Friday, December 23, 2011

Melting Ice with Salt

The origin for this activity was one of those classic questions that kids come up with, "Momma, why do they put ice on the road in the winter?" The follow up question that my older daughter came up with was, "Why do they use salt, though?  Couldn't they use sugar?"  For those of you lucky enough to live in a warmer climate where that isn't a question in your household, a similar one might be, "Why do you put salt in the pasta water?"  (I'll describe how you can do an experiment with second question in the Extensions section below.) 

Materials Needed:
  • 3 small plastic cups
  • crushed ice or ice cubes
  • measuring cups
  • spoon
  • salt
  • sugar
You could this activity a variety of ways depending on what type of ice you have available.  If you only have cubes, the best way would probably be to have your child count out the same number of ice cubes for each cup.  For crushed ice, you could either do a volume measurement (using the measuring cups) or, if you have a kitchen scale, you could measure out the same mass/weight of ice for each cup.

Process and Pictures:
Measure out an equal amount of ice for each of the cups, either by number of cubes, volume of crushed ice using a measuring cup or mass of crushed ice using a kitchen scale.  We used 1/2 cup for each of our cups but use what works for your cups.

Add about 1/4 cup of salt to one of the cups, 1/4 cup of sugar to another cup and don't add anything to the third cup.  Ask your child to predict which of the cups will melt the fastest.

Use the spoon to stir each of the cups.  Make sure to rinse the spoon between cups so that you are not contaminating the ice with salt or sugar from another cup.  Or use two different spoons.

Wait - the hardest part for the little ones.  If you are using science journals, then this would be a good time for your child to draw the experimental set-up in their journal.  Record how long it takes for each of the cups to melt.  This takes a while (1-2 hours +) so you will have to go do some other things and come back and check every 15 minutes or so.

From L to R: plain ice, ice with table salt, ice with sugar

Revisit the prediction your child made at the end regarding which cup had the ice that melted the fastest.

What's Happening:
Water typically freezes at 32 degrees Fahrenheit.  When table salt (NaCl - sodium chloride) is added to ice, it depresses the melting point; a 10% table salt solution will freeze at 20 degrees Fahrenheit and a 20% table salt solution will freeze at 2 degrees Fahrenheit.   Sodium chloride is an ionic solid which means that in water it will dissolve into two different ions, a positively charge sodium ion and a negatively charged calcium ion.  The ions interfere with the structure of the ice, making it more difficult to freeze.  Similarly, when you add table salt to boiling water it increases the boiling point of water making the pasta cook faster.  Other salts will similarly depress the melting point of ice; sodium chloride will actually only work well to about 15 degrees Fahrenheit whereas other salt (magnesium chloride, calcium chloride) is capable of bringing the freezing point down to a lower level. Interestingly, the sugar will also hasten the melting of the ice because it also interferes with the structure of the ice - it just won't melt it as fast because it lacks the ionic structure of salt.

A great extension to this activity is to make homemade ice cream.  Here's a good link with directions for kids to make them in plastic bags. Kids love this process, but be forewarned to have the kids use mittens or to hold the bags as they get cold enough to cause frostbite!

If you have a thermometer that will measure below 32 degrees Fahrenheit use this to measure the temperature of the different mixtures.  You could also add different types of salt (epsom salt, rock salt, table salt, etc.) and record the different temperatures that each mixture drops to.  Unfortunately, most kitchen thermometers do not go below 32 degrees Fahrenheit, but you may be able to find one at a teaching supply store.  A good place to find some cheap, classroom-style thermometers (and tons of other lab supplies) is American Science & Surplus.  I think their "small, two-way thermometer" is a great choice for kitchen science experiments.  Similarly, for older kids you could alter the experiment by exploring how different substances affect the boiling point of water.

Friday, December 9, 2011

Grow a Crystal Snowflake

Grow your own crystal snowflake ornament overnight!  This activity is fast and simple and a great example of crystal growth and shape.  From our rock collection activity last week, the girls are fascinated by crystals.  (Crystals are sparkly and they are girls - what else can I say?)  Because rocks are combinations of multiple minerals, the crystals are not able to grow in their true form as they would given optimal conditions.  We can create those conditions by creating a solution of borax in water and providing a surface (pipe cleaner) on which the crystals can grow overnight.  This activity results in a crystal snowflake that can be hung in windows or as an ornament.  Please be warned that the resulting crystals do look like rock candy but they SHOULD NOT be tasted! 

Materials Needed (for one snowflake):
  • recycled glass jar - pasta jar works well; the larger the jar the bigger the snowflake
  • boiling water
  • food coloring (optional)
  • stirrer - wooden spoon, etc.
  • pipe cleaner
  • 12" piece of nylon fishing line
  • pencil
  • borax - powdered cleaning agent, found with the laundry detergent at the grocery store

Process and Pics:
Cut pipe cleaner into smaller pieces that will fit inside the mouth of your glass jar.  In nature, snowflakes are always six-sided crystals so you will need three pieces for each snowflake.

