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Hydrosphere and the water cycle

الأسبوع 1 · الوحدة الأولى «النظام البيئي المائي» (افتتاحية ومقدمة) والموضوع ١-١ «الغلاف المائي على كوكب الأرض»: الأغلفة الأربعة ونسب الماء المالح والعذب والمتجمد ودورة الماء والنتح والإخراج · 3 دروس
2

The water cycle and what moves the water

Integrated Science Sec 1 Term 1, Unit 1: pp. 3–7
GoalDescribe the water cycle as an almost closed system and put evaporation, condensation, precipitation and infiltration in order.
New words
evaporation · liquid water turning into water vapour at its surfacecondensation · water vapour cooling into tiny drops of liquid water, which form cloudsprecipitation · water falling from clouds as rain or snowinfiltration · rain seeping into the tiny spaces of soil and rock, where it collects as groundwater
Explanation

Water on and near Earth keeps changing between solid, liquid and gas, and it moves from place to place along many paths. Together these paths form the water cycle (also called the hydrological cycle). It is an almost closed system: almost, because very little water is gained from space or lost to it, so the same water goes round and round. The Sun’s energy drives it. Water evaporates from oceans, lakes and rivers; evaporation happens at the surface of the water at any temperature, and the water does not have to boil. The vapour rises and cools, and condensation makes tiny drops that form clouds. Precipitation brings the water back as rain or snow. Some of it soaks into the soil and porous rock by infiltration and becomes groundwater, and some runs into rivers and returns to the sea. The cycle has no starting point, and over time it shapes the land, carries dissolved substances around and supports living things.

vapour risescools: condensationprecipitationflows back to the seaevaporationcloudrain and snowland and rivers
Worked examples
Follow one drop. It evaporates from the sea as vapour, rises and cools, condenses with other drops into a cloud, falls as rain on a hill, and then either soaks into the ground (infiltration) or runs down a river back to the sea. The drop has changed from liquid to gas and back to liquid, and the journey can start again.
Does water need to boil to evaporate? No. Wet washing dries on a line in air at 25 °C, and a puddle disappears on a mild day. Evaporation happens at the surface at any temperature; boiling is only a fast form of it.
Why is the cycle called “almost closed”? The water just changes place and state. Hardly any water arrives from space or leaves to it, so the total amount of water on Earth stays almost the same while the water keeps moving.
Class activity

Water cycle in a bag, with a grown-up. Put a little cool tap water with one drop of food colouring in a clear zip bag, close it and tape it to a sunny window. After a few hours, describe what you see on the inside of the bag. Draw the bag and add arrows and the words evaporation, condensation and precipitation where you think each happens. Say which part of the real cycle each part of the bag stands for.

Practice
1
Gas to liquid or liquid to gas: which one is condensation? Give an everyday example.
2
What is the name of the process in which rain soaks through soil, and what does it form?
3
Put in order: precipitation, evaporation, condensation.
4
Why do scientists say the water cycle has no starting point?