# Understanding Serial Dilutions and Concentration

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Understanding Serial Dilutions and Concentration
Activity 40 Follow-up Understanding Serial Dilutions and Concentration

One drop of red dye solution
One drop of water

Activity 40 Analysis Which is more dilute, Cup 1 or Cup 2? How do you know this? Cup 2 The color in cup 2 is lighter We know we mixed one part of the original solution to nine parts of water If Cup 1 has a concentration of one part in 10, and Cup 2 has 1/10 the concentration Cup 1, what is the concentration in Cup 2? One part per 100

Which cup has a concentration of one part of food coloring per one million parts of solution?
What is the number of the cup in which the solution first appeared colorless? What is the concentration of the solution in this cup? Hint: Express the answer for concentration as one part per ___. Usually it’s Cup 5, 6, or 7 (varies with techniques and ability to see color Cup 5 is 1 part per 100,000 Cup 6 is 1 part per 1,000,000 or 1ppm

Do you think there is any food coloring in Cup 8, even though it appears colorless? Explain.
Was there a direct observation (evidence) to support your conclusion) You might infer that there must be some food coloring present since it was in the original solution that became colorless with dilution An inference is a conclusion that is not based on direct observations Look at the tray with evaporated solutions You might conclude that particles are present in Cups 8 and 9, but there are not enough to observe directly

Imagine that the food coloring is a toxic substance
Imagine that the food coloring is a toxic substance. Do you think that diluting the substance with a lot of water will make it safe? A more dilute solution may not be harmful It will not be completely gone and may still cause problems In Activity 34, “Water Pollution”, you read some substances, such as bacterial contaminants, are harmful in very small amounts

The Willow Grove Story:
Activity 41 Willow Grove’s well water, Fenton River and Willow Lake were tested to make sure they met federal water-quality standards. The results showed Willow Lake water was turbid (cloudy due to presence of impurities), Fenton River water was slightly acidic, and the well water contained nitrates. Test for iron used potassium thiocyanate and hydrochloric acid---a reddish orange color indicated the presence of iron (test can detect 500 ppm or higher) Test for nitrates used hydrochloric acid and a powdered nitrate indicator---purple = nitrates Test for copper using ammonia---light blue = copper levels over 2 ppm

Results indicated: Willow Grove needs to treat the groundwater to remove nitrates---50% of the residents drink well water The town should figure out what is causing the acid in the river to prevent levels from rising The town should figure out what is making the lake turbid, but it is still safe since it is treated by the water district

Activity 42 Water Purification
Two treatment methods used by most water districts to remove contaminants before piping water to homes are coagulation and filtration. Coagulation involves using chemicals to attract contaminants into large clumps. The clumps settle to the bottom of the water tank. Filtration traps and separates solid contaminants from the water by making the water flow through filters. Alum was added to dirty water to coagulate Filtration tubes were filled with gravel, sand, and charcoal

Cleaning the Water at a Water-Treatment Plant

Title: Precipitating Specific Contaminants
Activity 45 Title: Precipitating Specific Contaminants

Read C-78 Problem: How can you use a chemical reaction to remove a contaminant from a solution? Hypothesis/Initial Thoughts:

Chemical Reaction…… What terms or ideas relate to this
Color change (of a solution) Energy change—temperature change, light, or generation of electricity Odor change Disappearance of an original substance (reactant consumption) Formation of bubbles (gaseous product) Formation of a precipitate (solid product) Change in the results of a chemical test (such as a pH change detected with an indicator)

NOT EVIDENCE Dissolving is of a chemical change. It is a physical change The solute can be reclaimed by a physical means of separation: evaporation of the solvent In this activity, dissolved copper chloride will represent a wastewater contaminant Dissolved sodium carbonate is the solution added to enable the contaminant to be precipitated out and removed CuCl2 (aq) + Na2CO3 (aq) CuCO3 (s) + NaCl (aq) Copper chloride + sodium carbonate copper carbonate + sodium chloride This is called precipitation which is different than precipitation that forms when water vapor condenses in clouds and falls to the earth

Data/Observation: Table: Precipitation Reaction
Make comparisons in your descriptions like, “lighter than or less than test 1” One pair do tests 1 and 2 Make observations before and after you mix them One pair do tests 3 and 4

Goggles on!

Decide which pair will do tests 1 and 2 and which will do tests 3 and 4
Follow the instructions on your table to determine the number of drops of water and sodium carbonate you add to Cups B and C A B C D E Test 1 or 3 Test 2 or 4

Write down what you see before you stir the mixture and after you stir the mixture. Be descriptive! Examples: blue, bluish-green, fluffy solid, cloudy, thick, darker blue than…, clear, pasty, fizzing, chunks, precipitate

Fold two filter papers, moisten them, and place them into two filter funnel openings.

Place the filter funnels onto your SEPUP tray so that the openings are over Cups D and E
B C D E

Using the dropper, transfer as much as you can of the material from Cup B to the filter in Cup D. Rinse the dropper and transfer Cup C material to E. A B C D E

Once the mixtures have filtered, observe the appearance and quality of the filter paper. Then put the filters in a paper towel and put them in the trash. Observe the appearance and quantity of the filtrate. Finish filling in the data table on your student sheet.

How could you test the filtrate to see if there is any copper chloride in it?
Add 1 one drop of sodium carbonate and observe any change. Continue to add a drop at a time pausing for observation until you reach 10.

Clean-up Everyone wash hands with soap and water.