To make chemistry interesting to students by showing them it is part of the real world, rather than being confined to reagent bottles and test tubes in the classroom laboratory. Students observe and do experiments showing them the surface tension of water, how detergents change this and how micelles form. They then make bubbles and observe the shape of the bubbles, their iridescent color, relative thickness of the top and bottom of the bubble, movement of water within the bubble, and longevity of bubbles.
4. Relate the polarity of the water molecule to the behavior of soaps and detergents.
5. Compare the size of bubbles
6. Compare the life span of bubbles, learn about gravity and that water evaporates very rapidly
7. Compare shapes and colors of bubbles
This project is based upon molecular force and the degree of surface tension, which depends on the amount of energy in the intermolecular forces. Liquids, like water, which are polar – have a positive and a negatively charged end, produce strong intermolecular forces and have a strong surface tension.
All molecules attract. In particular a molecule of liquid water attracts all the surrounding water molecules and is also attracted by them. Inside liquid water all of these attractive forces balance out (ie the liquid is stable as a liquid). At the surface, however, water molecules are attracted by the molecules below and to the side of them but there are only air molecules above. The air is a gas and the gas molecules are on average much further away from a surface water molecule than the other water molecules in a liquid (molecules in a liquid are closer together than those in a gas). As a result the liquid water molecules below the surface yield a force pulling the surface water molecules down into the liquid. There is also a sideways, force as water molecules on the surface attract each other. This makes the water surface act like cling wrap (the surface is called a meniscus) as it wraps up the liquid water. The surface force is called surface tension. You can see surface tension when a wet insect tries to climb out of the water: the surface tension pulls it in. Other insects, like water striders, exploit surface tension to stride or skate across the surface without sinking.
Soap bubbles are made up of soap or detergent molecules and water molecules. A soap or detergent molecule has a small polar (charged) head and a long nonpolar (uncharged) tail. The charged head group is attracted to polar molecules like water and the uncharged tail is repelled by water (it likes fat or greasy substances).
When you put a detergent into water, it forms a layer on top of the water. The charged head groups like water and stick into the water but the uncharged tails don’t like water and stick into the air. This is why molecules like detergents are calledsurfactants, or surface active agents. Replacing the water molecules on the surface with detergent molecules lowers the surface tension of the water. Indeed the ability of detergents to remove dirt is due partially to this decrease in water's surface tension: the lower surface tension makes it easier for the water to wet (or penetrate) into fabric.
If you keep putting detergent into water it will cover the surface completely and eventually move into the liquid water. Within liquid water, detergent molecules arrange themselves into little globules, called micelles, or layers, called membranes. In a micelle or membrane all the tails are pointing together towards the inside and all the head groups are pointing outwards towards the water. When detergents meet dirt (or oil or grease…) they surround the dirt with their tails pointing towards the dirt meet dirt and their heads pointing toward the water. Agitation (eg the washing machine) encourages the head groups to carry the dirt up to the surface of the water (scum on the surface) where it can be rinsed away. Soap bubbles are formed by soapy membranes. The membranes around cells are formed by phospholipids – biological soap molecules.the head groups to carry the dirt up to the surface of the water (scum on the surface) where it can be rinsed away. Soap bubbles are formed by soapy membranes. The membranes around cells in your body are formed by phospholipids – biological soap molecules.
Bubbles consist of air surrounded by films of soapy water. The detergent molecules line the interface between the air and the water. Their head groups are dissolved in the water and their tails point towards the air. The detergent molecules serve three purposes in the creation of bubbles. (1) They lower the surface tension of the water. For example, when water sprays from a tap small bubbles form, but they burst almost immediately because the high surface tension of the water pulls the water molecules in the bubble back into the main body of the water. The bubble wall gets too thin to remain intact and it quickly bursts. In soapy water the surface tension is much lower (by about 1/3) so the molecules of the bubble are less stressed and the bubble can survive longer. (2) The detergent molecules are very elastic and allow the bubbles to deform without breaking. The forces between detergent molecules are much weaker than those between water molecules and this makes the surface more elastic and more able to deform. (3) The detergent molecules also slow the evaporation of the water film and so extend the life of the bubbles.
Over time the water in the soap film will migrate, under gravity, to the bottom of the film or bubble and the film at the top will become thinner and eventually burst. The bubble’s life can be extended by adding substances to the water to make it thicker, or more viscous. These additives are things like glycerin and can include sugar, honey and gelatine (glycerin can be obtained from a pharmacy or cake decorating supplier and is used commercially to keep products such as make-up, fruit and cake icing moist). Bubbles will also pop if they touch the ground, clothing or a dry finger because the film will wet the surface.