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Chapter 1•Introduction to ChemistrySMALL-SCALE EXPERIMENT

MAKING OBSERVATIONS OF MATTER

Small-Scale Experiment for text Section 1.3

OBJECTIVES

  • Distinguish between the macroscopic and the microscopic worlds of matter.
  • Observe changes in matter and interpret the observations.

INTRODUCTION

Here’s an experiment you can do at home. You’ll need a page of a newspaper, a handful of pencils, some color crayons, and a hand lens. Place all the objects on a table, mix them up, and you’re ready to go! Your first task is to identify each object as a piece of paper, a pencil, or a crayon. “Wait a minute,” you say. “That’s easy!” Yes, it is easy because you can see the objects and identify each according to characteristics such as size or shape.

Now make a few scribbles onto the newspaper with a crayon and then with a pencil. Examine your work with a hand lens. What do you see? Perhaps you noticed that with an unaided eye the lines appear to be smooth. But under the hand lens, you can see that the lines are not smooth. You might have also noticed the fibers in the paper which aren’t noticeable without a hand lens.

Objects such as crayons and paper, which are large enough to see with the unaided eye, belong to the macroscopic world. Within every object that you can see are objects that are too small to see with the unaided eye. These objects belong to the microscopic world. Depending on their size, they can be seen with a hand lens, a light microscope, or an electron microscope. You will learn about electron microscopes when you study the structure of atoms in Chapter 4. You will also learn that there are particles within atoms that are submicroscopic, meaning that they cannot be seen with any microscope.

In chemistry, you will make observations of the macroscopic world and learn to explain your observations by describing what is happening at the microscopic and submicroscopic levels within matter. Making observations is a necessary step in any scientific method. The observations can help to answer existing questions or can lead to new questions. It is important to record your observations so that you can refer to them as you analyze the results of an experiment.

When a chemist describes an experiment, he or she includes a purpose, a description of the procedure, the results of the experiment, an analysis of the results, and any conclusions. For many experiments in this small-scale laboratory manual, a purpose and a procedure will be provided. You will need to record your observations, analyze the results, and draw some conclusions. In later experiments, you will have a chance to design your own procedures.

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PURPOSE

In this experiment, you will make observations and you will propose explanations foryour observations. You will attempt to explain your observations of the macroscopicworld in terms of changes that are occurring at the microscopic level. You will tear anewspaper and examine the torn edges under a hand lens. You will compare the behaviorof water and mineral oil on a plastic surface. You will observe the shape of rock saltcrystals and test the ability of rock salt to conduct an electric current when dry and whenwet. Finally, you will produce iodine and compare its behavior in water and in mineral oil.

This early in the year, your explanations for what you observe cannot be as completeas they would be later in the year. You may have more questions than answers. What isimportant is beginning to think like a chemist.

SAFETY

  • Wear safety goggles, an apron, and gloves.
  • Use full small-scale pipets only for the controlled delivery of liquids.

MATERIALS
rock salt (sodium chloride)
Small-scale pipets containing the following:
water / potassium iodide (KI)
mineral oil / sodium hypochlorite (NaClO)
EQUIPMENT
hand lens / conductivity testing device
rounded river rock / small-scale reaction surface

EXPERIMENTAL PROCEDURE

1. Obtain a page of newspaper and tear it in half from top to bottom. Now take one half and tear it in half so the second tear is perpendicular (at a right angle) to the first tear. Examine the torn edges with a hand lens and note the differences in the two tears.

2. Observe a large piece of rock salt with a hand lens. Then use a rounded rock to gently break the large piece into a few smaller pieces. Note and record what you observe.

3. Test the dry rock salt for its ability to conduct an electric current. Add a drop of water to the salt crystals and test the mixture for its ability to conduct an electric current. Record your results.

4. Place one drop of water and one drop of mineral oil about 5 centimeters apart ona small-scale reaction surface. Observe and note any differences in the behaviorof the water and oil. Examine the drops with a hand lens. Test each liquid for itsability to conduct an electric current.

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5. On a small-scale reaction surface, add one drop of potassium iodide solution to one drop of sodium hypochlorite. Note and record the color of the mixture.

6. Place two drops of mineral oil on a small-scale reaction surface and add one drop of water to the mineral oil. Then add one drop of potassium iodide solution and one drop of sodium hypochlorite to the mixture. Allow the mixture to stand for a few minutes while you observe and record what happens.

EXPERIMENTAL DATA

Record your results in Table 1.1 or in a copy of the table in your notebook.

Table 1.1

Procedure / Observations
1. torn paper
2. shape of salt crystals
3. ability of rock salt toconduct a current
4. oil and water on plastic
5. potassium iodide andsodium hypochlorite
6. mineral oil, water,potassium iodide, andsodium hypochlorite

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CLEANING UP

Avoid contamination by cleaning up in a way that protects you and your environment. Carefully clean the small-scale reaction surface by absorbing the contents onto a paper towel, rinse the small-scale reaction surface with a damp paper towel, and dry it. Dispose of the paper towels in the waste bin.

QUESTIONS FOR ANALYSES

Use what you learned in this experiment to answer the following questions.

1. Are the torn edges of a newspaper different depending on the direction of the tear? Describe any difference. What did the hand lens show about the fibers in a newspaper? A hypothesis is a proposed explanation for an observation. Make a hypothesis that might explain the differences in the tears.

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2. Describe the general shape of the crystals you obtained when you broke the rock salt into smaller pieces. Draw a picture of one of the crystals. What might determine the shape of a solid crystal? (Hint: Think about the particles within the crystal that you cannot observe.)

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3. Contrast the ability of dry rock salt and a rock salt-and-water mixture to conduct an electric current. Hypothesize about what might happen to rock salt when it dissolves in water, which allows the mixture to conduct an electric current.

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4. To offer an explanation for the behavior of mineral oil and water in Step 4, you need to know that particles in matter can be attracted to one another and that the level of attraction can vary with the particles. (You will study these attractions in Chapters 7 and 8.) For now, based on their behavior, which particles have a greater attraction for one another—the particles in water or the particles in mineral oil? Explain your choice.

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5. What is the color of iodine in water? What color is iodine in mineral oil? Use your observations from Steps 5 and 6 of the Procedure to explain your answers.

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NOW IT’S YOUR TURN

Consider the observations and explanations you made in this experiment. Makea list of questions that you might have about matter and changes in matter basedon your observations and the explanations you proposed for your observations.You don’t have to provide the answers to your questions.

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