Equal quantities (50 ml) of the following four samples of water are placed in four beakers of 100 ml capacity. Their boiling points are determined accurately using the same thermometer. Which sample of water will have the lowermost boiling point as compared to other three samples?

1. Basics of Matter: Boiling Point and Vapor Pressure (basic)

To understand the world around us, we must first understand how matter changes states. The boiling point is the temperature at which a liquid turns into a gas at a specific atmospheric pressure. At a microscopic level, as we heat a liquid, its particles gain kinetic energy and move more vigorously. Eventually, this movement becomes strong enough to overcome the interparticle forces of attraction holding the liquid together, allowing particles to escape into a gaseous state Science, Class VIII, Particulate Nature of Matter, p.105.

Crucially, boiling is not just about temperature; it is a tug-of-war between two pressures. Inside the liquid, molecules are trying to escape, creating vapor pressure. Meanwhile, the surrounding air (atmosphere) is pushing down on the liquid. Boiling only occurs when the liquid's vapor pressure increases enough to equal the ambient atmospheric pressure. This is why boiling happens throughout the liquid, unlike evaporation, which occurs only at the surface Physical Geography by PMF IAS, Tropical Cyclones, p.358.

The boiling point is highly sensitive to the environment. If you decrease the ambient pressure—such as by climbing a high mountain—the 'lid' of air pushing down is lighter. Consequently, water molecules can escape more easily, and the liquid reaches its boiling point at a lower temperature Physical Geography by PMF IAS, Geological Time Scale, p.43. Additionally, the chemical nature of the substance matters. For example, ionic compounds (like table salt, NaCl) have very high boiling points because the electrical forces holding their ions together are incredibly strong compared to the forces in simple carbon-based compounds Science, Class X, Carbon and its Compounds, p.58-59.

Feature	Evaporation	Boiling
Location	Surface of the liquid only.	Throughout the entire mass of the liquid.
Temperature	Occurs at any temperature.	Occurs only at a specific boiling point.
Speed	Slow process.	Rapid process.

Sources: Science, Class VIII, Particulate Nature of Matter, p.105; Science, Class X, Carbon and its Compounds, p.58-59; Physical Geography by PMF IAS, Geological Time Scale, p.43; Physical Geography by PMF IAS, Tropical Cyclones, p.358

2. Pure Substances vs. Mixtures in Water (basic)

To understand water, we must first look at how science classifies matter. In our daily lives, we might call a bottle of mineral water 'pure,' but to a scientist, it is actually a mixture. A pure substance consists of only one type of particle and cannot be separated into other kinds of matter by physical processes like filtration or settling Science, Class VIII, Nature of Matter: Elements, Compounds, and Mixtures, p.121. Distilled water is the best example of a pure substance in this context, as it contains almost exclusively H₂O molecules. Most water found in nature—whether it is in a well, a bottle, or the ocean—is a mixture. As water moves through the environment, it dissolves salts, minerals, and gases, becoming a solution of Total Dissolved Solids (TDS) Environment and Ecology, Majid Hussain, Environmental Degradation and Management, p.36. These added substances change the physical behavior of the water. For instance, in the process of distillation, we use the fact that different substances have different boiling points to separate them; pure water will always boil at a specific, constant temperature (100°C at standard pressure), whereas a mixture's boiling point will vary based on what is dissolved in it Certificate Physical and Human Geography, GC Leong, Fuel and Power, p.269.

Feature	Pure Substance (e.g., Distilled Water)	Mixture (e.g., Sea Water)
Composition	Fixed; only one type of particle (H₂O).	Variable; contains H₂O plus dissolved salts/minerals.
Physical Properties	Fixed boiling and melting points.	Boiling point increases as more solute is added.
Separation	Cannot be separated by physical means.	Can be separated using physical methods (evaporation, distillation).

When we add a non-volatile solute (like salt) to pure water, it creates a phenomenon called boiling point elevation. The dissolved particles interfere with the water molecules' ability to escape into the air as vapor. Consequently, you must apply more heat to reach the boiling point. This is why sea water, which has the highest salinity, requires a higher temperature to boil than distilled water Fundamentals of Physical Geography, Class XI, Water (Oceans), p.104.

