Lab 7: The Composition Of A Mixture
Introduction
Pure (true) substances are called elements and compounds. An element is the simplest form of matter that retains
unique properties. When elements react together to form compounds, the properties of the elements change because
they have undergone a chemical change. For example, sodium is a very reactive silver- colored metallic element and
chlorine is a yellowish-green poisonous and gaseous element. When these elements react together, they form the
compound sodium chloride, a white crystalline solid commonly known as table salt. Silicon is a silver-colored solid
metalloid and oxygen is a colorless gas. When silicon and oxygen are chemically bonded, they form silicon dioxide. This
compound forms the mineral quartz, which is one of the hardest substances in the Earth’s crust, as well as one of the
most abundant. The composition of compounds and elements is always the same throughout the sample, in other words
unique to that substance. In other words, it doesn’t matter where you find a quartz crystal on the planet; it will always
contain a 1:2 proportion of silicon to oxygen. In order to separate a compound into its individual elements, a chemical
change will be needed. This is a very difficult process, because it is relatively easy to form chemical bonds but rather
hard to break them. Atoms lose energy when bonds are formed, this is favored in nature. Just think about letting go of a
basketball…its natural tendency is to fall to the ground, not fly upwards. Breaking bonds requires the addition of energy.
This is why water’s hydrogen and oxygen atoms don’t fly apart and water stays water, even when heated past the boiling
point. Hydrogen and oxygen are more chemically stable together than they are apart. This is why atoms bond together in
the first place to form compounds.
Mixtures consist of two or more pure (true) substances, which are physically combined. The atoms or molecules are just
mixed together without them actually chemically bonding together. Mixtures can be divided into two basic types,
homogeneous and heterogeneous. In heterogeneous mixture the individual components are often still visible and are not
evenly distributed. A sample from one part of the mixture does not have the same composition as a sample from another
part of the mixture. Heterogeneous mixtures are chunky mixtures. Think about Italian dressing. You have oil, vinegar,
garlic, pepper and other spices that are all separated, and if you want to blend all of the flavors together, you need to
shake the bottle up to get them to mix more thoroughly. Heterogeneous mixtures like these can be separated by filtering
them because the visible particles are so large that they will be trapped by the filter paper. This similar to what happens
when a coffee filter is used when making coffee. The coffee-infused water contains particles that are so small that they
pass right through the microscopic holes in the filter paper, while the much larger coffee grounds stay behind.
In a homogenous mixture, the components are evenly distributed throughout the sample at the molecular level. A sample
from one part of the mixture will have the same exact composition as any other sample. Homogeneous mixtures made
from water and some other ingredient (called a solute) are called aqueous solutions. These do not need to be shaken
before using, as the size of the dissolved particles is so small that the natural, continuous motion of the liquid water
molecules keeps them evenly dispersed throughout the sample. The individual components are not visible, such as
when food coloring is added to water. Solutions cannot be filtered because the particles making up the mixture are so
small that they pass right through the filter paper.
In any mixture, the components or parts are only physically combined and NOT chemically combined. This means that
the components keep their own individual properties. These properties, such as the ability to dissolve or not to dissolve in
water (known as solubility), can be used to separate the components. Mixtures can be separated into pure substances
using physical changes. Each component will retain its unique properties before and after the separation.
In this lab, we will separate the components of a mixture using physical changes in order to determine the percent
composition of each component. We will exploit the differences in the physical properties of each component in the
mixture to accomplish this.
The percent composition of a mixture with components (parts) A and B is determined by the following relationship:
%A in mixture of A and B =
mass of component A x 100
mass of mixture A & B
In a two part mixture, such as A and B, the percent composition of component B in the mixture is readily obtained by
subtraction from 100.000%. In other words, if one component makes up 70.% of the mixture, the other component will
make up (100.000 – 70. =) 30.% of the mixture.
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Materials and Equipment:
Sample of sand-salt mixture
Watch glass
Two 100-mL or 150 mL beakers
Ring (Retort) stand
Stirring rod
2 Ring clamps
Bunsen Burner
2 Wire gauzes
Balance
Matches
Distilled water
Beaker tongs
Weigh boat
Procedure:
1) Get your goggles on and then obtain materials and
equipment. Sep up ring (retort) stand, ring clamps, 1
wire gauze and Bunsen burner as shown in figure 1
above. Place several scoops of the sand-salt mixture
in the weigh boat.
