Demo #2 – “Like Dissolves Like”

Water-Based vs. “Permanent” Markers

Useful in Units:  Solutions, Physical Chemistry, Organic, Bonding

Background and Uses

This is about as easy a demo to do as there is and it has good Real World Connections.  I first show the students this demo in the Solutions Unit to illustrate that “Like Dissolves Like”.  I use it again in the Organic Unit to re-illustrate the point.  This demo can be used to alter the Paper Chromatography Demo in the Physical Chemistry Unit to show general solubility issues and how they affect the Separation of Mixtures.

Alterations to try and pitfalls for this demo:  

  1. The more porous the object you write on, the harder it will be to remove the ink – don’t use paper.
  2. Don’t allow the permanent marker to stay on the object for an excessive amount of time – it may be difficult to remove completely.
  3. Acetone will dissolve plastic. Make sure the object you write-on is not dear to you.
  4. Try using Nail Polish Remover instead of Chemical Storage Acetone to show how students can remove Permanent Marker at home.

Concepts the Demo Illustrates:  “Like Dissolves Like”, Polar vs. Non-Polar, Oil-Based vs. Water-Based Mixtures

Where I found this demonstration:

This is one I came up with myself, though I’m sure that many other teachers have thought up and used this demo for an example of “Like Dissolves Like”.

Materials required: Permanent Marker, Water-Based Marker, Acetone, Small Dry Erase Board (or other object to write-on), Water, Paper Towels

Procedure:

  1. Write-on Dry Erase Board with both Permanent Marker and Water-Based Marker.
  2. Wash off the Water-Based Marker with a wet paper towel.  Show how the Permanent Marker still remains.
  3. Show how the Permanent Marker can’t be washed off with Water, but can with Acetone. (Using towel with Acetone)
  4. Discuss

What to illustrate with this demo:

This demo is great at illustrating the difference between Non-Polar and Polar Substances and how “Like Dissolves Like” (Non-Polar dissolves Non-Polar and Polar dissolves Polar).

To relate this to the Real World explain how the common term for Non-Polar mixtures is “Oil-Based” and for Polar mixtures is “Water-Based”.  This is why there are basically two types of paint: Oil-Based and Water-Based.  This is also why a mechanic might use gasoline to wash grease off his/her hands instead of water.

You could also go into how soap is the bridge between Non-Polar and Polar mixtures/substances or why some Organic compounds can be absorbed into the bloodstream through skin.  (Both the Organic compounds and the fat under your porous skin are Non-Polar – the fat dissolves the compounds.  Turpentine and Gasoline can cause Renal Failure if enough is absorbed.)

Some Safety Stuff: (This list may not be a full list of safety procedures.  Please always think ahead and use caution when working with any chemicals.)

  1. Acetone – it is recommended that you use gloves when handling acetone.
Video: coming soon
pdf version: Demo #2 – Like Dissolves Like
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Demo #1 – Magic String

Useful in Units:  Bonding, Physical Chemistry

Background and Uses

This is a demo I have used the first day of school.  It is simple and relays to my students that the class will be fun, but that they will need to be able to learn and have fun at the same time.  As simple as it is, students are amazed by this demo.  Parents are equally as amazed when I show it to them on Parents Night.

 The concept shown in this demo has a couple of different terms depending on what course and depth you are teaching the concept.  I remember in 7th grade technology and in Earth Science the concept being referred to as the “cohesion” of water.  This term is also commonly used in the real world.  In chemistry, we refer to this as “Hydrogen Bonding” – the intermolecular force that causes certain compounds to be very attracted to each other.  Hydrogen Bonding is the reason why water and some other compounds have higher than expected boiling points.

Alterations to try and pitfalls for this demo:  

  1. Make sure to soak your string for a little while ahead of time.  You want the string to be fully saturated with water.
  2. Have students make guesses as to why the “string” is so magical.  Try it with dry string and show that it is not the string at all.
  3. Make sure that the string is taught – it doesn’t work well if it is not.
  4. See how long a piece of string you can use and still get water from one beaker to another – the longer the string the more water that will be required.  (I’ve gotten it to work with string that is more than 20 ft. long with help from students (sorry, around 6 m)).

 Concepts the Demo Illustrates:

Cohesion, Hydrogen Bonding, Intermolecular Forces

 Where I found this demonstration:

Paul Scott, a science teacher from Niskayuna High School in Niskayuna, NY was the first person to show me this demo.  Paul was my “Science Methods” professor in the MAT program at Union College in Schenectady, NY.  He, more than any other person, molded my teaching philosophies to use demonstrations and science news as key components of getting students to think analytically while getting them excited about science at the same time.   The procedure I have written up is based on the demo Paul first showed me.

Materials required: 2 Beakers (250 mL or more), Water, String (2 to 3 ft.)

 Procedure:

  1. Fill 1 beaker with water and add string so that it has time to soak.
  2. Take wet string out of water.
  3. Hold the string between the 2 beakers and elevate the filled beaker higher than the empty beaker.
  4. Pour slowly – The water will follow the string from one beaker to the other dripping only very rarely.  See picture:

Magic String

Video of this demo:   http://youtu.be/G5hPFxGoRz8 

What to illustrate with this demo:

  1. That it is the water on the string, not the string itself, that is allowing the water to be transferred.  Try repeating the demo with dry string – the water will pour onto the table no matter how slowly you try to pour it.
  2. Water molecules are attracted to other water molecules.  It can be said that water has strong intermolecular forces, strong attractive forces, Hydrogen Bonding, or cohesion.
  3. The reason that the water can “walk” across this string is the same reason that water has a higher than expected boiling point.

PDF version of this demo:   Demo #1 – Magic String

Introduction – About ChemistryDemo.com

Hello and thanks for visiting.

My name is Bill Brown.  I have been a high school chemistry teacher in New York State for the past 12 years.  As a high school chemistry teacher, I love to present demonstrations that reflect the concepts that I teach.  I have collected many many books of demonstrations.  They vary from books on how to make concoctions with kids up to university level demonstration books.  Finding demonstrations in these books that are simple and straightforward and pertain to the concepts that I teach in high school chemistry is quite arduous.  Finding demonstrations that fit this criteria and actually work is more difficult still.

The goal of this blog is to present some demonstrations I currently use in my classroom and to create a dialogue that will help all of us improve in our ability to teach chemistry.  I will try to present each of the demos in such a way that you can download a .pdf version that would be worthy of printing and keeping in a binder so that  you can create a demonstration book of  your own.  Thanks for reading and sharing your ideas with me.