Sources: Science, Class VIII (NCERT), Nature of Matter: Elements, Compounds, and Mixtures, p.121; Environment and Ecology, Majid Hussain, Environmental Degradation and Management, p.36; Certificate Physical and Human Geography, GC Leong, Fuel and Power, p.269; Fundamentals of Physical Geography, Class XI (NCERT), Water (Oceans), p.104

3. Salinity and Oceanography (intermediate)

In oceanography, salinity is much more than just a measure of saltiness; it is a fundamental chemical property that dictates how water behaves. Technically, salinity is defined as the total content of dissolved salts in seawater, measured as the mass of salt (in grams) dissolved in 1,000 grams of seawater. This is expressed in parts per thousand (ppt) or the symbol ‰ Physical Geography by PMF IAS, Ocean temperature and salinity, p.518. The chemical presence of these salts changes the physical characteristics of water, including its density, freezing point, and boiling point.

One of the most critical chemical principles here is Boiling Point Elevation, which is a colligative property. This means the boiling point of a liquid depends on the concentration of dissolved particles, not their chemical identity. When a non-volatile solute (like Sodium Chloride, NaCl) is added to water, it lowers the vapor pressure of the liquid. Because boiling occurs only when vapor pressure equals atmospheric pressure, you must apply more heat to reach that threshold. Therefore, the higher the salinity, the higher the boiling point. This is why sea water (average salinity of 35‰) boils at a higher temperature than distilled water, which is the purest form of water with negligible dissolved solids.

In the natural world, salinity is not uniform. It is a dynamic balance between processes that add fresh water and those that remove it. For instance, the Arabian Sea exhibits higher salinity due to intense evaporation and limited fresh water influx, whereas the Bay of Bengal has lower salinity because of the massive discharge from rivers like the Ganga FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Water (Oceans), p.105. As you go deeper into the ocean, you encounter the halocline—a distinct layer where salinity changes sharply with depth, significantly affecting water density and vertical circulation Physical Geography by PMF IAS, Ocean temperature and salinity, p.520.

Sources: Physical Geography by PMF IAS, Ocean temperature and salinity, p.518; Physical Geography by PMF IAS, Ocean temperature and salinity, p.520; FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Water (Oceans), p.105

4. Thermal Properties and Daily Life Applications (intermediate)

To understand why different types of water boil at different temperatures, we must first look at what happens at the molecular level. Imagine a pot of pure water. As you heat it, the water molecules gain energy and try to escape into the air as steam. These escaping molecules exert an upward pressure called vapour pressure Physical Geography by PMF IAS, Tropical Cyclones, p.358. When this vapour pressure becomes equal to the pressure of the atmosphere pushing down on the water, the liquid begins to boil.

Now, imagine adding salt or minerals to that water. These dissolved substances are called solutes. In a solution, these solute particles occupy space at the surface of the liquid, effectively acting as "blockades" that prevent water molecules from escaping into the air Science, Class VIII NCERT, The Amazing World of Solutes, Solvents, and Solutions, p.136. Because fewer water molecules can escape, the vapour pressure drops. To overcome this and reach the boiling point, you have to provide even more heat energy. This phenomenon is known as Boiling Point Elevation.

The extent of this temperature rise depends entirely on the concentration of the dissolved particles. This is why we call it a "colligative property"—it doesn't care what is dissolved, only how much of it is there. In daily life, this explains why different water sources behave differently when heated:

Water Type	Solute Concentration (TDS)	Boiling Point Effect
Distilled Water	Negligible (Purest form)	Lowest boiling point (closest to 100°C)
Tap/Well Water	Moderate (Minerals like Calcium/Magnesium)	Slightly elevated boiling point
Sea Water	Very High (High Salinity)	Highest boiling point among common sources

Interestingly, this addition of solutes also increases the density of the water. For instance, a raw egg will sink in fresh water but float in highly salty water because the salt increases the water's density Science, Class VIII NCERT, The Amazing World of Solutes, Solvents, and Solutions, p.148. However, for our thermal study, the key takeaway is that more "stuff" dissolved in water means a higher temperature is required to make it boil.

