionic compounds because

It has a high specific heat
It has no colour
It has a high dipole moment
It has a high boiling point
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UPSC CAPF – 2024
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Water is an excellent solvent for ionic compounds because it is a polar molecule with a high dipole moment. An ionic compound is held together by strong electrostatic forces between positive and negative ions in a crystal lattice. Water molecules, being polar, have a partial positive charge on the hydrogen atoms and a partial negative charge on the oxygen atom. When an ionic compound is placed in water, the positive ends of water molecules are attracted to the negative ions, and the negative ends are attracted to the positive ions. These attractive forces between water molecules and ions (ion-dipole interactions) are strong enough to overcome the lattice energy of the ionic compound, causing the ions to separate and become surrounded by water molecules (hydration). A high dipole moment indicates strong polarity, which is crucial for this process. High specific heat, no colour, and high boiling point are characteristics of water but are not the primary reason for its ability to dissolve ionic compounds.
Water’s ability to dissolve ionic compounds stems primarily from its strong polarity and high dipole moment, which allows it to hydrate (surround and separate) the positive and negative ions.
The process of dissolving an ionic compound in water is called dissociation and hydration. The hydrated ions are dispersed throughout the solution. Water is often referred to as the “universal solvent” because of its ability to dissolve a wide range of substances, including many ionic and polar covalent compounds.

10. Which one among the following statements about matter is NOT correct ?

Which one among the following statements about matter is NOT correct ?

On increasing the temperature of solids, the kinetic energy of particles increases
The maximum temperature at which a solid melts to become a liquid at atmospheric pressure is called its melting point
Particles of steam have more energy than water at 100°C
Direct change of gas to solid is called deposition
This question was previously asked in
UPSC CAPF – 2024
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Let’s examine each statement:
A) On increasing the temperature of solids, the kinetic energy of particles increases. This is correct. Temperature is a measure of the average kinetic energy of the particles.
B) The maximum temperature at which a solid melts to become a liquid at atmospheric pressure is called its melting point. For a pure crystalline solid at constant pressure, melting occurs *at* a specific temperature called the melting point. Saying it is the *maximum* temperature at which it melts is imprecise or incorrect. Above the melting point, the substance is in the liquid state. For impure or amorphous solids, melting can occur over a range of temperatures, but the term ‘melting point’ usually refers to the start of the melting range or the specific temperature for pure substances. The phrasing “maximum temperature at which a solid melts” is misleading for a pure substance and not a standard definition for the melting point.
C) Particles of steam have more energy than water at 100°C. This is correct. When water boils at 100°C, it absorbs latent heat of vaporization to change state into steam at the same temperature. This latent heat increases the internal energy (primarily potential energy related to intermolecular forces) of the particles in the gaseous state compared to the liquid state at the same temperature.
D) Direct change of gas to solid is called deposition. This is correct. The phase transition from gas directly to solid is known as deposition or desublimation. The reverse process, solid to gas, is called sublimation.
Therefore, statement B is the incorrect statement due to its inaccurate phrasing regarding the melting point of a solid.
Understanding the states of matter, phase transitions, and the relationship between temperature, energy, and particle motion. Melting point is the specific temperature at which a pure crystalline solid melts at a given pressure.
Phase transitions like melting, boiling, sublimation, condensation, freezing, and deposition occur at specific temperatures (at constant pressure) for pure substances, involving the absorption or release of latent heat. The kinetic energy of particles is related to temperature within a single phase, while potential energy changes significantly during phase transitions.
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