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January 2017

Honors Chemistry 21 – Acids & Bases

For this lab I basically followed the Modern Chemistry lab: Household Acids & Bases but added a few things.  The lab is in the textbook and can be found in the online resources.  Its a simple enough lab with students testing various household items like dish soap, soda, lemon juice, vinegar, bleach, milk of magnesium, etc. to see if they are acids or bases.  The lab called for us to make a pH indicator out of red cabbage so I did that the night before to save time.  Its very easy to make, you just chop up a red cabbage and place it in a pan full of water.  Bring it to a boil then turn off the heat and let it cool.  Strain it, collecting the lovely purple water which is your pH indicator.  I found this great photo below that shows the color of the indicator for different pH.

red-cabbage-juice
Photo Copyright Fundamental Photographs, NYC, http://www.fphoto.com

I also happen to have a pH meter, litmus paper (red and blue) and regular pH paper, so I had the students test 5 different chemicals with as many methods as they could.  I also asked that they try to get a rainbow of colors with the cabbage indicator like in the photo above.  Here are some of their results.

This lab worked really nicely and the color changes were pretty dramatic.  Bleach turned the purple indicator a dark brown which quickly faded to yellow and eventually went clear (see photo with test tubes).   In most cases the  pH meter reinforced the pH values found with the cabbage indicator and the pH paper (which turns color like the cabbage indicator).  Litmus paper just tells you whether you have an acid or base.

I had the students watch two Crash Course videos before class.

Here’s a good video on sulfuric acid:

We had some extra time at the end of the lab so I demonstrate the power of hydrochloric acid by using it to dissolve aluminum foil, which is the same demo I talked about in yesterday’s post for the intro class.  The photo below shows the foil in muriatic acid (HCl) and the little bubbles of hydrogen gas starting to form.  img_8421

In this video you can see the hydrogen gas escaping and the acid looks like its boiling, but the beaker is very cold to the touch, its just a vigorous chemical reaction producing a lot of gas bubbles.  Near the end of the video you can see that the Al foil disappears completely.

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Intro Chem 18 -Chemical reactions

In today’s class we did Chapter 6, lesson 1: What is a Chemical Reaction from the American Chemical Society’s Middle School Chemistry curriculum. This lab starts by demonstrating combustion by burning a candle and then putting a jar over it to extinguish the flame. We had already done an earlier lab on combustion, so the kids already knew the reaction needed oxygen and fuel and produced water and carbon dioxide. We then looked at the reaction of methane and oxygen molecules to form carbon dioxide and water.  I had the students build models of the reactants with Snatoms or Zoomtool, then take them apart

and build the products from the same pieces.  I tried to make a point of showing how everything got used up to make the products, so no atoms were created or destroyed in the reaction, just rearranged.  This is the first time I’ve used the Snatoms and I really like them.  They go together much easier than the zoomtools since they use magnets.  The small black bars represent double bonds in the oxygen and carbon dioxide molecules.

IMG_8390.jpgOnce the students had done the models they colored the paper atoms from the curriculum and cut and paste them into their notebooks to show the reactants and products.  They then filled out three tables, each one a different combustion reaction,  where they had to figure out how  many carbon, oxygen and hydrogen atoms were on the reactant side and how many were on the product side of a chemical reaction.  This helped drive home that you have to have the same number of each type of atom on both sides.

After everyone was done with the paper work we went outside for a demonstration.  I cut very thin strips of aluminum foil and placed them in a test tube.  Then I poured muriatic acid (HCl), which you  buy at a pool store, into the test tube.  It takes a few minutes but you will start to see bubbles rising from the aluminum foil as the Al and HCl produces hydrogen gas.  Eventually it becomes come vigorous and the liquid turns dark grey (AlCl3 in soln) and suddenly the foil is gone!

Honors Chemistry 20 – Solubility

Unlike the last couple of labs, the Solubility Lab found in Ian Guch’s 24 Lessons that Rocked the World,  was a breeze and we actually finished early enough to do a bit of the middle school lab and go over homework!  img_8277

img_8281
I love this little pocket scale.  Its great for massing small quantities of chemicals.

The solubility lab tests whether water or isopropyl alcohol will dissolve more potassium chloride (KCl).  I bought NoSalt at the local grocery store for my source of potassium chloride.  It has a few other ingredients but it didn’t affect the experiment.  Students write down which they think will dissolve more KCl before we start, and most agreed that water would be the better solvent.  They measure 2 grams of the KCl into a small 50ml beaker and then added 10 ml of distilled water.  Stir for a minute then write down their observations.  They then pour off the liquid, keeping any undissolved KCl in the bottom of the beaker.   Beakers were then heated on a hot plate or butane burner (with very low flame) to drive off the remaining water.  Once the beaker were cooled, the beaker and remaining residue were massed.  Beakers were then cleaned, dried, heated briefly and massed again.  The difference in the masses is the mass of the undissolved KCl.  Then they repeated the experiment using 10 ml of isopropyl alcohol (91%) and found they had a lot more KCl left over in their beaker.
When I did the lab I found only about 3% of the KCl had dissolved in the alcohol, while over 90% dissolved in the water.

