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Tuesday, March 26, 2013

Mixtures and Solutions


I have to admit I was very a little terrified of this topic and how I was going to approach it with my kiddos. Don't misunderstand me. I LOVE SCIENCE! I am constantly trying out different science investigations both in class and at home and I understand most of the time the chemistry behind it and "how" it works I am very good at doing the research), but teaching abstract concepts such as: "It's there, you simply can't see it with the naked eye.", "It evaporated and is now in the atmosphere.", "The atoms rearrange to form 'new' products called precipitates.", etc. can become overwhelming. I wanted to make sure my kiddos developed a true understanding of the concepts, that I ignited their curiosity even further, and that their love of science wouldn't diminish because the concepts may be too foreign or difficult to tackle. I should have known better than to underestimate my kiddos. :)

The following are some of the investigations we have done so far:

Investigation #1 What is a Mixture
I first began by having a mixture of Chex Mex, mini-marshmallows, pretzels, peanuts, and M&M's in a large bowl. I asked the kids to tell me what they could see. They stated I had a mixture of items. I then asked them if I could separate them into like ingredients if I wanted to. They said I could and I asked them to prove it to me by doing just that. They separated the items (substances) that have been mixed together and of course got to eat them. I then told them that a mixture is any two or more substances that have been mixed together and can usually be separated back into the original substances. We then charted any mixtures that they may know:  chocolate milk, fruit salad, clothes in the dryer, coffee and creamer, stew, Skittles, etc. I did not differentiate between mixtures and solutions.
Students also had the opportunity to sort out pictures into either a mixture or a solution. They had to be ready to explain/justify why they placed the items where they did with a partner. Click on the picture for the sorting sheet.

Or you can access here as well.

Investigation #2 Separating Mixtures
In this investigation students mixed three different substances, Diatomaceous Earth (the remains of millions year old, sea-dwelling microorganisms that have decomposed into a predominantly silica-based substance), gravel, and kosher salt into three separate cups containing 50 ml of water each. They stirred and mixed the materials. They were then asked to separate all three substances using a screen. They realized only the gravel was able to be separated using the screen due to the size of the holes on the screen.


Students were then introduced to a different filter using a funnel and coffee filter. They were able to see that the coffee filter has tinier holes in it. When they tried to filter out the diatomaceous earth/H2O mixture and the salt/H2O mixture only the diatomaceous earth was able to be filtered the salt mixture went right through.



Students were asked to discuss why this happened. Some said that the salt melted, some said the salt disappeared, and some said that the salt had dissolved into the water. We then discussed that earlier we learned that mixtures can be separated into their original substances, therefore we should be able to separate the salt mixture. After much discussion, one group said that if we let the water dry up, maybe we could get the salt back because somebody in that group had once let saltwater from the beach dry up in a cup. This was a perfect segue into the terms evaporation and solution. Students then placed enough salt/H2O mixture into a lid and we let them out to dry.



 

 About 2-3 days later, we were able to observe salt crystals left behind on the lid after the water had evaporated from the solution. With magnifying lenses they noticed that the salt crystals are shaped like squares and have an x across them.



 They were so blown away by this! Explaining a solution to them, became much easier. They were able to understand that a solution is a special type of mixture that is clear and cannot be separated with a filter. Most solutions are separated through evaporation. We then went online to complete this sorting activity together. Click on the picture for the link.



http://www.rockingham.k12.va.us/resources/elementary/files/5mixturesolution.jpg



Investigation #3 Separating Dry Mixtures
In this investigation students mixed the same three substances used in investigation #2 into one cup and had to develop a thorough method of separating the three substances. Students easily identified that the way to separate the gravel was to use the screen. They then drew upon their previous work to add water to the powder and salt, use the funnel to separate the powder and let the remaining solution evaporate to get the salt back. This was beyond amazing to me at how they were making connections and figuring out the processes on their own.



