ingridscience

Tell students we will be making a simplified cake to understand the chemistry underlying making a cake.

Introduction as a demonstration:
Ask students what ingredients go into a cake, and for each one write up into colums as flavourings or ingredients needed for the structure of the cake. As flour, liquids and a rising agent are mentioned, mix these ingredients into a well of a muffin tin as a simplified cake batter: about 3 Tablespoons of flour, 3 Tablespoons of water, 1 teaspoon of baking powder. (Egg and milk in a real batter are needed for binding the ingredients together as they are cooked in the oven, but are omitted in our simplified cake batter.)
As discussion continues, the simplified cake batter will rise slowly, and students will start to notice.
Ask them which ingredient made it rise [the baking powder].

Introduction as a short activity for students:
Students each get a tray and a stir stick, and flour and water are in pots with their own scoop.
Ask students to mix a little flour (about 1/4 teaspoon) and water until they make a cake batter consistency, using their stir stick to mix them together.
Then they should add a small scoop of baking powder, mix it up, then wait and watch.
Their cake batter will slowly rise, and some of them will see bubbles appearing.

Continuing:
Tell students that baking powder has three ingredients - read them from the package:
1. cornstarch
2. sodium bicarbonate, which is baking soda
3. monocalcium phosphate and/or sodium aluminum sulfate, which do the same job as cream of tartar.
Ask students to figure out which two of these ingredients mix to make gas, by mixing small scoops of them with water in wells of their tray. If they add the powders to the tray first, the water can be added to mix them together, so the scoops can remain in the pots of powders.
The attached worksheet can help students keep track of what they find.
[Remove flour and baking powder from their tables, and add baking soda, cream of tartar and cornstarch to tables.]

Students should find that baking soda and cream of tartar make bubbles, but the other combinations of two ingredients do not. (Sometimes bubbles arise from the water being squirted in, but these soon pop, and do not count as bubbles being made.) Students may get different results from each other - tell them that this happens to scientists all the time, and they should simply both repeat the experiment until they get consistent results. Sometimes a powder may contaminate a nearby well to give differing results.

Use molecular models to figure out what gas is being made as the baking soda and cream of tartar are mixed and a chemical reaction happens to make gas.
Students are given HCO3 (baking soda) and H (in the cream of tartar). Ask the to rearrange the atoms to make two new molecules. Tell them that one molecule is water (H2O) and ask them to figure out the other one, by joining all the remaining atoms and bonds and filling all the holes in the atoms. They should arrive at CO2, which is carbon dioxide, a gas.

Summarize that when cakes are made with baking powder, the baking soda and cream of tartar (or other acid) in the baking powder mix together in the wet environment of the cake batter. They chemically react to make carbon dioxide gas, which makes bubbles which are trapped within the cake batter. We can see these bubbles as holes in cake. They make the cake light and fluffy.

Test the materials to determine how rigid, elastic and malleable they are. These are the properties of rigid, elastic and malleable materials:
Rigid materials do not deform easily.
Elastic materials can be deformed, then return to their original shape.
Malleable materials can be deformed, and do not return to their original shape.
(Note that technically, I should use the term "plastic" instead of malleable. But as the word plastic is used so often in another context, it would be confusing for students. Materials with plastic properties come in different types: malleable materials can be flattened easily, and ductile materials can be drawn into a long thread easily.)

The classification of the materials will depend on their thickness and shape, for example metal can be quite rigid if it is thick, or malleable if it is thin. Any answer is fine as long as the students are manipulating the materials and thinking about how they deform.
Our results: rigid materials were marble, rock, hard ball, bow tie pasta (spaghetti would be elastic too). Between rigid and elastic were metal strip, popsicle stick, plastic spoon, bouncy ball. Elastic materials were balloon, foam sponge. Between elastic and malleable were styrofoam and pipe cleaner. Malleable materials were play dough and tin foil. Between malleable and rigid is ping pong ball (though nearer rigid than on the photo).

If students are given several types of balls, ask them to see how elastic or rigid they feel, then based on that, predict how high they will bounce. The more elastic balls will generally bounce higher than more rigid balls. The elastic material stores energy as it is deformed, which is released again as it returns to its original shape, giving the ball energy to go higher.

At a molecular level, some of these properties can be explained.
Elastic materials are often made of long molecules that can bend and stretch, then return to their original shape e.g rubber. Elasticity may also be due to air pockets in the material that can be compressed e.g. foam.
Rigid materials will be made up of tightly-bonded molecules that are hard to move past each other. Sometimes rigid materials are brittle, so that with enough stress they break apart suddenly as the bonds are
Malleable silly putty has many weak bonds between long molecules that can be broken, then remade in new positions.

Make a slide with a drop of 2% milk, and place under the microscope.
Wait until the milk has stopped streaming around the slide, and the individual fat droplets in the milk stay within the field of view.
Look closely for the fat droplets jiggling around - caused by water molecules bumping into them!
Brownian motion was the first convincing evidence that atoms and molecules existed.