Twist the pipe cleaners together at the middle so that you have a six-sided snowflake shape. (Note: you could definitely do this with other shapes - I imagine red hearts would be really cute around Valentine's Day!)
Attach the fishing line to the top of one of the snowflake spokes.  Tie it at the middle in a double knot and then tie another knot at the top.  The resulting loop of fishing line will later serve as the hanger for the snowflake and crystals should not form on the nylon line.  The picture below shows the knots made with green yarn - note this is for visual purposes only - if this one were hung into the solution then crystals would form on the yarn as well as the pipe cleaner.

Boil enough water so that the glass jar is filled to within about an inch from the top.  Take note of how many cups of water this is because the amount of borax added will depend on the quantity of water.  Carefully pour the boiling water into the glass jar - this is an adult job!

If desired, add food coloring to the water in the jar.  We added about 4 drops for Maya's snowflake but the resulting crystals were not that colorful - more of the resulting blue color was from the underlying color of the pipe cleaner so you may need to add more.

Add borax to boiling water one tablespoon at a time, stirring to dissolve after each addition.  Because borax is slightly caustic, it is best if the adult measures out and adds the borax although you can let your child stir if you like.  You will need 3 tablespoons of borax for every cup of water.  After the last tablespoon is added, not all will dissolve - that is okay!

Use the loop to hang the snowflake from a pencil and lay the pencil over the top of the jar, inserting the snowflake into the solution.  It should hang in the middle of the jar.  If it is touching the bottom, you can carefully pull it up and either retie the top knot in the fishing line or use a clothespin to hold it at the correct level.  If it is too high, you can make an S-shape with another pipe cleaner and hang that over the pencil, attaching the snowflake shape to the S-shape to lower it.

Give your child time to draw the set-up in their science journal.  Sydney used my sample snowflake with the green yarn around it to trace around, which was a great idea!

Allow to sit for at least 8 hours or overnight for the crystals to develop. 

Remove the snowflake from the solution.  Rinse very quickly in cold water and place on a folded over paper towel to dry.  Hang and admire your snowflake!

What's Happening?
The borax (sodium borate) that is sold in grocery stores is in a powdered form but actually has a crystalline nature similar to salt and sugar.  Most crystals can be classified as one of seven shapes (or systems).  Given the right conditions, crystals will grow in the form of their distinctive shape.  Below is a picture of the seven basic shapes that crystals will grow into.  A few common examples: borax is monoclinic; sugar is hexaganol and salt (NaCl) is cubic.

In this activity, we we able to make those conditions happen by adding the borax to boiling water.  The hotter the temperature of the water, the greater the amount of borax that is able to be dissolved. The solution becomes "saturated" when it can't dissolve any more of the borax (solute) that you are adding.  For best results, we tried to make a "super saturated" solution - one that contains more of the solute that could be dissolved under the conditions.  (Remember that not all the Borax dissolved after that last tablespoon was added?)  As the solution cools, the water can hold less and less of the borax and the excess borax starts to crystallize out of the solution onto the pipe cleaner. 

Experiment with other types of crystals.  Both table salt and sugar will work, but they do take longer to grow (maybe a week).  Again, create a super saturated solution by adding your mineral to boiling water to the point where no more will dissolve in the water.  Alum, found in the baking good sections of the grocery story (its used in pickling), will also grow good crystals.  Here is a great list compiled by of other crystal growing experiments.  If you do grow more crystals then provide your child with a magnifying glass and a strong light and see if they can identify which of the 7 shapes they represent.

You can also build some models of the seven common crystal shapes.  I developed a crystal structure activity using gumdrops and toothpicks.  This would definitely work better with older kids, but Sydney and Maya both enjoyed building them with some help.  They are learning some of the geometry words, like "square", "cube", and "rhombus" at school, so if nothing else this was a great way to reinforce their understanding of those shapes.  If you want to build the shapes out of paper, Math Forum has a crystal activity where you print the shapes, cut them out, and attach them together with glue.  (And, I couldn't resist adding this: here's a recipe for homemade gumdrops, if you are so inclined!)

Need a Snow Day?And, of course, there a zillion things you can do for further exploration and fun with snowflakes.  I found a couple of really fun ones to share.  This tutorial from Instructables (one of my favorite sites ever) is great for cutting 6-pointed snowflakes out of paper.  Also, Snowdays from Popular Front is a fun virtual snow flake lab where you can design and "cut" your own snowflake, add a message to it, send it through email and share it with the pool of other snowflake designers.

Have fun with the science of crystals and snowflakes!

