Cells: Semipermeable Membranes

You don't have to get very far into your study of cells before your students are presented with the term "semi-permeable," used to describe cell membranes.  Just as easily as you can define it for your students, you can show them!

Before class mix together 1/2 cup of sand (or salt) and 1/2 cup of marbles (or dried beans, pebbles, or other objects larger than the holes in a colander).  Place the mixture in a beaker or glass jar.

When you get to semi-permeable membranes during class, show the students your mixture.  Then pour the mixture through a colander (make sure you have a bowl or pan underneath!).

The beans stay in the colander while the salt/sand passes right through.  Just like a cell membrane, particles that are small enough to pass through the holes do so and particles larger than the holes stay put.

Weathering: By Falling Water

One of the ways in which rocks are weathered (broken down into smaller pieces) is by falling water.

Place a bar of soap on a sponge.  Set the sponge in a sink under a faucet.

Use the faucet as usual for a day, letting the water hit the soap and observe the soap at the end of the day.  Or you could let the faucet drip on the soap for the duration of the class period.

The falling water knocks particles of soap free.  Similarly, rocks at the bottom of waterfalls (or in other locations where water falls over them) are weathered by the falling water over time.  Of course, this process takes a very long time since rocks are lots harder than soap!

Air Pressure: Egg into a Flask

This is another classic! 

In preparation for this demonstration, you'll need to hard boil some eggs.  When cool, peel them. 

You'll need a flask or bottle (Snapple bottles are a good size) with an opening that's smaller than the egg (the egg should be able to sit on top of the opening.  You may wish to grease the opening a little to help the process, but it usually isn't necessary.

To perform the demonstration:

Light a small piece of paper on fire with a match or lighter.  Drop the paper into the flask/bottle.  Quickly place the egg on the opening.  Watch.

The fire will extinguish when all the oxygen has been consumed.  And the egg will slowly work its way into the bottle/flask.

Since the oxygen has been consumed, there is less air in the flask, so the air pressure outside of the flask pushes the egg in. 

(At this point, you're probably looking for an additional picture.  My camera went on hiatus, so there is no picture, but I can tell you that my egg was too large for the bottle I had on hand, so even if I had a picture, it wouldn't look terribly different from the one above.)

There are ways to use air pressure to get the egg out, but I can never get them to work.  Instead, I break up the egg with a knife and dump it out.  Not as dramatic, but it works!  If you're interested in trying it for yourself, do a Google search and you'll find numerous sites with directions.

You'll probably want to try this demonstration once before performing, to check that your bottle/flask opening and egg are a good size match.  (See above).  Of course, eggs are all slightly different sizes, so there's no guarantee that you won't get the egg stuck in the neck of the flask, but it should give you a good idea!

Natural Things Swap Update

Swap boxes are getting packed up to head out to their recipients early next week.  (If you're not familiar with the Natural Things Swap, you can read more about it here).

Thanks to everyone who participated - each and every person who said they wanted to play along made me so happy!

If you missed out on this year's swap, you'll have a chance to play along virtually in the coming weeks - there will even be prizes! 

Keep watching for more information!

Moles: Challenge!

After discussing what a mole is (6.02x10^23 things), challenge students to bring in a mole of something.

Some examples to get you started...

A mole is...
...58 g of salt
...18 g of water


This is a good activity to do for Mole Day (celebrated October 23 - sorry to be late in sharing, you'll have to save it for next year!).

I would leave this as an extra credit opportunity for my students, as it's really beyond some of them. But, if you work with older or higher level students, go ahead and make them all do it!

Plants: Does it Matter Which Direction you Plant a Seed?

Take the basic procedure used for observing seed germination one step further.

Prepare two bags.  This time, across the bottom of each bag, mark 4 arrows, one up, one down, one to the left and one to the right.

Place a bean above each arrow, so that the concave part of the seed is pointing in the direction of the arrow.

Hang the bag on the refrigerator (or on another vertical surface) and leave the other on a flat surface and observe.

Observe what happens with the seeds in the different bags. 

Seeds rely on gravity to help them grow the "appropriate" way.  When the bag is hanging vertically, the seeds will orient their roots so they all go down.  Because the roots can't go "down" while the bag is laying flat, the roots in that bag will grow in whatever direction they can.

Erosion: A Flowing River

This is a good one to do outside, if you can swing it.  If not, it can be done inside, you'll just need to work harder to contain the water.

Materials:

--Paper cup

--Drinking straw
--Clay
-- Cookie sheet
--Loose soil or sand
--Jug filled with water (or other water source)

Prep Work:

Poke a hole in the side of the paper cup, near the bottom. 

Cut the straw in half and place one half in the hole.

Seal the hole, around the straw, with the clay.

For the Demonstration:

Lay the cookie sheet upside down on the ground and raise one end about 2 inches, using scrap wood, a brick, a pile of soil, whatever's convenient. 

Cover the cookie sheet with a thin layer of soil or sand. Place the cup at the raised end of the cookie sheet.

Hold your finger over the straw as you fill the cup with water. Remove your finger and watch the way path of the water.

Raise the end of the cookie sheet to about 6 inches, recover the sheet with soil and repeat.

What differences did the slope make in the erosion?

Cover the cookie sheet with a layer of soil again.  This time, place a small rock  in the soil in front of the straw. 

Fill the cup and watch what happens to the water as it encounters the rock.  You can place additional rocks along the way to change the direction of the river.