Plug into plant power

28th December 2001 at 00:00
Making vegetable clocks, onion-coloured T-shirts and conker soap are just some of the activities Ray Oliver describes

Exploration is at the heart of science and technology. Place some lemon juice directly on your tongue and you discover a painful effect rather rapidly. Fruit and plant material provide an unusual, and sometimes edible, focus for explorations. These natural materials allow us to extract dyes for a design and make assignment in technology, or produce soap from seaweed or even turn an orange into a battery. In the 19th century, they had a rather more robust attitude to practical work. Professor Aldini, a nephew of Galvani, made a battery from the severed heads of three oxen. Bovine batteries failed to catch on and would clearly struggle to meet contemporary safety standards. We shall just have to stick to citrus power.

A fruit battery All you need to make an electric cell is a liquid that conducts (the electrolyte) and two different metals. Connecting several cells together gives a fruit battery. Oranges and lemons contain a solution of citric acid. In lemons, the acid concentration is high enough to give the characteristic sharp taste. Cut the fruit into thick slices and push two different metals into each slice. You could try copper (coin ) with zinc or iron (paper clips ). Use connecting wire to join the metals in adjacent cells, copper to zinc, all the way round to form a circuit. A very low-voltage bulb or a meter will show how well the battery operates. Chemical reactions between the metals and the acid in the fruit juice cause electrons to flow along the wire: an electric current. The investigation can use a range of fruit and vegetables - potatoes work quite well. Pupils can consider the drawbacks of fruit batteries. For example, the voltage is unstable, the metals corrode and the battery stops working if the fruit dries out.

Citric acid from a lemon There is enough citric acid in lemon juice to form crystals. Other materials in the juice will contaminate the crystals. The coloured impurities can be absorbed by charcoal dust. They stick to the surface and can then be separated. Charcoal is used to purify many things, for example, raw sugar, air in gas masks and the shoe insoles that combat sweaty feet.

Squeeze the juice from a lemon and dilute 1:1 with water. Add about 1g of charcoal powder and boil for 10 minutes. Filter the hot mixture and leave the solution of citric acid to evaporate and cool, leaving colourless crystals. Pupils can then investigate the properties of citric acid by dissolving the crystals and testing with indicators such as litmus (a material obtained from lichens). The acid will also react with metals to give hydrogen and with carbonates such as chalk to give carbon dioxide.

Taste tests Some people seem to have an unusually acute sense of taste - there may be a gender bias in this. Dilute some fresh lemon juice with water in various ratios such as 1:10, 1:50 and 1:100. Carry out taste tests to find the person who can detect the taste at the lowest concentration (largest dilution). Look for any evidence of a gender bias in the class results.

Fruit to dye for In technology, work on textiles usually includes practical work on finishing techniques in a design and make activity. Fruit and other plant materials are the source of many natural dyes for textiles. Dyes can be extracted and then applied to fabrics to investigate a range of colour effects, colour fastness and resistance to fading in daylight. The focus could be to produce a good colour for a cotton T-shirt or a woollen scarf.

Making it stick There is a problem with using many natural dyes. The colour may be good but it often does not stick well to the fabric. The answer is to treat the fabric first with a mordant, a material that helps fix the dye to the fibres. There are lots of possible mordants; many contain aluminium compounds. Examples include tea-leaves, club mosses, rhubarb leaves, lemon juice and alum.

Alum is the double salt aluminium potassium sulphate and is the easiest mordant to use. Pupils could compare mordants, say alum and rhubarb leaves. The key chemical in rhubarb leaves is the poisonous acid, oxalic acid (ethanedioic acid). For alum, use a mordant solution of 10g in 100ml of water. For rhubarb, boil the chopped leaves in water for an hour and strain the mordant solution. The fabric samples can be strips of cotton or lengths of wool. The samples need to be boiled in the mordant solution (10-40 minutes) and rinsed before applying the natural dye. One feature of this investigation can be to find whether the amount of boiling affects the quality of the dyeing.

Choosing the dye Various roots and leaves will make dyes. To prepare natural dyes, follow these steps:

* cut into small pieces * grind with sand in a pestle and mortar, using hot water alone or mixed with colourless methylated spirits (which is flammable) * filter to give the dye solution


Plant Mordant Colour

Blackcurrant Alum Violetpurple

Redcurrant Alum Red-brown

Onion skins (boil) Alum Orange

Plum Alum Yellowpink

Sunflower Alum Yellow

Logwood Alum + washing soda Purple

Applying the dye

The cotton or wool strips can be soaked in the cold dye solution or boiled. Pupils can experiment by varying the amounts of dye used and the temperature chosen. If the dyed fabric is part of a focused practical task it will be important to test the colour fastness. Try cutting a sample into four pieces. Investigate the effects of rinsing, using detergents, and exposure to daylight. Keep one piece in the dark to compare.

