Water Cycle

Hydrological Cycle

The hydrological cycle is also known as the water cycle.

The water is recycled again and again through the process of evaporation, condensation and water transfers such as surface run-off.

Step 1 Evaporation- Water changes to a gas and evaporates to the air.

Step 2 Condensation: Water condenses into clouds.

Step 3 Precipitation- Water falls from the clouds as rain, snow, sleet or hail.

Step 4 Surface runoff- Water goes to lakes, river, oceans or Aquifers(underground rivers- percolation)

Step 5 Transpiration- Some water gets absorbed by the plants for photosynthesis and evaporates in the plants.

1 of 5

Hard and Soft Water

Hard water is water that contains an appreciable quantity of dissolved minerals (like calcium and magnesium). Calcium hydrogen carbonate is the main cause of hard water. It forms when rain falls on rock of limestone and chalk. These are made of calcium carbonate which is not soluble in water. But rainwater contains CO2 disolved from the air, which makes it acidic. So it reacts with the rocks to form calcium hydrogen carbonate which is soluble an ends up in taps. The reaction is:

H2O + CO2 + CaCO3 ---> Ca(HCO3)2 

2 types of hardness:

  • Temporary- Hardness can be removed.
  • Permanent- Boiling does not affect it.

Soft water is treated water in which the only ion is sodium.

How can you tell if its hard or soft? Soap will give you a clue.

  • If soap lathers easily it means the water contains very little calcium and magnesiums compounds. It is soft water.
  • If a greyish scum(layer of dirt) and hardly any lather shows that larger amounts are present. It is hard water.

The scum forms because the calcium and magnesium compounds react with soap to give and insoluble product:

calcium sulphate + sodium stearate(soap) ---> calcium stearate(scum) + sodium sulphate

2 of 5

Why do we soften our water?

When it boils down, the major difference between hard and soft water can best be seen while doing household chores. Hard water is to blame for dingy looking clothes, dishes with spots and residue, and bathtubs with lots of film and soap scum. Even hair washed in hard water may feel sticky and look dull.

From hard to soft:




4.Reverse Osmosis

How does water become hard? When it is filtered by rocks.

3 of 5

Benefits of using hard water?


  • Good for teeths and bones
  • Reduces risk of heart diseases
  • A coating of CaCO3 inside pipes, boilers and radiators helps to prevent corrosion.


  • When the coating develops into a scale it must be removed from time to time to prevent blockages.
  • Used up more soap than soft water does.
  • It leaves a messy scum, and its tougher on laundry.

4 of 5

Benefits of using soft water?


  • Clothes get cleaner
  • Laundry uses less soap and can be done at lower temperatures
  • Less scum in your bath tub
  • It leaves no scale in pipers or boilers


  • Cantain more sodium ions than hard water does ( sodium is linked to heart disease)
  • Dissolves metals as lead (poisonous) and cadmium ( linked to hipertension)
5 of 5


No comments have yet been made

Similar Chemistry resources:

See all Chemistry resources »