# 4 September Science 8 Evidence for the existence of atoms? Dissolving, saturation, density, crystallization, chemical reactions, Brownian motion

Look at this picture of salt crystals that have come out of solution (that have “precipitated”). What do you notice?

Look at the results of your set up to evaporate the water from the saturated salt solution. Also look at the dishes which included other substances. What do you notice?

The intention was to keep track of the salt added to 100 ml of water until the solution was saturated. That is, until no more salt would dissolve. Then a sample of the saturated solution was used to determine the density. Make two histograms to help you analyze the results. One histogram looks at the amount of salt (in grams) added to 100 ml of water. The other examines the density of a saturated salt solution. Follow graphing guidelines. Remember that you will need to choose intervals or bins for the values. You might want to try several to see which sized interval/bin gives the best picture of the distribution of results. Compare the class results to standard values (See the last paragraph of this reference).

https://www.sigmaaldrich.com/content/dam/sigma-aldrich/docs/Sigma/Datasheet/6/71376dat.pdf

How would you interpret the class results?

Read this chapter on Water with your partner–help each other with new words, new ideas, etc. Discuss the diagrams and what you think they show. Collect and clarify your questions.

Start keeping the following table of observations, evidence, argument, and explanation.

Consider these big questions (KEEP IN VIEW-KIV QUESTIONS AND IDEAS):

• What can we observe about the structure and behavior of matter?
• How can we explain what we observe (at the scale of our senses) by structures, entities, and behaviors at a scale we cannot see?
• How can such explanations be tested?
###### THEORY

With your partner, get a microscope, slide, and small piece of copper. Place the copper on the slide and bring it into focus on the lowest power. Try to take a picture with your ipad. When you are ready, we will put a drop of silver nitrate solution on the copper piece on the slide. Continue to observe carefully. Take pictures as you can. What do you see happening.

Here is a depiction of the reaction. S means solid. Aq means aqueous–in water–a solution:

copper + silver nitrate → copper(II) nitrate + silver
Cu(s) + 2AgNO3(aq) → Cu(NO3)2(aq) + 2Ag(s)

The numbers give the ratio of atoms that combine. Notice that the same number and kind of atoms appear on each side of the reaction. This is why this expression is called an equation.

Try a similar observation with a piece of zinc. Here is the reaction equation:

Zn(s) + 2AgNO3(aq)Zn(NO3 )2 (aq) + 2Ag(s)

If time, we will try to observe Brownian motion. If no time, next class.

We will examine Brownian motion of fat globules in milk. See what you can find out about Brownian motion. The technique will be described in class.

See the videos showing Brownian motion of fat globules in milk. Apologies for the camera motion. The videos would have been better with a fixed camera. Nevertheless, it is possible to see the random movement of the fat globules. How is this movement interpreted in light of the atomic/molecular theory of matter? (Note: a microscopic fat globule is composed of a truly enormous number of too-small-to-see molecules–according to the theory.)

Video 2