Investigation #4 Salt  Saturation
Students were asked to dissolve salt in water until no more salt will dissolve. They marked the level of the water without salt using a sticky note. Using a  balance, they weighed the water and noted it weighed approximately 50 grams. They then added one 5ml spoon of salt at a time into a container that had 50ml of water. After each spoonful they shook the container and observed for evidence of salt particles. If so salt was observed they continued to add salt. As soon as they observed salt they stopped. They now had a saturated solution--the solid substance can no longer dissolve. They then used the balance to measure how much salt in grams was added to achieve saturation.




50 grams of H2O + ?salt grams= ?grams of solution
 Once they had the new weight they were easily able to do the math to find out the grams of salt by doing the inverse relation of addition.



Investigation #5 Epsom-Salts Saturation
Students repeated the prior investigation to determine if it would take the same, less, or more amounts of Epsom-salts to reach saturation. They discovered that it took more grams of Epsom-salts. I then asked how I could separate the Epsom-salts for the solution and they immediately knew that we had to use evaporation. They again placed some solution on a lid and let it evaporate. They made their observations a couple of days later  and noticed the crystals left behind looked different then the kosher salt.


 



One group mixed had an epsom salt saturation with food coloring. After evaporation, this was their result.
Another group had baking soda saturation with food coloring. After evaporation, this was their result.



                                   ultraepsom.com


 Students observing the salts with a microscope.



Investigation #6a Chemical Reactions-Fizzing
Students were asked to consider whether citric acid, baking soda, and calcium chloride would make solutions in water. Students made three solutions using 50ml of H2O and 5Ml of each substance. Students discovered that all but the baking soda easily dissolve in 50ml of water, making them more soluble in H2O than baking soda. Students were asked what might they do to get it to completely dissolve and some suggested to add more H2O. The groups added another 25ml of H2O and confirmed that indeed it dissolves in H2O. Some students also noted that when they mixed the calcium chloride, the solution became warm. I then explained that this is a property of calcium chloride. As it dissolves, energy as heat is released. 

The Fizz Test
Cup#1 Students mixed 5ml each of calcium chloride and baking soda in 50ml of H2O.  They observed some fizzing with some white stuff in it. when asked if it is a solution, students were torn. Some said it was not a solution because of the white stuff left behind and others said it was a solution with white stuff in it. I accepted both arguments.  Save the contents of this cup for a later investigation.

Cup#2 Students mixed 5ml each of calcium chloride and citric acid in 50ml of H2O. Students noted this made a solution-not very exciting to observe.

Cup#3 Students mixed 5ml each of citric acid and baking soda in 50ml of H2O. Whoa! Watch out! This one is very exciting to observe.  There is lots of fizzing and there is a solution once the fizzing stops. Sorry about the lack of pictures. I forgot my phone on this day. 

Once students have settled down, discuss with them what they think caused the fizzing of cup 1 & 3. Someone will suggest it is gas. When fizzing is observed in a liquid it is gas escaping and coming to the surface. You can further explain that the bubbles in soda are gas escaping the liquid and then provide the chemical name of the gas--CO2 (carbon dioxide).


It is now time to head outdoors! I am an avid follower of Steve Spangler the Science Guy. His website, store, and blog are the absolute best for fun, exciting, and engaging science learning experiences. One of my kiddos' favorite is the flying canisters. In our experiment the canisters didn't really fly, but the lids sure did. We used the #44 container as provided in our FOSS kits half way with water one Alka-Seltzer. As stated on the site, "this is an explosive lesson in air pressure and motion," as well as gas escaping the liquid-CO2.

Plastic Vials
                             http://www.unitednuclear.com/index.php?main_page=index&cPath=25_31









Another experiment we did (I did-they observed), was the Mentos Diet Coke Geyser, as seen on Steve Spangler's site. This was such a blast, but I do want to clarify that some scientists argue it is not a chemical reaction, but instead a physical reaction. Either way CO2 is formed in massive amounts and the view is awesome--besides the kids can't stop talking about it!





(Excuse my crazy hair-it was Crazy Hair Day!)



