What it shows
Water used in the kitchen can sometimes be reused on plants, but soap can harm them. Growing seedlings with different kinds of used water shows which can be safely reused.
The science
Rice wash carries starch and small amounts of nutrients that can feed soil microbes. Soap and detergent can damage root cells and clog the soil. Comparing growth shows which water improves or harms plants, and why untreated drain water must not be used on food crops.
Materials
Cost: under Rs 50. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Bean or mung seeds
- Four pots with soil
- Rice-washing water, mild soapy water, vegetable-wash water and tap water
- Labels and a measuring cup
- A ruler
- A notebook
Safety
For this project: Use only mild washing-up water diluted well. Never use bleach, toilet cleaner or strong detergents. Wash hands after use, and do not eat the test plants.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Label the four pots tap, rice-wash, vegetable-wash and diluted soapy water.
Plant five seeds at the same depth in each pot.
Water each pot with 50 ml of its own water every day.
Keep all pots together in the same light.
Measure the height of the tallest seedling every three days.
Note leaf colour and any signs of spoiling or smell.
After three weeks, compare the final heights and fresh mass.
Graph the growth of each group.
What you should see
Tap and rice-wash water give normal or better growth, while the soapy pot shows poorer growth or yellow leaves. The graph supports a clear rule for safe reuse.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What is greywater?
Used water from sinks and washing, but not toilets.
2Why might soap harm plants?
It damages cells and changes the soil.
3Why save water this way?
It reduces the demand for fresh water.
4What must be avoided?
Chemicals, bleach and oil.
Take it further
Measure the soil pH after each treatment to see how the water changes the soil.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
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