Skills Practice Labobservation

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Skills Practice LabOBSERVATION

Separating Substances in a Mixture

Introduction

A mixture is a combination of one or more pure substances. A heterogeneous mixture is a mixture in which the substances are not uniformly mixed. Because the substances in a mixture may have different physical properties, these properties can be used to separate the substances from the mixture. In this experiment, you will examine several substances to determine their physical properties. Then you will develop your own methods for extracting these substances from a mixture, and carry out an experiment to test those methods.

Objectives

Identify key physical properties that could be used to extract a substance from a mixture.

Develop methods for extracting substances from a mixture.

Evaluate the effectiveness of your methods, and recommend improvements.

Materials

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• beaker

•bowl with mixture of substances

•craft stick

•filter screens (2)

• filter paper

•graduated cylinder

•hot plate

•iron filings

•magnet, strong

•rocks

•oil

•pepper, cracked

• pipette

•plastic-foam cups (6)

•sand

•sieve or colander, small

•spoon, plastic

•sugar

•towel, small

•water

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Safety

Procedure

1.Learning the physical properties of a substance can help you decide how to separate it from other substances. Observe the physical properties of each of the following substances: cracked pepper, iron filings, rocks, sand, sugar, oil, and water.

Caution: Do not taste or eat any of these substances.

Separating Substances in a Mixture continued

2.For each substance, answer the following questions in the Physical properties column of the chart on the next page: Does this substance dissolve in water? Does it float? Is it magnetic? Are its pieces large or small relative to the pieces of other substances? Is it a solid, a liquid, or a gas?

3.Look over the properties you recorded in step 2. For each substance, determine which characteristic would help you best distinguish it from the other substances. Choose from among the following properties: size, shape, density, state of matter, solubility, boiling point, and magnetic attraction. Record a distinguishing characteristic on the chart for each substance.

Data Table: Separation of Substances

Substance / Physical properties / Distinguishing characteristic / Method of separation
Pepper
Nuts
Sand
Iron filings
Water
Sugar
Oil

Separating Substances in a Mixture continued

4.You have been given a bowl containing a mixture of all the substances you have observed: pepper, iron filings, rocks, sand, sugar, oil, and water. Formulate hypotheses about how you could separate each substance from the rest of the mixture using the materials provided for you in the lab. It may help to look over the materials list. If you are working in a group, discuss your proposed methods with the others in the group and decide together on a method for each substance. Describe your proposed methods in the “Method of separation” column in the table.

5.Have your teacher approve the methods you have described, then conduct an experiment to test your methods.

Analysis

1.Organizing dataList the substances in the mixture in order from the simplest extracted to the most difficult extracted.

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2.ClassifyingWas this mixture a homogeneous mixture or a heterogeneous mixture? Was it a suspension, a colloid, or a solution? Justify your choice.

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Conclusions

1.Evaluating resultsDo you think that your components that have been separated are pure? Explain.

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2.Evaluating methodsHow could you improve your methods to better separate each substance?

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Separating Substances in a Mixture continued

The Centrifuge

Read the following paragraphs, and complete the exercises below.

Suspensions settle out

If you add sand to a container of water and shake it up, you create a suspension. In a suspension, relatively coarse solids are suspended in a liquid. The sand settles to the bottom of the container. Why?

The particles of sand are much more dense than the particles of water. Because of this density, gravity exerts a much stronger force on the sand particles than on an equal volume of water. As a result, the sand displaces the water below it. Eventually, all of the sand sinks to the bottom.

Colloids are more difficult to separate

Unlike the particles in a suspension, the particles in a colloid, such as blood, do not settle out naturally. The reason is that the difference in the force of gravity on different particles is not enough to pull the small particles in a colloid down through the liquid.

A centrifuge is a machine that uses a strong force to separate out the particles in colloids. Such a method is often used to separate cells and platelets from the yellow liquid plasma of blood. A centrifuge spins the container holding the colloid around an axis. The part of the container closest to the axis changes direction more quickly than the other end of the container. As a result of inertia, the denser particles are pulled to the other end of the container.

Extensions

1.______Research and communicationsA centrifuge is a device that uses centrifugal force to separate substances of different masses. Centrifuges are often used by biologists and biochemists to separate parts of cells or biological molecules of different masses. Research how centrifuges work, and write a short paragraph explaining what you learn.

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2.If a suspension of sand and water were at zero gravity, would it separate? Explain your answer.

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