Investigation #6b Chemical Reactions
Students are given back Cup#1 from the #6a investigation and asked what they think the "stuff" in the bottom of the cup may be. I had one student guess it was chalk-Yippee!!! I then confirmed and explained that the white stuff is a new substance that is formed when calcium chloride and baking soda are mixed together in water. This new substance is not soluble in water, so it settles to the bottom of the cup. When a solid substance forms and settles out of the water, it is called a precipitate.

Even though many of may students had already been saying that they knew it was a chemical reaction, I decided to introduce it when all the parts of the chemical equation had been observed. It then became easier and clearer that the substances we started with are called reactants and the new substances that form are called products.
We discussed the chemical changes that took place in cup#1 as the evidence that indeed a chemical reaction had occurred: temperature change, CO2 formation, and chalk (calcium carbonate) precipitated.

It was finally time to introduce Atoms & Molecules. Most of my students are familiar with atoms and the notion that everything in the world is made of atoms. We discussed it last year when we learned about electrons and protons in electricity & magnetism unit. They are also familiar with the periodic table and some element names because of H2O and CO2 in regards to photosynthesis. It is wonderful when things connect. I reintroduced a molecule as two or more atoms combined. Again it ties back to H2O and CO2. In the weeks to come we will be digging deeper into the periodic table, its 90 naturally occurring elements, and their associations/properties on the table.

Investigation #6c Reaction Products
Students were asked to separate the contents of cup#1. They knew exactly what to do! They used the funnel and filter paper to separate the chalk (set it aside to dry). I told them one of the properties of chalk is that it reacts with vinegar. I took a piece of chalk (it was red) and added 50ml of vinegar to find out for sure. I told them they would be able to test their substance on the filter paper once it was dry to make sure it was chalk--TaDa! It was chalk.

Chalk reacting with vinegar-fizzing=CO2.
Top view after evaporation.

I then asked them if they thought the clear liquid was a solution and if so how we could test it. They said to let it evaporate and if crystals formed it was a solution--One of my students asked if the substance that remains after the solution evaporates are always crystals? What a wonderful mind. To their amazement they soon discovered that the solution had salt. They knew it was salt because of the square shaped figures with and x's across them. This brought about so many questions. they wanted to know how salt came about if it was not added as a reactant. Again this was the perfect segue into how atoms rearrange to form new substances in a chemical reaction. It was also a perfect time to explain that atoms are destroyed during reactions and other molecules are created. No atoms are left behind and no atoms are  added from other places. The same amount of atoms on one side equals the same amount of atoms on the product side but in different combinations (molecules).

CaCl2+ 2 NaHCO3 = 2 NaCl+CO2+CACO3+H2O


With the use of small, different colored circles from the die-cut machine. students got the opportunity to practice rearranging atoms and realized there had to be two molecules of baking soda in the reactants in order to balance out the equation, producing 2 molecules of salt and H2O. They really liked that challenge.


 :)

All these Mixtures and Solutions investigations can be found by clicking on the FOSS link along with teacher prep videos, and PDF's.



4 comments:

  1. Hi there :) I realise this post is almost 12 months old but I was wondering how you mastered your Mentos & Diet Coke experiment. I can't seem to work out how to get the Mentos into the bottle in one hit without getting soaked!! Any ideas would be great. Thanks :)

    ReplyDelete
    Replies
    1. Hi Kelly, I was unable to find a way around that either. The first time I did it with the kids I got completely soaked--the kids loved that! If you go onto http://www.stevespanglerscience.com/geyser-tube.html you can purchase a geyser tube (very cheap) that allows you to do it without getting soaked. My kids loved it! We experimented with different amounts of Mentos which created different geyser heights. It was definitely one of the fondest memories for my kiddos that year. Let me know how it turns out.

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    2. You can string the mentos with a needle and thread and drill a whole in cap of 2-liter bottle to put it through. Hold the mentos out of the soda by using the string until you are ready for the reaction to occur. You will have plenty of time to get away without getting wet and sticky. My students were even able to do this and have a competition to see which soda worked the best. They were all able to set off explosion at the same time using this method. Just FYI

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