# Liquid State Properties > Liquid state for JEE Chemistry: vapour pressure and boiling, viscosity and surface tension, temperature dependence and the hydrogen-bond anomalies of water. - Canonical URL: https://prepelephant.com/topics/jee/chemistry/liquid-state-properties - Exam / course: JEE · Subject: Chemistry - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Liquid State Properties", PrepElephant, https://prepelephant.com/topics/jee/chemistry/liquid-state-properties ## Direct answer A liquid sealed in a flask reaches equilibrium when the rate of evaporation equals the rate of condensation; the pressure exerted by the vapour then is the equilibrium vapour pressure, fixed by temperature alone and untouched by the liquid's volume, the surface area or the container size. Boiling begins when this vapour pressure matches the external pressure, which is why water boils at 373 K at 1 atm but far lower on a hill station. Two more transport properties complete the chapter: viscosity (resistance to flow, falling with temperature) and surface tension (contracting force per unit length, also falling with temperature), both vanishing at the critical temperature; water's anomalies — densest at 277 K, ice that floats, an abnormally high boiling point — all trace back to hydrogen bonding. ## What you must remember - **Vapour pressure determinants:** only temperature; adding more liquid or changing flask shape does nothing, a fact the exam tests through misleading data. - **Boiling point definition:** the temperature at which vapour pressure equals atmospheric pressure — at 2 atm water boils near 393 K (pressure-cooker logic), and on a Himalayan pass nearer 363 K. - **Evaporative cooling:** the fastest molecules escape first, so the residual liquid loses energy and cools; evaporation is always a cooling process. - **Viscosity behaviour:** eta falls as temperature rises in liquids (thermal energy defeats cohesive forces), the exact opposite of gases, where viscosity rises with temperature. - **Surface tension arithmetic:** gamma is force per length (N/m); work done = gamma × increase in area, the reason drops assume spherical shape (minimum area for a given volume). - **Capillary rise:** h = 2 gamma cos(theta) / (r rho g) — narrower tube, higher rise; concave mercury meniscus gives depression instead because cos(theta) is negative. - **Critical ceiling:** both vapour pressure distinction, viscosity and surface tension vanish at the critical temperature, above which liquid and gas are one phase. - **Water's hydrogen-bond fingerprints:** maximum density at 277 K (ice's open lattice collapses on melting), boiling point of 373 K against H2S at 213 K, and high specific heat. ## Reading evaporation against boiling Consider two glasses of water: one in shade, one in the sun. Both evaporate, but the sunlit one empties first because vapour pressure climbs steeply with temperature — roughly doubling near room temperature for a modest 10-15 K rise. Neither boils in an open room, because boiling demands vapour pressure equal to about 760 mm Hg, reached only at 373 K in the open; yet both glasses "boil" instantly if you put them under a vacuum bell jar, since the external pressure collapses below the liquid's vapour pressure. This single reasoning chain answers every JEE-style question on boiling at altitude, on pressure cookers cutting cooking time (water near 383-393 K at 2 atm denatures food faster), and on why a wet cloth on a bottle cools it. Add the surface-tension lens: hot soup spreads and wets better than cold because gamma has dropped, and detergent works by slashing gamma so the water penetrates fabric fibres. ## Where marks leak The chapter sits inside states of matter, which was removed from the JEE Main syllabus in the 2023 rationalisation; JEE Advanced retains the liquid state in its physical chemistry section, and NEET-UG still examines it. Within the retained questions, three errors recur: claiming vapour pressure depends on the amount of liquid (it does not), stating that viscosity of liquids increases with temperature (that is the gas trend), and forgetting that both surface tension and viscosity trend to zero at the critical temperature, not at the boiling point. A classic Advanced trap asks why glycerol is more viscous than water — three -OH groups per molecule give it an extensive hydrogen-bond network, a structural answer worth more than a definition. ## Frequently asked questions ### Why does a liquid cool during evaporation? The highest-energy molecules escape preferentially, lowering the average kinetic energy of those left behind; temperature is that average, so it falls. ### What exactly is the boiling point? The temperature at which the liquid's vapour pressure equals the external pressure, so bubbles of vapour can survive and rise within the bulk liquid. ### Why does liquid viscosity fall while gas viscosity rises with temperature? Liquid flow is resisted by cohesive attractions, which thermal agitation overcomes; gas flow is mediated by momentum transfer between molecules, which speeds up as molecules move faster. ### Why is water denser at 277 K than at 273 K? Melting collapses ice's open hydrogen-bonded lattice, and the compact liquid keeps contracting until 277 K, after which thermal expansion wins — hence ice floats. ### Which liquid properties vanish at the critical temperature? Surface tension and viscosity effectively vanish, and the meniscus disappears, because liquid and vapour become a single indistinguishable phase.