It took us a while to do all the activities, but we finally completed our salt study today.
We love any excuse to look at stuff with the microscope, so we compared basic table salt to fancy "fleur de sel" sea salt. It was interesting to see the difference in the size and regularity of the crystals. The sea salt crystals were bigger, more diverse, and had little specks in them. The ordinary salt was very white, and the crystals were very similar in size and shape.
Just for fun, we carefully put drops of water on the microscope slides and watched the crystals dissolve.
Then we made our own salt crystals. We dissolved as much salt as we could in a small amount of water, which (I just learned) is known as a "saturated solution." I suppose we could have poured the solution on the paper, but it just seemed more "scientific" to use a pipette! We used black construction paper to make it easier to see the salt crystals forming.
We put one plate in front of our fireplace, and one plate in our unheated storage shed. While we waited for the water to evaporate, we read The Story of Salt by Mark Kurlansky. We really enjoyed reading A Cod's Tale a few months ago, and this book was just as fascinating. He has a real talent for taking a seemingly "bland" topic and making it "appetizing!" (Sorry.)
We checked on our crystals occasionally. The plate on the left was in front of the fire, and it was dry within 45 minutes. The plate on the right was in the cool shed and it dried overnight. Can you see a difference?
The microscope confirmed our predictions: the crystals that formed quickly in front of the fire were much smaller than the crystals that formed more slowly. We documented the difference in our science journals. It's fun to draw salt crystals, by the way. They really do look like little jewels up close!
Then it was time to attempt to freeze some salt water.
We made sure that both cups of water were the same temperature. They both were 71.3 degrees. Then we stirred salt into one cup.
After an hour and a half in the freezer, the plain water had a large crust of ice on top, and a little bit of liquid water underneath. The salt water was full of slushy ice, but didn't freeze solid. I forgot to get a photo, but basically, the fresh water looked like ice, and the salt water looked like a Slushee.
Finally, we tried the egg and salt experiment. It's hard to see in this photo, but the egg did sink in the plain water.
We took turns stirring salt into the water. I think the ripples look cool in this picture.
The salty water looks a bit milky. Super gently put the egg in the solution.
And it floated!
This experiment is a great introduction to studying density. Super and Dude offered several theories about what made the egg float in the salt water, and they almost got it, but didn't quite have the vocabulary to explain it. I really wanted them to make the connection on their own instead of just explaining it to them, so I asked them to imagine tossing a beach ball and a bowling ball into a swimming pool. Of course the bowling ball sinks, and the beach ball floats, but why? The water is dense enough to float a ball full of air, but not dense enough to hold up a dense bowling ball.
To illustrate the point, we looked up the Dead Sea on Wikipedia, and there is a great photo of a man floating in the water reading the newspaper. He's holding his head up, which would make it impossible to float in an ordinary lake. But in the dense, salty water of the Dead Sea...
Finally, we went back to the egg experiment and I had them hypothesize about why the egg floated on the salt water. Their explanations were perfect. I typed while they narrated our experiments and the results; their narrations will go in their science journals. We hadn't discussed density before, but they really got the concept after seeing it in action!
This was a fun activity, and I probably wouldn't have thought of it if I hadn't seen the suggestion on Harmony Art Mom's wonderful blog. If you haven't visited yet, you should take a look!