The mystery and history of soap
What's the best substance to clean your clothes with? Fat or oil, of course! That doesn't sound right? Well, how about adding some ashes to the oil? Sounds worse, doesn't it? But that's the basis of soap, and people have been using it to clean themselves and their belongings for thousands of years.
No one knows who first started using soap. There are recipes for soap on Sumerian clay tablets dating from 2500 BC, but the recipes don't say what the soap was used for. Later cultures used a similar mixture - as hair gel.
One story has it that Roman women were doing laundry in the Tiber River some 4,000 years ago when some fat and ashes from animal sacrifices upstream washed into the water and then seeped into the clay of the riverbank. Women found that their clothes cleaned more easily with the ashes-fat mixture in the clay.
The sacrifices were performed at Mt. Sapo, which resulted in the word "soap." It's a nice story, though probably not true. It is likely, however, that soap's discovery was accidental. Who would think of using ashes and oil to get things clean? People knew it worked long before they could explain why.
One way that soap cleans is by reducing water's surface tension. What's that? Water molecules are attracted to one another. On the surface of the water, the molecules are attracted to the water, not to the air. They are pulled toward the rest of the water. This pull is called surface tension. It's why water beads up on surfaces.
Surface tension makes it hard for water to wash away dirt. The water tends to stick to itself, not to the dirt.
Soaps are made from fats and oils. More specifically, they are made from the fatty acids in fats and oils. This is done by treating them with a strong alkali, which causes a chemical reaction. That's where the ashes come in. (An alkali is the opposite of an acid. Just as lemon juice is slightly acidic, plant ashes are slightly alkaline.) Plant ashes first provided the alkali needed to make soap. Today the alkalis can be made commercially.
If you look at the ingredients for a bar of soap, you might see potassium hydroxide or sodium hydroxide. These are the alkalis that react with fatty acid molecules. The molecules that are formed are called "surface active agents" or surfactants. Surfactants break down water's surface tension to make the water "wetter," so it can react with dirt more easily.
One side of each soap molecule is attracted to water. It is called the hydrophilic (water-loving) end. The other side of the molecule is attracted to oil and grease and is repelled by water. It is the hydrophobic (water-hating) end. The water-hating end "grabs" the grease, and the water-loving end pulls the grease away from whatever you're trying to clean and toward the water. The soap holds the grease in the water until it is rinsed away.
Soapmaking was an established business in Europe by the 600s. In the American colonies, the first soapmakers arrived in 1608 on the second ship from England to reach Jamestown, Va. But for a long time many colonists and pioneers made their own soap. They boiled fat with wood-ash lye. (Lye was made by letting rainwater trickle through a barrel of wood ashes.) Lye soap was smelly and scratchy. By the 1850s, soapmaking was one of America's fastest-growing industries.
Then, in the early 1900s, the first detergents were created. Instead of using fat or oil, detergents are made synthetically, created chemically from a variety of raw materials. By 1953, detergents outsold soaps in the United States and now can be found in soap bars as well as laundry and dishwashing agents. Each person in the US uses an average of 30-1/2 pounds of detergents and soaps each year. About 10 million tons of soaps and detergents were produced globally in 1998.
Mostly, soap is for cleaning. But it can be for fun, too. Check out the activities on the facing page.
Bubbles are a good example of surface tension. The water molecules are drawn together. They form into the shape that gets them as close together as possible around the air inside the bubble. That shape is a sphere.
If you want to impress your friends, tell them that you can predict when a bubble will pop. Here's how: Watch the top of the bubble closely. When a black band begins to form on top of the bubble, announce that it is ready to pop! Blow several bubbles and tell which one will pop first. The black band forms because the bubble wall becomes thinner before it pops. Gravity is pulling the moisture downward. Less light is being reflected at the top, and this results in a black band.
Here's a bubble-liquid recipe from the Soap and Detergent Association: Combine 4-1/2 cups water with 1/2 cup of hand dishwashing detergent and 1/2 cup of corn syrup or glycerin. Source: SDA website (see below).
Make a triangular boat out of aluminum foil and cut a hole near the back as shown. Fill a bathtub or sink with an inch or two of water. Float the boat on top of the water, then put a drop of dishwashing detergent into the hole. The boat will move forward across the water. Make boats of different sizes and shapes. Which one moves fastest? The boats push forward as the detergent breaks the surface tension, causing the water to spread out at the back of the boat. You'll need fresh, soap-free water for each race. Source: My father and I made these boats together when I was a little girl.
You need a straw, some sugar, and soap to make two toothpicks move in water. Fill a bowl with water and have two toothpicks ready. Then take a drinking straw and dip one end in a little sugar. (The sugar sticks better if you get that end of the straw wet first.) Dip the other end in a few drops of dishwashing detergent. Float the two toothpicks on the water. Leave enough space between them so you can dip the straw in the gap. First, dip in the end of the straw that's coated with sugar. The toothpicks will move together. Then put in the soapy end, and the toothpicks will move apart.
Why? The sugar absorbs water. Not much, but enough to move the toothpicks toward each other as water moves into the sugar. The soap on the other end of the straw lowers the surface tension of the water so that it moves away from the straw and pushes the toothpicks outward. Source: Adapted with permission of Sterling Publishing Co., Inc., from 'Bathtub Science,' by Shar Levine and Leslie Johnstone, © 2000.
You can use surface tension to make a paper clip float on water. Fill a small bowl with water. Rub a paper clip against your face or fingers. The oil from your body will help repel the water. Use a fork to gently lower the paper clip onto the surface of the water. It will gently float there. Put a drop of dishwashing detergent on your finger and put it in the water near the paper clip. The clip will move away, pushed by the water as the surface tension decreases. After dipping your finger in the water a few times, the paper clip will drop to the bottom of the bowl. The surface tension that held it on the top has been removed by the detergent. Source: Exploratorium website (see below).
• For more fun with bubbles, check out the SDA Kids Corner at the official site of the Soap and Detergent Association: www.cleaning101.com/sdakids. You can also explore bubble fun and science at the Exploratorium site: www.exploratorium.edu/ronh/bubbles/bubbles.html.