Head office, Rana Pratap Bagh, Delhi011 2721 1512+91 97116 99511

Science project idea

Diaper crystals: how much water do they hold, and what does salt do?

The white powder inside a baby diaper is a polymer that soaks up many times its mass of fresh water and turns to gel. It absorbs far less from salty water, which shows how dissolved salt changes the way it works.

Make it yourself8 numbered steps, the materials and the science, all free.Go to the stepsGet a ready-made kitReady-made models, kits and spare parts in the store under Chemistry.Browse ChemistryAsk on WhatsAppAsk the head office whether a model or kit for this idea is available.Send a message

What it shows

The white powder inside a baby diaper is a polymer that soaks up many times its mass of fresh water and turns to gel. It absorbs far less from salty water, which shows how dissolved salt changes the way it works.

The science

Sodium polyacrylate has long chain molecules with many charged sites that attract water by osmosis. In pure water, water enters the polymer network and swells it into a gel of 100 times its mass or more. In salty water the outside solution is already rich in ions, so there is less difference to pull water in and the polymer swells much less. Plant growers use the same crystals to hold water in soil.

Materials

Cost: under Rs 200. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.

  • A clean unused diaper or a packet of water-absorbing crystals from a garden shop
  • Three plastic cups
  • A measuring cup or syringe
  • Common salt
  • A kitchen scale
  • A spoon
  • A tray and a plastic bag for waste

Safety

For this project: An adult must cut the diaper open and shake out the powder outside. Avoid breathing the dust and keep it away from eyes and mouth. Never put the swollen gel down the drain, because it blocks pipes; put it in the dustbin.

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 rules

Step 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.

  1. Weigh out 1 g of the powder into each of three cups labelled A, B and C. If you have only crystals, use 1 g each.

  2. Add 100 ml of plain water to cup A and stir once. Watch the powder swell and become a gel.

  3. Dissolve one spoon of salt in 100 ml of water and add it to cup B.

  4. Dissolve three spoons of salt in 100 ml of water and add it to cup C.

  5. After 10 minutes, tip each cup over a sieve or cloth and measure how much liquid does not get absorbed.

  6. Calculate the water absorbed by each gram of powder by subtracting the leftover from 100 ml.

  7. Compare A, B and C in a table and draw a bar chart.

  8. Put a pinch of the gel in a bowl on a sunny shelf and note how it dries over a few days.

What you should see

Cup A absorbs most or all of the 100 ml and becomes a thick gel, while the salty cups absorb noticeably less, the 3 spoon cup the least.

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 the powder made of?

A polymer called sodium polyacrylate.

2Why does salt reduce the absorption?

Salty water already has many ions, so less water is drawn into the gel.

3Where else are such polymers used?

In garden soil crystals and in sanitary products.

4Why must the gel not go down the drain?

It swells and blocks the pipes.

Free ebook: Viva Questions for Science Projects

Take it further

Grow two seedlings, one in soil with the crystals and one without, and compare how often they need water.

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

WhatsApp us