1. Developing
Observation
Skills
Textbook pg 15
Observing and Inferring A chemist's ability to make careful and accurate observations is developed early.
The observations often are used to make inferences. An inference is an explanation or interpretation of
observations.
Problem Oil, the fat in milk, and grease belong to a class of chemicals called lipids. What can you infer
about the addition of detergent to milk?
Materials
petri dish (1),
large graduated dropper (graduated cylinder)
whole milk, water,
vegetable oil in plastic cup
four different food colorings,
toothpick (1),
dishwashing detergent
beaker
shot glass
Procedure
1
Obtain materials.
2.
Add water to the cover of a petri dish to a height of 0.5 cm (roughly 20 mL). Add 1 mL of vegetable oil.
3. Dip the end of a toothpick in liquid dishwashing detergent.
4. Touch the tip of the toothpick to the water at the center of the petri dish. Record your detailed
observations.
5. Add whole milk to a second petri dish to a height of 0.5 cm (roughly 20 mL)
6. Place one drop each of four different food colorings in four different locations on the surface of the
milk, as shown in the photo. Do not put a drop of food coloring in the center.
7. Repeat steps 3 and 4.
Analysis
1. What did you observe in step 4 (water and vegetable oil + soap)?
2. What did you observe in step 7 (milk +soap)?
3. Do you think the detergent is reacting with the oil or with the water?
4. Identify ALL variables (Independent, dependent, control and constants (3 minimum)
5. Oil, the fat in milk, and grease belong to a class of chemicals called lipids. What can you infer
about the addition of detergent to dishwater?
6. How would the results of this experiment be useful in real life situations? Think of the oil spill in the
Gulf of Mexico.