We finished this lab in about an hour so I had the students do the same lab the middle school did yesterday, dissolving chemicals in water and watching the temperature as it dissolves to see if its exothermic (gives off heat) or endothermic (absorbs heat).  I had each group just do one chemical, either potassium chloride or calcium chloride (DampRid, purchased at Lowes).  The temperature change is pretty drastic for both of those and after the frustrations of the last couple of labs it was nice to do some short and simple labs.

Intro Chem 17 – Exothermic & Endothermic

Today’s lab went great, it was fairly simple but had pretty impressive results and the students did a great job with it.  The lab, Temperature Changes in Dissolving, is from Chapter 5, Lesson 9 of the American Chemical Society’s  free middle school chemistry curriculum.  Before we started the lab I wrote ‘exothermic’ and ‘endothermic’ on the white board and asked if anybody knew what the words meant.  Nobody answered, so I asked if they could look for clues in the words and immediately there were shouts of ‘temperature’ for thermic and ‘outside’ for exo.  One students said bugs have exoskeletons which means its on the outside and we have endoskeletons – skeleton is on the inside.  So I explained how exothermic means something is giving off or releasing heat and would be warm to the touch while endothermic means heat is being absorbed.  I then did a few demos, including ‘hot ice’, which is super saturated sodium acetate.  There are many recipes for this on the internet/youtube, basically you have a small amount of water and dissolve a huge amount of sodium acetate into the water at high temperature.  Then you let it cool.  I poured the super saturated solution on to a petri dish containing a few crystals of solid sodium acetate so that the solution crystalized immediately upon contact. You can see the broken stalagmite in the photo below and the glass jar behind it where the solution crystalized before I could finish pouring it.  The reason I did this demo today is that when the sodium acetate turns back into a solid it becomes very hot, hence the term ‘hot ice’ for this demo.  Its actually the same stuff you find in hand warmers and instant hot packs.  The heart shaped HotSnapZ in the photo below is an example of an instant hot pack. I also had an instant cold pack on hand and you’re supposed to activate it by squeezing and popping the internal bag of water inside the cold pack but we just cut it open so we could see what was going on and dumped it in a large beaker.  It became very cold when the pellets (probably ammonium nitrate) dissolved in the water. img_8261

The lab itself involved dissolving 2 grams of different chemicals (potassium chloride,

calcium chloride, sodium bicarbonate and sodium carbonate) in 10 ml of water and recording the initial temperature and the maximum or minimum temperature reached as each of the chemical dissolved.  Two of the solutes were exothermic and two were endothermic and the changes in temperature were very noticeable, one increased by almost 20 degrees Celsius!  So overall a very nice lab. img_8267

I showed the following Ted talk on cold packs at the end of class.

Honors Chemistry 19 – Conductance of solutions

This lab goes along with Chapter 13 on ionic aqueous solutions in Modern Chemistry.  This chapter discusses how ionic compounds dissociate in water into their different ions and therefore the solutions should conduct electricity since it contains freely moving charged particles.  Molecular compounds, like sugar, do NOT dissociate into their components so there are no charged particles floating around to conduct electricity.  The lab, II-2: Conductance of Ionic & Molecular solutes,  is in the home scientist chemistry kit manual CK01A.  img_8049The students populate a 24 well reaction plate with 6 different chemicals at 4 different concentrations, 1.0M, 0.5M, 0.25M and 0.125M.   Its pretty easy to mess this up if you’re not paying attention so it helps to have lab partner reading out the instructions and double checking where everything is going.  Once the reaction plate is populated the students used a digital multimeter to measure the resistance of each solution. img_8050

For the solutions with ions the resistance should decrease with higher concentration.  For molecular solutions the resistance should be high for all concentrations.  Measuring all the resistances took quite a while because you had to dip the probes in tap water, dry them off, dip them in distilled water, and dry them off again, between every well!  It also took a while for the resistance to settle down.  Once they have all their data, students plot the conductance (1 over the resistance) versus concentration for the 6 chemicals.  Hopefully they’l see that some compounds had a resistance that depended on concentration, while some (molecular) did not.

Here are the videos I suggested the students watch before class:

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