2) Measure and record the mass of a dry, clean beaker.
Record
all
your
group’s
measurements
and
calculations in data table #1.
3) Add a sample of the sand-salt mixture to the beaker.
Measure and record, the mass of the beaker and
mixture.
Determine the mass of the sand and salt
mixture by subtracting the mass of the empty beaker
and record.
4) Using your other beaker, measure about 30 mL of distilled water. Add this water to the beaker with the sand-salt
mixture and stir for 1 minute. Carefully decant (pour off) the water mixture LEAVING THE SOLIDS BEHIND IN
THE BEAKER. Repeat rinsing and decanting 2 more times.
5)
Using beaker tongs, place beaker with the solids on the ring clamp with the wire gauze and cover with the watch
glass. Gently heat to avoid splattering the solid while drying.
6) When the contents of the beaker with the solids are dry, remove beaker using beaker tongs and place on the 2 nd
wire gauze to cool for 5 minutes. Measure and record the mass of the beaker with the solids left.
7) Place sand in the waste beaker. (It can be reused.) Wash up equipment and return to designated area.
8) Calculate the percent salt and percent sand in your sample.
Report your calculations on the overhead at
assigned group number.
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Lab 7: The Composition Of A Mixture
Name:
Group:
Minutes:
Grade:
Objective:__________________________________________________________________________________
Data Table #1
Teacher Check:
Description
Measurement
Mass of beaker (g)
What is the precision of the balance?
Mass of beaker and mixture (g)
________________________________
Mass of beaker and solids left after heating (g)
Calculations: all calculations must include a numerical Set-up (number plugged in with units), original answer
and answer rounded to correct significant figures.
Calculation #1: Mass of Mixture Used:
Formula
Numerical Set-Up and Original Answer
Answer rounded to correct
Significant Figures
Mass of beaker & mixture
- mass of empty beaker
grams of mixture used
Calculation #2: Mass of Solids left:
Formula
Numerical Set-Up and Original Answer
Answer rounded to correct
Significant Figures
Mass of beaker & solids left
- mass of empty beaker
grams of mixture used
Teacher Check:
Calculation #3: Percent Sand in Mixture
Formula
Numerical Set-Up and Original Answer
Answer rounded to correct
Significant Figures
% sand =Mass of solids (#2) x 100
Mass of Mixture (#1)
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Calculation #4: Percent Salt in Mixture:
Formula
Numerical Set-Up and Original Answer
Answer rounded to correct
Significant Figures
Use as many places pass the
decimal for 100 as your answer for
calculation #3
% salt = 100.00% - % sand
Teacher Check:
Questions: Complete sentences required for questions #1-8, Must show work for #9. Do on loose-leaf and attach
to these papers.
1.
Describe in detail what you did to separate the sand and salt from each other in the original mixture.
2.
What difference in properties between the sand and salt allowed you to separate them like this?
3.
Which component of the mixture was present in the beaker after step 5 in the procedure?
4.
Why can you use a physical change to separate the sand and the table salt mixture?
5.
Why can you not use a physical change to break apart the sodium and the chlorine in the table salt,
sodium chloride?
6.
Describe another method you could use to separate sand from a mixture of salt and water in this lab.
7.
This particular mixture of sand and salt has been investigated repeatedly over the last few years. One
group has reported that the percent composition of sand in their sample of the mixture was 74.3 %.
Another group reported the percent sand to be 95.0 %. Based on this information and the results you
got, explain whether this mixture of sand and salt is homogeneous or heterogeneous.
8.
A student repeated the same experiment to separate table sugar (sucrose) from a mixture containing
sand and table sugar. Using the data below, calculate the percent table sugar in the sand and table sugar
mixture.
Description
Measurement
Mass of beaker (g)
44.256 g
Mass of beaker and mixture (g)
59.446 g
Mass of beaker and solid (sand) (g)
55.225 g
Remember to include formula, numerical Set-up (number plugged in with units), original answer and answer
rounded to correct significant figures for ALL calculations.
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