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.
The students will be able to:
1. Discover what "surface tension", means
2. Hypothesize the outcome of the experiments,
3. Analyze experimental data and conditions,
4. Relate the polarity of the water molecule to the behavior of soaps and detergents.
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.
Basic Bubble Recipes
Two common recipes are:
Recipe A: 1 to 3 parts liquid dishwashing detergent + 6 parts water
Recipe B: 1 to 3 parts liquid dishwashing detergent + 6 parts water + 1 to 4 parts glycerin (Too much glycerin makes the bubble mixture too heavy and prevents the formation of bubbles).
You will have best results if you let the soapy solution rest for a couple of days, but if you are impatient, you can use it immediately. A cold solution makes longer lasting bubbles so keep the bubble mixture in the fridge for best results. To minimize evaporation make bubbles in shady areas when the air is as still as possible. They will also last longer if the air is very humid, for example after a storm. Stay in open areas so the bubbles won't run into dry objects and keep you bubble tools really wet with bubble solution to make the thickest films possible.
Optional: powdered gelatin, golden syrup
If solution gets in a student's eyes, instructors should wash the student's eyes with clear water.
You should have a bottle of vinegar and a mop or paper towel handy to clean up any spills on the floor. Newspapers can also be used to clean up.
After the investigations, students should wash their hands to remove any soap solutionI
For each demonstration below:
Describe what you are going to do.
Ask the students to write their predictions of what they think is going to happen in their notebooks or on a worksheet.
Do the experiment.
Ask the students to describe what actually did happen (notebook).
Ask the students to try to explain what happened (notebook).
Think of ways to test whether the students’ explanations are correct (notebook).
Demonstrations to be done by the teacher
1. Float a paper clip on water in a Petri dish. You can do this by carefully placing the paper clip onto the water, or, as an almost foolproof alternative, place the paper clip on a fragment of tissue, float the tissue on the water, and as the tissue gets wet and sinks, the paper clip will stay floating.
Why is the paper clip floating?
This shows that the surface tension of the water is “strong” enough to hold up the paper clip.
Get the students to squat down to get eye level with the water. Observe what is happening. Is the paper clip a little bit down in the water? Or does the paper clip totally rest on top? Can they see kind of a surface or skin-like appearance on the very top of the water?
Ask the students what they think will happen if a drop of detergent is added to the Petri dish. Now add a drop of detergent to the Petri dish away from where the paper clip is. The paper clip will instantly sink.
Why does the paper clip sink? What did the detergent do?
Share ideas. If no one comes up with it, share that when we added the drop of detergent, we interrupted the pull and tug of the water molecules on top and below. This broke up the force and skin-like surface on top, so the paper clip dropped. If you like: the detergent spreads over the surface of the water and reduces its surface tension, the surface tension is no longer “strong” enough to hold up the paper clip.
2. Make a loop of 5" of silk thread. Float in a low, wide container of water. Touch a bit of wet soap or put a drop of detergent into the water inside the silk loop. What happens to the thread?
It becomes a circle.
Why does the thread become a circle?
The soap or detergent spreads out over the surface of the water and reduces the surface tension inside the loop, but not outside. The detergent wants to cover as much surface area on top of the water as possible – for a given area coated with detergent, the circle has the smallest possible circumference OR for a given perimeter the circle encloses the largest possible surface area.
What would happen if detergent were added to the outside of the loop rather than the inside?
The thread would form a blob on the surface of the water. 3. Sprinkle black pepper over the surface of water in a Petri dish. What happens?
The pepper forms a “glob” on the surface of the water.
Why does the pepper sits in a glob/lump on top of the water?
The pepper is made up of non-polar molecules that don’t interact with the water but do interact with each other. The pepper initially clumps together to stay as far away from the water as possible and as close to the other pepper grains as possible.
Add a drop of detergent to the center of the Petri dish. What happens?
The the pepper spreads out over the surface of the water.
Why does the pepper spread out?
The detergent spreads out over the surface of the water and reduces the surface tension of the water but, more importantly, the non polar end of the detergent molecules interacts with the pepper, spreading it over the surface. 4. Support a piece of window flyscreen over a pan or bowl. Slowly pour water onto the screen. What happens?
Plain water will bead up on the screen.
Why is it hard for the water to go through the screen?
Now pour over water with detergent in it. What happens?
The detergent mixture will fall through the mesh.
Why does adding detergent make it easier for the water to go through the mesh?
The detergent lowers the surface tension of the water so it is no longer “strong” enough to “seal” the holes in the mesh. Student activities:
1. Surface Tension
Put the students into groups of at least two. Give each group an eyedropper and a five cent coin. Ask them to predict how many drops of water/soapy water will fit on the coin without spilling, they will also investigate another liquid: milk, . Distribute plastic cups of water and water+detergent (it doesn’t really matter how much detergent) and give each group two eye droppers and two five cent coins. First have the student find out if the plain water will spill off the five cent coin. After the students have a chance to observe the surface tension of the water on the coin; the surface tension of the water will form a “dome” of water over the coin – get them to draw this in their experimental note books or on the worksheet, ask them to count the number of drops of water and of soapy water they can put on the five cent coin making sure that the drop on the end of the eye dropper does not touch the liquid on the five cent coin. One student can drop and the other can count and observe. Get them to write this number in their notebooks and to repeat this 3 times, switching roles if they want. Were they surprised at how many drops they could add?
Ask them what would happen if they added a drop of detergent to the plain water on one of the five cent coins. Get them to test their prediction.
The detergent lowers the surface tension of the water so it is not strong enough to stay in a dome and it will run off the five cent coin. Get the students to repeat this with another liquid: milk/lemonade/oil/orange juice and come back and combine the results.
Questions to ask them:
What is surface tension?
Do you think that the surface tensions of the liquids are the same?
Why do you think a bug can stay on top of the water?
Why can the five cent coin hold more drops of some liquids than of others?
How does surface tension and density differ?
Were your predictions correct? If not why do you think it was different?
Can you think any ways you have experienced surface tension?
2. Magic Milk
Give each student a Petri dish with milk in it. Put drops of red, yellow and blue food colouring in the milk to make a triangle. Ask the students to predict what will happen when detergent is added to the middle of the Petri dish.
Get them to put 4-5 drops of detergent into the middle and record what they see. They can draw a picture in their notebooks or on the worksheet.
Ask them what they think is happening.
The detergent coats the fat in the milk making it into globules called micelles. As the globules form the watery part of the milk swirls and the red, yellow and blue colours are mixed together.
What is the important thing in this experiment – the milk or the detergent? Get them to repeat the experiment with no detergent, with water rather than milk, with skim milk and with cream. Which works best?
What do they think will happen if they used vinegar instead of detergent?
3. Capillary action
Cut out a flower from the template. You can get your students to colour it in if you like. Fold the flower so the petals cover up the smiley face. Put some water into a Petri dish and ask the students to predict what will happen when they put the folded up flower on top of the water. Now get them to float the folded flower on the water and watch what happens, get them to record this in their notebook or on a worksheet. Ask your students what they think has happened. For example you can get them to see if the paper is wet (does the dry flower unfold by itself).
The water molecules interact with the fibres in the paper, being pulled into the fibres by capillary action. As paper unfolds as the fibres swell with water.