Sources: Science, Class VIII NCERT, The Amazing World of Solutes, Solvents, and Solutions, p.136; Science, Class VIII NCERT, The Amazing World of Solutes, Solvents, and Solutions, p.137; Science, Class VIII NCERT, The Amazing World of Solutes, Solvents, and Solutions, p.148; Physical Geography by PMF IAS, Tropical Cyclones, p.358

5. Colligative Properties: Boiling Point Elevation (exam-level)

To understand why liquids boil at different temperatures, we must first look at the boiling point from a molecular level. It is the temperature at which the vapor pressure of a liquid equals the surrounding atmospheric pressure. At this point, particles gain enough kinetic energy to overcome interparticle forces of attraction and escape the liquid state to become vapor Science Class VIII, Particulate Nature of Matter, p.105. Pure substances, like distilled water, have a fixed boiling point because their molecules are uniform.

However, when we add a non-volatile solute (a substance that does not easily evaporate, like salt or minerals) to a pure solvent, the situation changes. These solute particles occupy space at the surface of the liquid, effectively 'blocking' some of the solvent molecules from escaping into the air. This reduces the vapor pressure of the liquid. To make this solution boil, we must provide more heat energy to push the vapor pressure back up to match the atmospheric pressure. This phenomenon is known as Boiling Point Elevation.

This is a colligative property, meaning it depends strictly on the concentration (number) of dissolved particles rather than their chemical identity. Therefore, the more 'impurities' or dissolved solids a water sample has, the higher its boiling point will be. For example, sea water, which has a high concentration of dissolved salts (salinity), will always have a higher boiling point than pure distilled water Physical Geography by PMF IAS, Hydrological Cycle, p.329.

Water Type	Solute Concentration (TDS)	Boiling Point Relative to Pure Water
Distilled Water	Negligible / Zero	Lowest (Standard 100 °C)
Tap / Mineral Water	Low to Moderate	Slightly Elevated
Sea Water	Very High	Highest

Sources: Science Class VIII, Particulate Nature of Matter, p.105; Physical Geography by PMF IAS, Hydrological Cycle, p.329; Science Class X, Carbon and its Compounds, p.58

6. Solving the Original PYQ (exam-level)

This question perfectly illustrates the application of colligative properties and the fundamental concept of boiling point elevation. As you have learned in your building blocks, the boiling point of a liquid is not an immutable constant; it increases when non-volatile solutes, such as salts and minerals, are added to a solvent. This occurs because solute particles occupy the surface area of the liquid, hindering solvent molecules from escaping into the vapor phase. To reach a vapor pressure equal to the atmospheric pressure, the liquid requires a higher temperature. Therefore, the more "impure" or saline the water, the higher its boiling point will be.

To arrive at the correct answer, you must evaluate the four samples based on their Total Dissolved Solids (TDS) or salinity levels. Sea water (Option D) contains the highest concentration of salts, followed by well water (Option C) and bottled mineral water (Option B), which contains added electrolytes. Reasoning logically, since boiling point elevation is directly proportional to the concentration of dissolved particles, the sample with the fewest impurities will experience the least elevation. (A) Distilled water, being the result of evaporation and condensation, is the purest form among the choices with negligible solute concentration, as noted in FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT). Thus, it will have the lowermost boiling point, closest to 100°C.

UPSC often uses common terminology as a trap. Many students might incorrectly choose "bottled mineral water" by confusing potability (safety for drinking) with chemical purity. In chemistry, "pure" means the absence of any solute, whereas mineral water is intentionally supplemented with salts. Another trap is failing to distinguish between the highest and lowermost boiling points; while sea water is the most extreme in terms of salinity (as detailed in Physical Geography by PMF IAS), the question specifically asks for the sample that boils at the lowest temperature. Always focus on the relationship: Minimum Solutes = Minimum Boiling Point Elevation.