Think about the last time you washed dishes, cleaned your clothes, or watched a car go through a car wash. What did you do? What did you see? Did you just use water to wash the dishes, or did you add some special liquid to the water? Did the car ever look like it was covered in foam? The dishes, the car, and your clothes all need a little more than just water when you clean them; they need detergents!
A detergent is a substance whose molecules (that is, its smallest building blocks) dissolve in water (see our blog on solubility), and also helps dissolve dirt and stains that would otherwise not dissolve in water. They can do this because they are amphiphilic (AM-fi-FILL-ick), which means that part of the molecule is hydrophilic, or water loving (likes to dissolve in water), and part of the molecule is hydrophobic, or water hating (does not like to dissolve in water). Because part of the molecule likes water, you can dissolve the detergent in water. But because the other part of the molecule doesn’t like water, the detergent would also like to grab onto something else that doesn’t like water.
Let’s look at what happens when you wash the dishes: When you begin, your dishes are dirty with lots of leftover food particles that may not come off easily. This is because they may not like water very much: things like egg yolks, butter, and mayonnaise are more hydrophobic than hydrophilic because they are mostly made of fats, and fats don’t like water!
If you place these dishes in a sink full of water, the fats will stick to the dishes, and usually won’t come off easily. They do this because they would rather be stuck to the dishes than float away into the water.
However, if you place the dishes in a sink full of water with added detergent—such as a typical dish washing liquid—the fats will slowly come off the dishes. The hydrophobic part of the detergent is slowly grabbing onto the fats, while the hydrophilic part still wants to be dissolved in water. This way, the detergent lifts the fat off the dishes, and allows it to be dissolved in water.
Finally, remember the car in the car wash covered with foam? The foam is caused by the detergent; in fact you probably see this type of foam when you wash dishes too. Detergents foam in this way when they are mixed with air—the hydrophobic part of the molecule would rather touch the air than the water, so the hydrophobic parts of the detergent line themselves up so they all touch the air, keeping them away from water. This is how the bubbles are formed!
TRY IT!!
Here’s what you’ll need:
1. 2 small drinking glasses, or 2 small glass bowls
2. 2 teaspoons butter
3. ½ teaspoon dish detergent
4. Water from a kitchen sink
Here’s what to do:
1. Measure one teaspoon of butter into each glass.
2. Smear the butter all around the walls of the glass with your fingers.
Here’s what you’ll need:
1. 2 small drinking glasses, or 2 small glass bowls
2. 2 teaspoons butter
3. ½ teaspoon dish detergent
4. Water from a kitchen sink
Here’s what to do:
1. Measure one teaspoon of butter into each glass.
2. Smear the butter all around the walls of the glass with your fingers.
3. Fill one glass up to the rim with water only.
4. Measure ½ teaspoon of dish detergent into the other glass, then fill it with cool water up to the rim also.
4. Measure ½ teaspoon of dish detergent into the other glass, then fill it with cool water up to the rim also.
5. Allow both glasses to sit on your kitchen counter for 20 minutes
6. After 20 minutes, pour out the water from each glass, and rinse both glasses gently with cool water.
What do the glasses look like after you rinse them? Do they still have butter in them? Do they look the same, or different? Try to sketch in your notebook what the glasses look like, and make notes about the differences between them. Be sure to write down what you did!
DETERGENT CHALLENGE!
Try this experiment again, but use several different dish detergents to see which one works best!
· Be sure to have an identical glass for each different detergent
· Be sure to use the same amount of butter in each glass
· Be sure to use the same amount of detergent and water in each glass
· If you have a scale, record the weight of each glass and butter before and after soaking in water or water and detergent. Then you can compare detergents by determining how much weight (butter) the glasses lost after rinsing. LET THEM DRY BEFORE YOU WEIGH THEM THE SECOND TIME!
Image licenses:
GNU Free Documentation License
6. After 20 minutes, pour out the water from each glass, and rinse both glasses gently with cool water.
What do the glasses look like after you rinse them? Do they still have butter in them? Do they look the same, or different? Try to sketch in your notebook what the glasses look like, and make notes about the differences between them. Be sure to write down what you did!
DETERGENT CHALLENGE!
Try this experiment again, but use several different dish detergents to see which one works best!
· Be sure to have an identical glass for each different detergent
· Be sure to use the same amount of butter in each glass
· Be sure to use the same amount of detergent and water in each glass
· If you have a scale, record the weight of each glass and butter before and after soaking in water or water and detergent. Then you can compare detergents by determining how much weight (butter) the glasses lost after rinsing. LET THEM DRY BEFORE YOU WEIGH THEM THE SECOND TIME!
Image licenses:
GNU Free Documentation License