(Milk is an emulsion, a type of colloid - tiny fat droplets held in the watery parts of milk. The fat droplets are all small enough to stay in the milk, and not settle out as a separate layer.)

This activity adapted from pdf: https://www.acs.org/content/dam/acsorg/education/resources/k-8/science-…

Give students tubs of glue making materials, scoops, water bottles, trays, stir sticks.
Ask them to make the best glue possible, mixing the ingredients in a well of the tray. They should test their glue by putting a blob of it on a cardboard strip, pushing a marble into it, then hanging the strip upside down - the longer the marble holds, the better the glue. Students should count seconds to see how long their marble sticks (one mississippi, two mississippi...)
Distribute worksheets/use board space for students to record the recipes they make, so that they can keep track of their own changes, try others' recipes and start to quantitate the data.
A long-lasting recipe can be taped upside down over the edge of the table so other recipes can be tried while it is hanging - note the time that it was taped along with the seconds counted up to that point. When the marble drops, calculate the seconds elapsed. At the final discussion calculate the stick time for those still sticking - do the math together to calculate the number of seconds from the number of minutes.
Discussion around the data can include how to more accurately measure out the quantities (e.g. use standard teaspoon measures) and to time the sticking-times (use a stop watch), for fair comparison.
Students may well discover that their mixtures work better once they have dried out a bit, for good discussion on how commercial glues often require a drying time until they are full strength.

Acting out the glue molecules:
Flour-and-water glue forms in the same way that dough forms: gluten proteins and starch molecules of the flour are bonded together by water molecules.
Divide the students into two groups, A and B. Each group will make a glue molecule.
Four students in group A link hands in a line to make a long starch molecule. Then other groups of four students make starch molecules, to make a total of two starch molecules in group A and two starch molecules in group B (using a total of 16 of the students in the class).
The remainder of the students will be water molecules.
Ask the two starch molecules in each group (A and B) to line up facing each other, with a space between them.
Then ask the water molecules in each group stand between the starch molecules, and reach out their arms to touch both starch molecules, forming 'bonds' between the starch molecules. As more bonds form, the starch molecules are more fixed in place. In the same way, the glue the students made got thick as water was added to the flour.
Optional: race the As and the Bs. Which starch molecules in flour can combine with water to make a glue the fastest? First make your starch chains, then link them with water molecules.

Discuss how these and other glues work:
The long molecules in some of the ingredients (starch molecules in flour and cornstarch, and protein molecules (gluten in flour and casein in milk powder)) reach into the tiny cracks in the cardboard and hold onto it, like fingers reaching into cracks in a wall. Starch also forms chemical bonds with the material it is touching.
The smaller sugar molecules of the icing sugar is not so good at making a glue, unless it is made really thick.
Glues we buy in a tube have these and other mechanisms that make them work - sometimes long tangled molecules dry out, sometimes a chemical reaction is most important, sometimes surfaces are welded together.

Mechanisms of all types of glues:
The molecules of a glue need to be good at sticking to each other and to the material(s) it is glueing together. There are several molecular processes at work.
Adsorption - the glue and the material have charged molecules that attract each other. It is a weak attraction, but with many of these bonds they can hold the glue and surfaces of the material together.
Mechanical - the long molecules of the glue creep into the tiny holes in the surface of the material(s) and hold them together.
Diffusion theory. The adhesive can diffuse into the surface and vice-versa, with molecules swapping over at the join and mingling together.
Chemisorption - there is a chemical reaction between the glue and the material. (not the mechanism for the glues made in this activity)
From www.explainthatstuff.com/adhesives.html

This activity is from the Children's Museum of Houston: https://www.cmhouston.org/classroom-curriculum/stretch-your-potential-2

Fold the elastic band flat.
Tape one end of the flattened elastic band to the end of the skewer, making the tape really tight around the skewer, so that the elastic band is held on firmly.
Slide the straw onto the other end of the skewer.
Lay the free end of the flattened elastic band over the straw and tape tightly to secure the elastic band to the straw, but without taping the straw to the skewer.

Hold the jumper upright on the desk and pull the straw down to the desk so that the elastic band is stretched.
Make sure your head is back (not over the jumping stick).
Let go of the device in one go, by separating your fingers wide quickly.
It will jump in the air by a metre or more.

Students need to be instructed on using it safely - do not jump it towards anyone.

Students enjoy games with their jumping sticks (see photos) after discussion of the science.

For energy transformation discussion:
Ask students to think about the chain of energy transfer that happens as the toy is used.
Chemical energy - from our hands as they pull down on the toy
Motion energy - as the toy is pulled down
Elastic energy - as the elastic band is stretched
Sound energy as the device is released and the straw hits the top of the stick
Motion energy - as the elastic band pushes the toy upwards
Gravitational energy - as the toy is up in the air
Motion energy - as the toy falls back down

For chains of forces discussion:
Draw on the board the toy with step-by-step forces.
Our fingers hold the toy (with the force of friction) - a push
We push the toy down on the desk with a downwards force
The elastic band stretches - a pull - as we push down
When we let go the elastic band pulls
The straw moves upwards and pushes against the top of the toy
This pushes the whole toy into the air
Gravity eventually pulls the toy back down

Hand out stacks of paper to each student.
Ask them to build a tower as high as they can with their paper. They can fold and tear the paper as they wish, but get no more materials.
Encourage them to exchange ideas (as adult engineers do the same).
Point out the different styles of towers, and the various ways that students reinforce their structure to make it stable e.g. rolling sheets to make strong columns, creating stability by leaning sheets against each other etc.

