ingridscience

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 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) are able to reach into the tiny cracks in the cardboard and hold onto it, like fingers reaching into cracks in a wall. (The smaller sugar molecules of the icing sugar is not so good at making a glue, unless it is made really thick). This mechanical mechanism is just one of the ways that real glues work.

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.

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, 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.

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.

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.

Discuss what sound is: molecules vibrating back and forth, which bump the next door molecules, passing the vibration through the air (or liquid or solid). When those molecule vibrations reach your ear, they cause your eardrum to vibrate, which transmits all the way to your inner ear, where tiny hairs move. The moving hairs initiate an electrical signal which gets sent via nerves to the auditory section of your brain. Only then do you perceive the sound.

How low or high a note is depends on the frequency (or speed) of the vibrations. Faster vibrations sound higher, and slower vibrations sound lower. The rate of the vibrations are measured in Hertz, or Hz.

Tell students that different people can hear different frequencies - some can hear higher than others, and some can hear lower. This activity collects data on the frequencies that students and adults can hear.
Demonstrate what kind of sound they will be hearing, by turning on the tone generator and the connected speaker, and running up and down the frequencies.

Start data collection. Start the frequency generator at the highest, slowly lower the frequency, and ask students to put their hand up when they start to hear the tone. If at any point, it is hurting students' ears, lower the volume (it may hurt students while adults cannot hear it at all).
When students raise their hands, write down the frequency you are at, and ask students to do the same (see photos). Keep lowering the frequency, and keep writing frequencies down, until all present in the class can hear the tone. (It is good to start high, so that students are less likely to feel inadequate at not hearing the tone - adults will likely be the last to hear.)

Then collect data on the lowest frequency humans can hear. Continue to lower the frequency and ask students to raise their hands when they no longer hear it. Note that the tone may get very quiet, so be sure to raise the volume on the phone and speaker, and if necessary, move the speaker around the classroom sot that students can put their ear to it. Record the lowest frequencies perceived on the board. (Note that the limits of the speaker may define the lowest note that can be played.)

Discuss the results. There is a range of frequencies that students can hear, due to variability in their ear physiology. Show students a diagram of the inside of the ear and the path that the sound takes, to show the complexity of sound perception. The ear drum is vibrated by sound in the air, which transmits these vibrations through the bones of the middle ear to the liquid in the cochlea canal of the inner ear. When the hairs in the cochlea vibrate, they generate an electrical signal that neurons transmit to the brain - only then do we perceive the sound. Adults are usually unable to hear the higher frequencies that students can hear, as during aging humans lose the inner ear hair cells that are sensitive to high frequencies.

Compare to other animals. Show students a diagram of animals’ hearing range compared to humans. Humans are able to hear frequencies ranging from 19,000Hz (19KHz) down to 30Hz. Bats can hear higher frequencies than humans (called ultrasound), up to 115KHz, and use these frequencies for echolocation. Elephants can hear lower frequencies than humans (infrasound), down to 17Hz.
Try these webpages with animal frequency ranges: https://upload.wikimedia.org/wikipedia/commons/thumb/5/5d/Animal_hearin… http://www.philtulga.com/AnimalHz.gif http://elephant.elehost.com/About_Elephants/Senses/Hearing/infrasounddi…