SSC
The temperature at which the solid, liquid and gaseous phases of a substance can exist together in equilibrium is called the:
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Answer: Option C — Triple point
Detailed Solution
The triple point of a substance is the unique combination of temperature and pressure at which all three phases of the substance, solid, liquid and gas (vapour), can coexist in thermodynamic equilibrium. At this point, the substance can change between any of the three states with only tiny changes in temperature or pressure.
For water, the triple point occurs at a temperature of 273.16 K (0.01°C) and a pressure of about 611.66 pascals (about 0.006 atmosphere). At this point, ice, liquid water and water vapour exist together. Because it is so precise and reproducible, the triple point of water was used until 2019 to define the kelvin, the SI unit of temperature: one kelvin was defined as 1/273.16 of the thermodynamic temperature of the triple point of water. Since 20 May 2019, the kelvin has been redefined in terms of a fixed value of the Boltzmann constant.
The triple point is shown on a phase diagram, a graph of pressure against temperature, where the lines (curves) separating the solid, liquid and gas regions meet at one point.
Other important points on a phase diagram:
The critical point is the temperature and pressure above which the liquid and gas phases cannot be distinguished; for water, it is about 374°C and 218 atmospheres. Above the critical temperature, a gas cannot be liquefied by pressure alone.
The normal boiling point is the temperature at which a liquid boils at one atmosphere of pressure (100°C for water).
For carbon dioxide, the triple point lies at about 5.1 atmospheres, which is above normal atmospheric pressure. This is why solid carbon dioxide (dry ice) changes directly into gas (sublimes) at ordinary pressure, without melting into a liquid.
Why not the other options?
- ✗The boiling point is where liquid and vapour are in equilibrium at a given pressure.
- ✗The critical point is where liquid and gas become indistinguishable.
- ✗The dew point is the temperature at which air becomes saturated with water vapour.
Quick tip
Remember: Triple point = solid, liquid and gas coexist (water: 273.16 K, 0.01°C, about 611 Pa); critical point of water = 374°C; dry ice sublimes because CO2's triple point is above 1 atm.
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Heat & Thermodynamics — Preparation
This question tests your understanding of Heat & Thermodynamics, a key topic in the General Knowledge section. The detailed solution above uses a step-by-step approach to show the reasoning, and where available, a shortcut that helps you solve similar questions in under 90 seconds during the actual exam.
This topic covers heat and temperature, temperature scales and conversions, thermometers, thermal expansion, specific heat and latent heat, the transfer of heat by conduction, convection and radiation, and the basic laws of thermodynamics. Questions commonly ask about the temperature at which the Celsius and Fahrenheit scales read the same (-40 degrees), why water is used in car radiators (high specific heat), how heat reaches us from the Sun (radiation), and why we feel cold after sweating (cooling by evaporation). Practise converting between Celsius, Fahrenheit and Kelvin. Practise the heat and thermodynamics questions below, written in the style of the general awareness sections of exams such as SSC, banking, railways, CUET GAT, MAH CET and CLAT. Each question comes with a detailed explanation that adds related facts, so every question helps you revise more than one point.