Before the activity, securely tie the end of each string to the elastic band, to make a ring of strings extending outwards.

Place the cups downwards on the floor and spaced apart.
Hand each student the end of each string, and ask them to space out so that they can open and close the elastic band by collaboratively pulling on their strings.
Ask them to use the elastic band and string contraption to arrange the cups into a tower with a two-cup base with one on top.
They may need to be reminded to stay at the end of their strings.

Students need to work closely together, to discuss who should pull or loosen their string, in order to pick up each cup and place it. Placing the top cup will be hardest and they may try several times. They may drop a cup on its side, and will need to figure out how to right it together.

Encourage respectful teamwork, and that no idea is a bad idea.

Gather 30m or further from a flat wall, and demonstrate by a single loud clap, or one bang of the wooden blocks together, that there is an echo. The sound of the clap/blocks, reaches the wall and bounces back to us, so that there is a delay between the initial sound made and the sound heard after it bounces off the wall.
If you are in the middle of a gravel field with other buildings around, you may hear other echoes.

Discuss with students how we can hear the echo(s) as the sound returns from the building walls, and can maybe sense its direction, we cannot discern much more. Animals that echolocate can discern the shape of the objects which echo sounds, by the shapes of the sound waves that their sensitive ears receive.

Discussion on echolocation:
Some animals (e.g. bat) are able to use the echo to measure how far away prey is, as well as the size and shape of objects to navigate in a dark cave.
Some blind people use echolocation, by clicking their tongues and listening for the echo back to find out where objects are (just like bats or marine animals). https://www.youtube.com/watch?v=WHYCs8xtzUI
Some blind people can so precisely tell where objects are using echolocation that they can use this method for biking!
https://www.youtube.com/watch?v=RGZaaYj6qzA
Experts in blind echolocation can even listen to a recording of tongue clicks echoing, and state what objects were there when the recording was made!

To measure the speed of sound:
By timing how long the delay is and measuring how far away the wall is, we can measure how fast sound travels.
This works best with a longer distance of 40 or 50m from the wall.

Show students how long a metre is using the tape measure, then ask them to see how long their stride must be to measure a metre. Once their stride is calibrated to a meter, ask them to pace out the number of strides (or metres) to the wall.
Write down this number [52m and 58m for two of our students]. Double the number to find out how far the sound must travel to the wall and back [110m for us].

Meanwhile, another student needs to bang the wood blocks together so that the echo from the first bang coincides with the echo from the second bang. Ask them to keep banging the wood at this rate, so each bang coincides with the echo from the previous bang. The time between the bangs is the time it takes the sound to travel to the wall and back.
To be somewhat accurate in how long it takes the sound to travel the distance, ask the student to keep banging the blocks together at the same rate, while another students times 10 bangs of the blocks. [Our students measured 3.42 seconds for 10 bangs.]
Divide this number by 10 to find the time for one bang i.e. the time for sound to travel to the wall and back [0.342 seconds for us].

Now do some math: if the sound travels x metres to the wall and back, and takes y seconds, the sound is travelling at x/y metres in one second - this is the calculated speed of sound. [We calculated 110/0.342 = 321.6 metres per second, approximated to 320m/sec.]

The actual speed of sound in air is 343m/sec, so this method is not bad for calculating the speed of sound.

Make a hole in the centre of two cup bottoms.
Push one end of the string through a hole in a cup, then tie a knot on the inside of the cup so that the string is secure. Repeat with the other end of the string and the other cup.
Pairs of students each hold a cup, and move apart until the string is taught. They can then communicate by talking into one cup and putting the other over an ear. The sound is pretty decent, as long as the string is taught.

Discuss how the sound is transmitted: the cup catches the sound and transmits it to the string. Sound is a vibration of molecules, so the vibration of the air molecules in the cup causes molecules in the string to vibrate. The vibrations transmit along the string to the other cup, where they cause vibrations in the air of the second cup. This moving air reaches the ear, where the vibrations are transmitted into the ear, where they are converted to electrical signals that are passed to the brain along nerves.

Optional: do not tell the students that the string needs to be taught and allow them to investigate how the telephone works best, with slack or taught string.
Optional: experiment with different string diameters and different cup sizes and different string lengths.
Optional: play the telephone game, by making a circle of students and string telephones between them: one student passes a message down a string telephone to the next student, who uses the next telephone to pass the message they hear on to the next student, continue until all the students have received and passed on the message, until the last student tells the group what they heard (similar to the game where the message is whispered between participants). See how garbled a message becomes from the first student to the last - discuss why: the string telephones are not perfect at transmitting sound, and so some words are hard to make out.