chemistry lab help

Lab 2: Separation of Mixtures

Note: On the pre-lab video, I’m pointing to the last spot on the plate when showing how the Rf

value is calculated. I should have been pointing to the 2 nd

spot.

Lab Materials:

Materials for a 4 component mixture—need prior approval before making and separating the mixture

Will need additional supplies based on your separation scheme

Food colors (yellow, green, blue in a 5:1 dilution, i.e. 5 drops of water to 1 drop of food coloring)

Scissors

Coffee filters

Large and small Plastic cups

Measuring cup (1 cup)

Table salt

Tape

Paper towel

A 0.1% NaCl solution (Mix 1 tsp of salt with 2 cups of water. You will not need all of this solution.)

Toothpicks

Pencil

Ruler

Separation of Mixtures

Name: _____________________________ Date: _________

Introduction

Matter as we have learned can be classified either by its physical state, i.e. solid, liquid, or gas, or

by its composition. If we classify matter according to its composition, we can categorize any

material as a pure substance or a mixture. Mixtures contain two or more types of atoms or

molecules in variable proportions and can be either homogeneous or heterogeneous in nature.

Homogeneous mixtures have a uniform composition throughout where heterogeneous mixtures

have regions with different compositions. In this lab, you will first practice classifying matter.

This will then be followed by two exercises. The first involves designing an experiment to

separate a 4 component mixture. To complete this, you should learn several methods in which

mixtures can be separated. The second exercise involves the separation of food dyes using a

technique called chromatography.

Chromatography literally means color separation and relies on the difference in relative affinities

of the compounds to be separated for a liquid solvent and a solid material. In this lab, you will

place a small amount of each food coloring sample on a strip of paper and then allow the solution

to “wick” through the paper carrying the dye with it. Each pigment in the dyes will have a

different preference for the solution vs. the paper. Consequently, each pigment will travel a

different distance up the paper during the procedure. While paper chromatography is not

particularly useful for separating substantial quantities of materials, it is ideal for identifying the

different components of a mixture. 1

1. Directions: For the examples below, identify each item listed as an element, compound, or a

mixture. If the item is a mixture, specify whether it is homogeneous or

heterogeneous.

a. Platinum ____________________

b. “Clean” Air ____________________

c. Sugar ____________________

d. Chicken noodle soup ____________________

e. Pop ____________________

f. Oxygen ____________________

g. Seawater ____________________

h. Italian salad dressing ____________________

i. Salt ____________________

j. Iron ____________________

k. Banana split ____________________

l. Gasoline ____________________

m. Carbon ____________________

n. Methane ____________________

o. Water ____________________

2. Separating a 4 component mixture

You will need to make and then separate a 4 component mixture using items found in your kitchen.

Do not mix any chemicals until you have submitted your mixture and separation scheme to the

instructor and received approval from the instructor. Please remember your procedure should be

thorough enough so another person could easily follow your instructions and obtain each of the

components. You should also be aware that receiving approval from your instructor does not

mean the procedure will work. Multiple tries may be necessary…remember our discussion of the

scientific method. If you have not received approval from your instructor by the deadline

stated in the syllabus, you may NOT complete this question.

a. Describe below your procedure for separating the 4 component mixture.

b. Assuming the masses below, what is the percent of salt in the original mixture?

Mass of salt = 1.29 g Initial mass of mixture = 6.83 g

Mass of Cheerios TM

= 0.75 g Mass of iron fillings = 0.23 g

Mass of rice = 2.91 g

c. What is the student’s percent recovery?

3. Paper Chromatography 1 (Make sure to watch the pre-lab video to help with this section.)

Preparing the Sample:

Take a coffee filter and cut it into a long rectangular strip. Draw a pencil line ~0.5 inches from one

end. Place a small drop (preferably this small → ☻) of each color dye evenly spaced on the pencil

line using a toothpick. Do NOT spot near the edge of the filter paper. To help keep the spot small,

gently blow some air on the paper as you try spotting it with the food coloring. You might find

that you need to spot the filter paper multiple times to get enough of the food coloring on the paper.

Developing the Sample:

Tape the top part of the paper (the end opposite of the samples) to a pencil, so the end of the paper

almost touches the bottom of a large plastic cup. Pour some salt water into the cup, but make sure

the solution in the cup does NOT go above where you spotted your samples. The liquid should be

immediately drawn up the paper carrying the colored spots with it. Allow the liquid to rise as far

up the paper as possible, and then remove the paper strip. Place the coffee paper on a paper towel

and mark where the liquid stopped. This mark will be used in determining the solvent front. When

the paper has dried sufficiently, mark the center of each colored streak with the pencil. For each

lane (the track made by an individual spot), measure the distance from the pencil line to the center

of each spot. Record this length in the table below. Next, measure the distance from the pencil line

to the solvent front. Enter the values in the data table. Note that you will not have a streak of

every color for each food dye.

a.

b. Convert the distances for each streak that you observe into a retention factor (Rf) by dividing each streak distance by the solvent front distance for that lane. For example, if the blue streak

from the blue spot travels 4.2 cm and the solvent front in that lane went to 6.6 cm: 4.2 cm ÷ 6.6

cm = 0.64 (no units). Please show your calculation in the table below.

c. What do you notice about the different streaks observed and their Rf values? Do you think that the food dyes have anything in common?

References: 1 Written by Dr. George Whitwell, Associate Professor of Chemistry at NCWC

Food

color

Yellow streak

distance

Green streak

distance

Blue streak

distance

Solvent front

Yellow

Green

Blue

Food

color

Yellow streak

retention factor

(Rf)

Green streak

retention factor

(Rf)

Blue streak

retention factor

(Rf)

Yellow

Green

Blue