Compare what happens to what happened in the previous experiment. Repeat steps three and four, only this time use hard water instead of purified water. Are the results different from those obtained with purified water? If so, in what ways? Other powders may be available to test instead of talcum powder, to see whether the type of powder makes any difference.
If you do test any of these, what differences do you find? Again using a clean beaker with purified water, try to float a fine sewing needle on the surface by carefully lowering it into the beaker, avoiding breaking the surface with your fingers, and dropping it from as close above the surface as possible. Once you have a needle floating, add a small drop of detergent to the water, but away from the needle. What happens?
Teaching notes This series of brief experiments on the surface tension of water, and the effects of detergents and soaps on this, can serve as an introduction to the phenomenon of surface tension, with a discussion of results leading into simple theory. Show Fullscreen A diagram of the forces between water molecules at the surface and centre of a liquid.
Show Fullscreen A diagram of the forces enabling a needle to float on water. Show Fullscreen A diagram of a detergent or soap molecule, which is responsible for breaking down surface tension. Show Fullscreen A diagram showing detergent molecules in a beaker of water, some lining the surfaces and other forming clumps. Additional information This is a resource from the Practical Chemistry project , developed by the Nuffield Foundation and the Royal Society of Chemistry.
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The main difference between soap and detergent is the ingredients. Soaps are made with biodegradable ingredients such as oils, lye and fats. Detergents are made with synthetic chemicals, such as surfactants, optical brighteners and perfumes. Both clean clothes well, but detergents are more effective on stains and with brightening clothes. However, soap is kinder on the environment and skin. Soap is a combination of fat or oil, water, and finally, an alkali or basic salt 1.
The cold process takes a room-temperature lye solution and mixes it with an oil. This thickens and heats up before being poured into a mold where it solidifies into a bar of soap. This is a quicker way to make soap. How does soap work? It removes germs and dirt. So when you wash your hands, the soap molecules bind with both water and oil which carries dirt and germs on our hands. When you rinse your hands, the soap removes the germs along with the water.
Soap comes in a bar form or a liquid form which is usually dispensed from a bottle with a pump. You can buy it in the store or make your own. Laundry detergents , unlike soap, contain more synthetic ingredients 2. The main difference is that detergents contain manmade sulfates, usually derived from petrochemicals, which cleans clothes, hair, hands, and dishes when it comes in contact with water. Detergents nowadays contain lots of other chemical ingredients that have lots of benefits.
The most common examples of such compounds are soaps and detergents, four of which are shown below. Note that each of these molecules has a nonpolar hydrocarbon chain, the "tail", and a polar often ionic "head group".
The use of such compounds as cleaning agents is facilitated by their surfactant character, which lowers the surface tension of water, allowing it to penetrate and wet a variety of materials. The oldest amphiphilic cleaning agent known to humans is soap. Soap is manufactured by the base-catalyzed hydrolysis saponification of animal fat.
Before sodium hydroxide was commercially available, a boiling solution of potassium carbonate leached from wood ashes was used. The pin is no longer supported and so sinks to the bottom! You can measure surface tension yourself by making your own button balance, like the one used by the famous nineteenth century home experimentalist, Agnes Pockels. You will need:. You can set up the balance in one of two ways, as shown in the diagram above.
You'll soon find out which one works best for you. The lollystick is used for the lever and the nylon thread has the advantage of not soaking up water and influencing the balance. The piece of card can be suspended from the lollysick with the nylon thread to act as a counterbalance pan. To use your balance:. With your button balance, try measuring the surface tension of a range of liquids and comparing them. For example: cold water, salt water, warm water and soapy water.
You can also try changing the size of the button used or the material it is made out of. About us. Contact us. Soapy Science In this activity, you can: find out how detergents work , carry out four bubble-making experiments , explore surface tension and measure it by making your own button balance. How do detergents work?
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