This chapter follows water through its three states and the processes that change one into another. It first establishes that ice and water are two forms, or states, of the same substance, differing only in behaviour — water flows and splashes while ice does not. Investigating where water from puddles and wet utensils disappears, the chapter shows water does not seep through a steel plate but instead turns into an invisible gas called water vapour; this conversion of water into vapour is called evaporation, which happens continuously even at room temperature (drying clothes, mopped floors, and sweat are examples). A second mystery — water droplets forming on the outside of a cold glass tumbler — is explained by condensation, the conversion of water vapour in the air into liquid water when it meets a cold surface, confirmed by an increase in mass on a weighing balance while the water level inside does not fall. The chapter compares the three states: ice (solid) keeps its shape and does not flow or spread; water (liquid) flows, spreads, and takes the shape of its container while keeping a fixed volume; water vapour (gas) spreads to fill all available space and has no fixed shape. States change with heating and cooling: melting is solid to liquid, freezing is liquid to solid, evaporation is liquid to gas, and condensation is gas to liquid. Evaporation is faster with a larger exposed area, in sunlight or warmth, with more air movement, and slower when humidity is high; evaporation also causes a cooling effect, as seen in earthen pots, surahis, and the pot-in-pot cooler. Finally, condensation high in the atmosphere forms clouds around dust particles, which grow into raindrops; this whole circulation of water between the Earth and the atmosphere is the water cycle, and the chapter urges us to use water wisely.
Ice and water as statesEvaporationCondensationStates of water and changes of stateCooling effect, clouds, and the water cycle
Key terms
States of water
The three forms of water — ice (solid), water (liquid), and water vapour (gas) — which are all the same substance in different states.
Evaporation
The process of conversion of water into its vapour state, which takes place continuously even at room temperature.
Condensation
The process of conversion of water vapour into its liquid state, as when vapour in the air meets a cold surface.
Melting and freezing
Melting is the conversion of a solid into a liquid; freezing is the conversion of a liquid into a solid.
Cooling effect of evaporation
The cooling produced when water evaporates, as in earthen pots and surahis that keep water cool.
Water cycle
The circulation of water between the Earth's surface and the atmosphere through evaporation, condensation, and rain, hail, or snow.
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