Keeping it clean Despite assertions to the contrary, water is not very good at making things wet. The effect is most obvious on a polished surface such as a car, or a greasy surface such as the skin. The high surface tension of water makes it hold together in drops rather than spreading out to cover a surface. Soaps and shampoos improve the "wettability" of surfaces by lowering the surface tension of water.

Alkali from the ashes People have been preparing soaps from plant material for hundreds of years. The two essentials for soap making are an alkali and an oil or fat. Traditionally, burning plants such as barilla (Salsola sativa) gave an ash rich in alkali. Pupils can experiment with the ashes from a variety of plants, including seaweeds, to check for alkalinity. Simply stir the ash with water, filter and test with an indicator.

Soap-making Safety glasses essential. This crude soap will contain some sodium hydroxide. It would give you the cleanest hands ever - without any fingerprints. Avoid touching it.

The alkali sodium hydroxide works well in soap-making. It is also known as caustic soda for a good reason. It attacks skin and the eyes and can even strip paint from wooden doors. For the second ingredient use olive oil. Samuel Pepys, writing in 1659, offers an alternative recipe. He suggests, in the interests of household economy, wood ash and waste kitchen fat. Sounds promising.

* Mix 15ml olive oil with 50ml dilute sodium hydroxide solution in a beaker * Cover with a watch glass to reduce loss of liquid * Boil and then simmer for about 30 minutes * Cool. Add about 20g of salt to release the crude soap from solution, a process known as salting out.

* Test the gelatinous soap for its ability to make a lather with water.

Horse chestnut soap The fruit of Hippocastanaceae, the horse chestnut or conker, has medicinal uses documented as far back as the 16th century. Conkers contain saponins, chemicals which form lathers when shaken with water. The technical name for soap-making is saponification. Extracting saponins from conkers is very straightforward.

* Cut off the brown outer layer * Chop the inner part of the fruit into small pieces * Cover with water in a beaker and boil for 10 minutes * Cool. Filter the mixture * Shake a sample of the solution with water to give a long-lasting lather. Compare with the lathers produced by commercial soaps.

The hair story Most textiles are woven from fibres. Human hair is a widely available renewable resource but little used, even for the penitent's hair shirt. The technology and science of hair provides opportunities for investigations.

Strength and stretch of hair:

* Clamp one end of the hair and tie a weight holder to the other end. Add weights until the hair snaps. A single hair should support 70g-90g.

* A single wet hair may stretch up to one and a half times its normal length: check it.

Shampoo magic and myth Many commercial shampoos feature natural ingredients, fruit extracts and oils designed to improve the hair. A class survey of shampoo labels and favourite aromas will indicate which natural products are favoured. The active cleaning agent in most shampoos is a surfactant. These are double-ended molecules. One end is attracted by water and the other by grease and dirt on the hair. Using a group of hairs from one source, investigate the effect of a shampoo on the strength of the hair.

* Soak a bundle of hairs in a small amount of the chosen shampoo.

* Rinse and leave part of the sample to dry on a kitchen towel.

* Repeat the strength test (above) using wet and dry hair samples.

Some shampoos are described as pH-balanced or non-alkaline. Alkalis have pH values above pH 7, neutral. Such shampoos claim to provide the best conditions for cleaning hair. The forces of attraction that give hair its strength work best in mildly acidic conditions, about pH4. Neutral water has a pH of 7. Try the effects of lowering the pH (more acidic conditions) on hair strength.

* Soak the hair sample in an acid such as citric acid or orange juice.

* Test the wet strength of the hair. If the sample is rinsed in water (pH7) the pH will rise towards neutrality. Pupils may conclude that pH-balanced shampoos are less beneficial than they once thought.

Ray Oliver is a former science teacher


The Craft of Natural Dyeing - Glowing Colours from the Plant World by Jenny Dean (Search Press)

Your Yarn Dyeing by Elsie Davenport (Select Books)

Soap through the Ages by R. Lucock Wilson (Unilever, 1963)

The Right Chemistry? What it's all for by Jeffrey Hancock (Hodder amp; Stoughton)

The Encyclopedia of Medicinal Plants by Andrew Chevallier (Dorling Kindersley)

Chemistry in Everyday Life. The World of Science, edited by Peter Furtado (Guild Publishing)

Vegetable Substances The Library of Entertaining Knowledge by Charles Knight (1833)

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