In the Sorting Salt and Pepper video we saw that we could mix salt and pepper into a pile and then separate them easily by using the static charge on a balloon. That is one way to separate salt and pepper, but there are many others. How many can you think of? Don't read any more until you have spent some time thinking of as many different ways as you can.
We start with a pile of mixed salt and pepper. How are we going to sort them out? Well, start by thinking about how salt and pepper are different. Think about how you could use those differences to separate the two materials. How many different ways can you think of? Take some time, and really think about it before you go on. Make a list, and then compare it with the answers below. If you thought of one that I missed, let me know.
After you give it some thought, here are some answers.
The first difference that many people noticed was that salt will dissolve in water, but pepper will not. Quite a few of you suggested putting the salt and pepper in water. The pepper floats on the surface, while the salt sinks to the bottom and then dissolves. You can then evaporate the water to recover the salt. Carolyn not only tried that, she sent me some wonderful photos of her experiment. Jeffrey suggested adding a drop of dish detergent to get the pepper to rush to the side of the bowl, making it easier to collect.
Another difference is their density. The grains of salt are heavier than the flakes of pepper. We used that with the static charge on the balloon, attracting the lighter pepper flakes. Other ideas on using density included:
1. Either coating the salt or using a liquid that the salt would not dissolve in, such as oil. The salt sinks and the pepper floats.
2. Blowing or using the wind to blow the lighter pepper away from the heavier salt.
3. Taping or shaking the mixture, so the denser salt settles to the bottom, leaving the pepper on top.
4. Placing the mixture on an incline and vibrating it. The denser salt will move downwards faster.
5. Throwing the mixture across a long sheet of newspaper. The air resistance will stop the pepper quickly, while the heavier salt will fall on the other side of the paper.
6. Dropping or throwing the mixture onto a piece of paper, the denser salt will bounce more, leaving a pile of pepper with salt around it.
7. Use other sources of static charges, such as cellophane tape or a plastic comb to attract the lighter pepper.
The salt and pepper also have different sizes and shapes, which lead to more idea.
1. Using a screen or sieve to let the smaller pepper flakes fall through, while the larger salt grains are trapped.
2. Rod came up with a great idea of placing the mixture between two panes of glass. With a little pressure, the salt grains are held in place, but flat flakes of pepper fall out.
You can also use adhesion. As we saw in the Spoon on Your Nose video, adhesion can cause materials to stick to some things, but not to others. Pepper sticks to most cling wraps, but the salt usually does not.
All the entries boiled down into these eleven basic ideas, but I am certain that there are more. If you think of more, let me know. I am delighted that so many of you put on your thinking caps and sent in your ideas. My goal is to get people thinking about science, and actually playing with it themselves. This challenge seems to have done that, so look forward to more of them in the near future.
Have a wonder-filled week.