Chemistry/Identifying and Making Mixtures

IDENTIFYING MIXTURES VERSUS PURE SUBSTANCES AND MAKING MIXTURES

PURPOSE:The purpose of this experiment is to learn how to identify a substance as a pure substance or a mixture, in addition to learning how to make various types of mixtures.

MATERIAL:

Set of 11 test tubes, labeled A to K.

2 or 3 watch glasses

2 or 3 dropping pipets

2 or 3 small rubber stoppers

hot plate

Hot Hand™

SPECIAL NOTES — READ BEFORE PROCEEDING!

a) This experiment requires you to answer questions based on exactly what you see. If you make poor or incomplete observations, you will not be able to answer the questions correctly.

b) A proper observation describes everything that you see, including:

• whether all, none or only part of a substance dissolves.
• the colour of everything and especially whether a substance is CLEAR (so that you can see through it) or TRANSLUCENT (so that it looks “milky” or “cloudy”), or OPAQUE (so that no light can pass through).
  

c) If you mix a chemical with water or another chemical, and you see

• TWO SEPARATE LIQUID LAYERS OR A SOLID LAYER ON THE BOTTOM AND A LIQUID LAYER ON TOP, the mixture is a MECHANICAL MIXTURE.
• ONLY ONE CLEAR LIQUID, the mixture is a SOLUTION. (If a solid and a liquid are mixed and only a few specks of solid remain undissolved, you can assume that the rest of the solid would dissolve with a little more shaking and therefore the mixture is forming a SOLUTION. If a quarter or more of the solid remains, the mixture is a MECHANICAL MIXTURE and the original substance was a MECHANICAL MIXTURE of solids.)
• A CLOUDY OR MILKY LIQUID AND NO SEPARATE SOLID OR LIQUID LAYER, the mixture is a SUSPENSION.
  

d) To properly shake a liquid, the test tube must be NO MORE THAN 3/4 FULL. Put a rubber stopper in the test tube to be shaken and quickly shake the liquid back and forth between the stopper end and the bottom end of the test tube. Continue for the required time.

e) In this experiment, IGNORE ANY BUBBLES THAT YOU SEE IN THE LIQUIDS. They are just trapped air and are not significant.

Procedure: 1. Use a dropping pipet to suck up some of the liquid in test tube A and put ONE DROP of the liquid on a watch glass. Dispose of the remaining liquid in the pipet by squirting the liquid into the sink and washing out the pipet with water. Place the watch glass on a hot plate for two minutes, remove the watch glass using a Hot Hand, and describe what you observe. 2. Use a dropping pipet to suck up some of the liquid in test tube B and put ONE DROP of the liquid on a watch glass. Dispose of the remaining liquid in the pipet by squirting the liquid into the sink and washing out the pipet with water. Place the watch glass on a hot plate for two minutes, remove the watch glass using a Hot Hand, and describe what you observe. 3. Use a dropping pipet to suck up some of the liquid in test tube C and put ONE DROP of the liquid on a watch glass. Dispose of the remaining liquid in the pipet by squirting the liquid into the sink and washing out the pipet with water. Allow the liquid to sit for one minute, occasionally blowing on the liquid, and describe what you observe. 4. Examine the contents of test tube D. Describe what you observe. Add enough distilled water to the test tube until the tube is about 3/4 full. Stopper the tube and shake it for one minute. After one minute, describe what you observe. Dispose of the test tube contents in the “DISPOSAL BEAKER” provided by your teacher. 5. Examine the contents of test tube E. Describe what you observe. Add enough distilled water to the test tube until the tube is about 3/4 full. Stopper the tube and shake it for one minute. After one minute, describe what you observe. Dispose of the test tube contents in the “DISPOSAL BEAKER” provided by your teacher. 6. Examine the contents of test tube F. Describe what you observe. Add enough distilled water to the test tube until the tube is about 3/4 full. Stopper the tube and shake it for ten seconds. After ten seconds, describe what you observe. Dispose of the test tube contents in the “DISPOSAL BEAKER” provided by your teacher. 7. Examine the contents of test tube G. Describe what you observe. Add enough distilled water to the test tube until the tube is about 3/4 full. Stopper the tube and shake it for five seconds. After five seconds, describe what you observe. Dispose of the test tube contents in the “DISPOSAL BEAKER” provided by your teacher. 8. Examine the contents of test tube H. Describe what you observe. Add enough distilled water to the test tube until the tube is about 3/4 full. Stopper the tube and shake it for one minute. After one minute, describe what you observe. Dispose of the test tube contents in the “DISPOSAL BEAKER” provided by your teacher. 9. Examine the contents of test tube I. Describe what you observe. Add enough distilled water to the test tube until the tube is about 3/4 full. Stopper the tube and shake it for one minute. After one minute, describe what you observe. Dispose of the test tube contents in the “DISPOSAL BEAKER” provided by your teacher. 10. Examine the contents of test tube J. Describe what you observe. Examine the contents of test tube K. Describe what you observe. Carefully, pour the contents of test tube J into test tube K. Quickly stopper test tube K and shake for a few seconds. Observe the contents of the test tube for two minutes and describe what you observe as time passes. 11. Your teacher will demonstrate what happens when (a) sand in water is filtered through a filter paper, and (b) salt water is filtered through a filter paper. Describe what you observe.

DISCUSSION:

1. Look at the observations you made when the liquids in test tubes A, B and C are allowed to evaporate. Which of these test tubes, if any, contain solutions? Which, if any, contain mechanical mixtures? Which, if any, contain a suspension? Which, if any, appear to be pure substances?

2. According to your observations, does test tube D contain a pure substance or a mixture? Does test tube E contain a pure substance or a mixture?

3. Look at the observations you made when the liquids in test tubes F, G, H and I are mixed with water. After water is added, which of the mixtures, if any, are solutions? Which of the mixtures, if any, are mechanical mixtures? Which of the mixtures, if any, are suspensions?

4. Look at the observations you made in Procedure step 10. A chemical reaction occurred in the test tube. Did the reaction produce a solution, a mechanical mixture or a suspension? Explain why you believe your answer is correct.

5. Look at the observations you made in Procedure step 11.
(a) Which of the two mixtures is a mechanical mixture and which is a solution?
(b) Is it possible to separate a mechanical mixture by filtration? Explain why you believe your answer is correct.
(c) Is it possible to separate a solution by filtration? Explain why you believe your answer is correct.