ingridscience

To make classic slime.
Make 50% white glue in water. Give students 2 Tablespoons each in a cup.
Mix 1 tspn borax into 1 cup water. Give students 1 Tablespoon each in a cup.
Students mix the cups together and stir quickly until combined.
(For measuring everything individually, use recipe 2 below.)

Play with the slime: it can flow between the fingers, but when it is suddenly pulled it breaks.
Chemical explanation of how slime is made:
The glue contains long molecules (called polymers). When borax is added it makes permanent (covalent) bonds between the glue molecules, called cross-links. These cross links form a branching web of glue polymer molecules, giving the slime its thick texture.
The following activity compares two different kinds of slime/silly putty, with different amounts of cross linking.
Chemical explanation of how silly putty behaves:
There are other bonds between the slime molecules (called hydrogen bonds), which are weaker bonds and can easily break and reform. When the slime is pulled slowly, a few of the hydrogen molecules break, but then reform with another adjacent polymer. As these hydrogen bonds continually break and reform, the slime stays in one piece but can flow and change shape. However, when slime is pulled on suddenly, many hydrogen bonds are broken at once, so it breaks apart.

To make two different slime/silly putty recipes, with different consistencies
Ask students to work on the tray, as this activity is messy.

First make the classic slime recipe as follows:
Give students their materials: 4tspns of glue in a cup, an empty cup, a squeeze bottle of water and a tablespoon measure, borax powder in a cup and a 1/16tspn or equivalent measure (I use two scoops of a very small measure).
Instructions for students:
To an empty cup, add 1 Tbspn water and 1⁄16 tspn borax. Mix well.
To 4 tspns glue in a cup, add 1 Tbspn water. Mix well.
Pour the borax mixture into the white glue mixture. Mix well.
Lift the blob out of the cup, and into a small baggie.
Mould with hands through the bag to mix completely.
Pull the blob out of the bag, leaving any liquid behind.
Mould with hands until smooth.

Allow students time to play with their slime.
If slime is not stretch enough, gradually work in some more water.

Discuss how and draw how the borax molecules cross link (or bridge) the long glue molecules, binding them together so that the glue is more solid but can still flow.

Then make another slime recipe:
First tell students that they will add a lot more borax to this recipe and ask what more cross links of borax will do to the texture of the slime that they make [it will be more solid].
Do the experiment:
To an empty cup, students add 1 Tbspn water and ½ tspn borax. Mix well.
Pour the borax mixture into the cup of 4 tspns white glue. Mix well.
Lift the blob out of the cup, rest for 5 minutes.
Mould with hands into a ball.
Roll in cornstarch to make less sticky.

This recipe makes a much more solid ball, which can be bounced.

Review the consistencies in terms of the molecules that make silly putty (see last photo):
In a chemical reaction, the borax molecules bridge (or "cross-link") the glue molecules together, turning the two liquids into something more solid.

Other background chemistry: White glue is a polymer, which is a long chain of repeating units. Other polymers are nylon and plastics, as well as naturally occurring rubber. Polymers can be cross-linked at any of the repeating units along their chain, so the amount of cross-linking determines how solid they become.

Slime is a non-Newtonian fluid - its viscosity changes depending on how much force is applied. (Standard Newtonian fluids only change viscosity with changes in temperature.) When a sharp force is applied to slime it becomes rapidly more viscous and behaves more like a solid.
More info on slime: http://www.acs.org/content/dam/acsorg/education/resources/highschool/ch…

Show students a chunk of dry ice (only hold with thick gloves, as it will give cold burns to skin). It is carbon dioxide in its solid state and is very cold, -80ºC! The solid carbon dioxide does not stay solid very long in a warm room, but instead of turning to a liquid then a gas, it turns straight into a gas - called “sublimation”. The gaseous carbon dioxide can be seen as white clouds surrounding the solid chunk of dry ice.

For each student group around a tub of warm water, drop in a few nuggets of dry ice. The students must not touch the dry ice.
The warm water will dramatically speed up the state change from solid carbon dioxide to gas: large bubbles of carbon dioxide gas form in the water. At the surface, warm water that has evaporated comes in contact with the cold carbon dioxide gas and re-condenses into tiny droplets, forming white clouds. The white clouds, a mixture of water droplets and carbon dioxide gas, spill over the sides of the container and fall to the ground (as carbon dioxide is heavier than air).

If the reaction slows before the dry ice is used up replace the water with more warm water.

If you allow most of the dry ice to sublimate in the tray, small pieces will float to the surface of the water, and spin and zoom around. As gas projects from one side, it pushes the dry ice it in the other direction (Newton's Law of action and reaction). If the gas comes from an angle it will start it spinning.
It behaves as a hovercraft does - the gas escaping from the bottom of the dry ice piece makes a layer of gas between the dry ice and water, so that it can skim with very little friction over the surface of the water.
The small pieces are safe to touch briefly, so students can redirect the direction of the dry ice pieces with a light touch.

Students are instructed to walk around the classroom, and touch different surfaces (e.g. metal, paper, wood, plastic, other objects they find).
Each time they touch a surface, they should count to three and then record how warm it feels. Record on their worksheet (attached below): warm, cold, or in between?
Discuss results as a class. Generally metals will feel colder, and insulators such as plastic and wood, and especially cloths or fur, will feel warmer. BUT without guidance as they touch each object, students will generate a variety of results (see photo).

Discuss what is happening:
Your hand is warm. Some materials can take that warmth away better than others. Metal is a good conductor, and the heat of your hand flows through the metal easily, so your hand loses heat to the metal and feels cooler. Other materials (good insulators such as plastic, wood and cloth) do not take the warmth away very easily, so your hand still feels warm.

