Lab ___: 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 mixtureWatch glassTwo 100-mL or 150 mL beakersRing (Retort) standStirring rod2 Ring clampsBunsen Burner

2 Wire gauzesBalanceMatchesDistilled waterBeaker tongsWeigh 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 ___: The Composition Of A Mixture

Name: Group: Minutes: Grade:

Objective:__________________________________________________________________________________

Data Table #1 Teacher Check:

What is the precision of the balance?

________________________________

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 MixtureFormula Numerical Set-Up and Original Answer Answer rounded to correct

Significant Figures

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Description Measurement

Mass of beaker (g)

Mass of beaker and mixture (g)

Mass of beaker and solids left after heating (g)

% sand =Mass of solids (#2) x 100 Mass of Mixture (#1)

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

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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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