Thursday, December 1, 2011

Creating a Rock Collection

My girls both love rocks and frequently come home from our outdoor adventures with pockets stuffed full.  Recently, they've taken to destroying, I mean harvesting, rocks from our decaying asphalt driveway.  The top layer of asphalt has come off at some spots and the underlying rock layer is exposed.  (My hubby and I are having issues justifying the expense of repaving...)  Rocks are fascinating to kids and they are a great way to explore some basic geology.  A rock collection is a great way to turn those finds your kids come home with into a learning experience.

Materials Needed:
  • bucket
  • digging tool
  • magnifying glass
  • sorting containers (we used plastic egg and apple cartons we pulled from the recyling bin)
  • desk lamp
  • rock and mineral guide (we used the DK Pocket Guide for Rocks and Minerals as well as a one page sheet (Google Doc) I put together comparing the different types of rocks)

Process and Pictures:

Start off by collecting a variety of rocks.  The girls collected some from around our property but you could find them a variety of places.  Outcroppings or hillsides are better places to find interesting rocks than flat land in general.  (Assuming, of course, that you don't want your child picking apart your driveway.)  Give them a bucket in which to put their finds.  You can also provide them with a magnifying glass to examine their rocks.

Bring the rocks inside and have your child spread them out in front of them.  It is helpful to have both a magnifying class for observing the rocks more closely and also a desk lamp to provide some extra light.  If you have a rocks and minerals guide this would be a great time to share it with your child; they are often fascinated by the beautiful pictures that the guides often contain.  Talk about the properties of the rocks that they can observe.  Some rocks may have visible crystals.  Some may be very monochromatic while others may have many colors present in them.  You may find a rock that has fragments of other rocks in them.  You can also have them observe the rocks using their other senses such as feeling how rough or smooth they are.  
Ask your child to sort the rocks. There are a variety of ways to sort, depending on the inclinations of your child.  One way to sort which works especially well for the youngest child is by color, which is what my younger child wanted to do.  We used a large, apple container for Maya's collection - the impressions were big enough to fit a large number of rocks.  The colors are indicative of the minerals present, which you can tie back to the mineral pictures in the guide if you want.  For younger children, you could also sort it into what they can imagine using the rocks for, similar to the book "If You Find a Rock".  Sort the rocks into groups based on what your child can imagine using them for: skipping, wishing, writing, etc. 

Another way to sort is by the crystals present in the rocks. Sydney is obsessed by the "sparkles" in the rocks - so we talked about what crystals are and how they grow.  (We may have to grow some crystals in the near future she is so fascinated by them...)  Sydney started out by sorting the rocks into those with visible crystals and then from there sorted them by those with similar color and texture.  We used a 2 dozen egg container for Sydney's collection - the impressions were smaller but able to accommodate her collection with a greater number of groups.

For older children, see if you can sort the rocks by type: igneous, sedimentary or metamorphic.   This is not necessarily an easy process as experienced rock hunters and geologists can spend hours debating the characteristics of various rocks, but you may be able to sort them based on some very general ideas related to how they are formed.  (See 'What's Happening' section below for formation information.)

Put your collection into a safe spot so that future finds can be put into the containers. You can also take pictures of the rocks to put into the child's science journal and make labels for the containers.

What's Happening:
Rocks are all around us.  They are a combination of two or more minerals.  (For more information on minerals, see the 'Extensions' section below'.)  They are classified by how they are formed into either igneous, metamorphic or sedimentary rocks.  They are then further identified based on the minerals that are present in the rocks. 

Igneous rocks are formed as molten lava cools and often have interconnected crystals of different minerals.  If the crystals are big, then the rock cooled slowly so they were probably formed inside the early.  If the crystals are small, then the rock cooled quickly on the surface of the earth.  Igneous rocks also may have pores with the gases escaped as the rock cooled.  Metamorphic rock is rock that has been transformed due to heat and pressure.  They are often denser than igneous rock and their crystals may have separated into bands.  Sedimentary rocks are rocks that have been created from fragments of other rocks that have been cemented together.  If you can see small pieces of other rocks or, even better, fossils, then you probably have a sedimentary rock.  The one page sheet I created has some pictures of common rocks for each of the three types, or you can also refer to this rock chart from the United States Geological Survey to learn more.  I also found this on-line rock key that the author has successfully used with elementary age students that you might want to give a try.


We really enjoyed the book, "Let's Go Rock Collecting" by Roma Gans.  It starts out with kids going rock collecting but talks about the types or rocks and even the rock cycle.  A good intro for young scientists out there and Maya even selected it as a bedtime reading book several days in a row.

Another thing to do that would be interesting, especially with older kids, is to see if you can identify the minerals that are present in the rocks.  For this a guide is especially helpful.  Geologists look at the following characteristics to aid in mineral identification:
  • color
  • streak
  • transparency
  • luster
  • hardness
  • cleavage
  • fracture
  • specific gravity
  • crystal form
You can find more information on each of these characteristics on the "Rocks for Kids" website.  
Maya testing the streak of one of her rocks.