Higher level discussion of results in terms of what the molecules are doing:
The molecules in your finger are moving faster than the molecules in the room-temperature materials. Because metal is a good conductor, the heat from your finger is transferred to the molecules in the metal. This decreases the motion of the molecules in your skin and makes your skin feel colder.
The molecules in your finger are moving faster than the molecules in the plastic/wood/cloth. But because plastic/wood/cloth is a poor conductor (a good insulator), the heat energy from your finger is not transferred to them, so your skin stays feeling warm.
Video of the molecule movement here: http://www.middleschoolchemistry.com/multimedia/chapter2/lesson1#conduc….

See heat sensitive sheets for a better activity on conductors and insulators.

Prepare the strips for the activity:
Add a small smear of vaseline to the end of each strip. (If you are using spoons, try and find ones with flat handles, and add the vaseline to the handle end.) Make sure you use the same amount each time and add it in the same spot. (Using an applicator, such as a coffee stir stick, will help to make the amounts more consistent - see photo.)
Push a penny, or a nickel, into the vaseline on each strip.

Each table group can have a set of strips with pennies, and a coffee cup.

Add just-boiled water to fill the coffee cup until quite full, then simultaneously add the metal, plastic and wooden strips to the water with the pennies pointing upwards. (If you are using metal and plastic spoons, place the wider scoop end into the hot water). Make sure the strips are sloped outwards by the same degree, so that this is not a variable in the penny falling off. For most students, it is best if the teacher does this step, to make it as fast and consistent as possible.

Students record on worksheet (see attachment) which penny falls off first, second and third. (See photos for my usual order of pennies falling off.) Some pennies may stay stuck for longer than you want to run the activity, but make sure at least one has fallen off before stopping. If the water has cooled before some pennies have fallen, take out the strips, replace the water with new just-boiled water, and return the strips to the cup - students will become quite engaged in the "race" as the last pennies in the class fall.

Metal strips are expected to release the penny first, but some experiments may differ. Plastic and wood release the penny later.
Record the class results, to find out what happens most of the time, and to use for discussion.

Discuss the mechanism:
Heat moves up the strip by conduction. Once the heat energy reaches the vaseline it melts it and causes the penny to fall off. The different materials conduct heat at different rates: metals conduct heat the fastest, wood and plastic much slower.

Discuss the molecular mechanism with older students:
The molecules of the water are moving around fast. As they bump into the end of the strip that is immersed in the water, they transfer their energy to molecules in the strip, which also start to move around faster. The molecules at the bottom of the strip bump the molecules higher up the strip and their heat energy is transferred too, so spreading the heat energy up the strip. In different materials, the molecules are more or less able to transfer heat energy to the neighbouring molecules, so the rate of heat transfer varies. When the heat energy reaches the vaseline it melts it (the molecules of the vaseline move faster as they change from solid to liquid). The melted vaseline can no longer hold onto the penny, so the penny drops.
The movement of heat when molecules transfer energy between each other by colliding with each other is called “conduction”.

Metals are better heat conductors than plastic and wood. A material that is not a good conductor is an "insulator".

See heat sensitive sheets for a simpler activity on conductors and insulators.

Set-up prior to experiment:
Stand a desk or table in an open area of the classroom. Arrange the four cans on the desk so that they can support the tub at each corner. Fill the large tub with cold water and stand it on the four cans so that it is stable. Wait for the water to become completely still before proceeding. Boil the kettle of water, so that it is quick to boil again. (Outdoors, hot water from a good quality thermos will work fine. Heat pads that get very hot also work, though not as well.)

Demonstration:
Ask all the students to sit in a circle around the tub, so that they can see through the sides of the tub.
Suck up a little food dye into the pipette, then very slowly and carefully lower the pipette into the water and deposit a pool of food dye on the base of the tub. Slowly remove the pipette from the tub, so as to disturb the water as little as possible. (A second pool of food dye was used as a control in the photos above, but this is optional.)
Get the styrofoam cups ready - use one, or stack them, until they just slide under the tub.
Bring the kettle to the boil again, then immediately fill the a styrofoam cup (stack) with boiled water. Slide the cup(s) under the tub, and leave it directly below the pool of food dye.
After a few seconds, streams of food dye should start to flow upwards from the food dye (see last photo above).
Make sure all the students are able to see the food dye streaming upwards before continuing discussion. You may need to carefully wipe condensation from the outside of the tub for a clearer view.

Explanation:
The hot water in the styrofoam cup heats up the water and food dye directly above it, making the molecules here move faster as they gain heat energy. This group of fast moving molecules flow upwards in the water (because they are less dense than the surrounding cooler water). They take heat energy with them, and are moving by "convection". The visualized convection currents are beautiful as they trace out the curving patterns of heated water.
Convection is the movement of a group of higher-energy molecules through a liquid, or a gas. Convection is how heat moves around the air in the classroom.

For a lesson on the Sun, this demonstrates the convection currents that carry hot gas (not liquid) from the centre to the surface of the sun.
Diagram of section of the sun with convection zone: https://www.nasa.gov/wp-content/uploads/2023/03/655928main_solar-anatom…
Each granule has a bright centre, which is the hot gas rising through a thermal column. The granules’ dark edges are the cool gas descending back down the column to the bottom of the convective zone. (From https://education.nationalgeographic.org/resource/sun/.)
Many, separate convection cells, form the granulation patterns on the surface of the sun.
Image of convection cells in the sun: http://astrobites.com/wp-content/uploads/2012/07/kauf18_4.jpg
Video of granulations on the sun's surface: https://www.youtube.com/watch?v=hXEYbovTUr0 http://solarscience.msfc.nasa.gov/images/SVST_granulation.mpg