May/June 2017 – Vol. 29 No. 7

Chemicals and Their Properties Lab

Posted: Tuesday, March 1st, 2011

by Jeff  Bradbury and Patricia Buchanan

Name ____________________

Date _____________________

Partner’s Name ____________

Question: The work of a crime scene investigator (CSI) is to identify certain found liquids or solids in an investigation, how could we find out the identity of an unknown substance?

Purpose: To learn how to classify matter and its changes.

Part 1 Introduction:

Background Information:

1) A pure substance is a sample of matter that has a definite, fixed composition, and the same properties throughout the sample.  Each pure substance has its own set of characteristic properties.

Physical properties can be observed without changing the chemical composition of the substance.  Common physical properties that can be measured without changing the chemical composition are color, melting point, boiling point, solubility, and density.  Taste and odor are usually considered to be physical properties although they depend on the physiology of the person making the observations.

Physical changes do not cause a change in chemical composition, but only a change in appearance.  When a substance simply changes from a solid to a liquid, liquid to a gas or vice-versa (liquid water to solid water, for example) without a change in composition, it is a change in state.  This is a physical change.  (The three physical states are:  gas, liquid, and solid).

Chemical properties describe the ability of a substance to react and change into another substance with a different chemical composition.  The new substance would have new properties.

2) Evidence that a chemical change is taking place includes:

a.  A color change

b.  A solid product, a precipitate is formed

c.  A gas is formed

d.  Energy such as heat, light or electricity is produced.

Introduction:

In today’s lab you will observe some physical properties of elements and some physical and chemical changes. You will be working in groups of 2.

Procedure, Observations, and Data:

SAFETY!

Safety goggles must be worn at all times

Hydrochloric acid (HCl) and sodium hydroxide (NaOH) can harm eyes, skin, and clothing. Handle with care. Any acid spilled on the skin should be rinsed with a large volume of water for 15 minutes.

Wash your hands before you leave the lab.

Do not take reagent bottles from the reagent bench to your work station.

Do not pour anything back into a reagent bottle. Offer extra chemical to another student

Do not stick anything into a reagent bottle.

Always clean glassware and spot plates with distilled/deionized water.

Using Chemical and Physical Changes to Identify an Unknown Substance:

At the reagent bench are samples of 7 common substances.  Your instructor will assign you an unknown, which is the same as one of the 7 known substances.  Based on the observations of the chemical and physical properties of the known and unknown substances, you will determine the identity of the unknown.

Procedures 3-5 do not need to be done in order.  You will perform all of the tests on the unknown that you perform on each of the known samples.

1. Observe each sample, including the unknown, and then record your observations as to the color, texture and any other important properties in table 1.

2. In a spot plate place small pea-sized amounts of each substance in 2 different rows of wells. Be careful to note which substance is in which well. Do not use more than a small pea-sized amount, or it will be difficult to perform the tests. (On a paper towel write the name of each substance in the order it is placed in your spot plate, and place this paper towel next to the spot plate.)

3. a. Into the first row of wells of the different substances, put about 1 ml of de-ionized water.  See if the substances dissolve completely or partially, or change in any other way. Record any evidence of change that occurs in table 1.

b. In the row of wells that contains the substances mixed with water, put 3 drops of universal indicator. Record any evidence of change in table 1.

4. In the other row of wells containing the samples put 3 drops of dilute acetic acid (vinegar) and record any evidence of change in table 1 below.

5. Cover your wire gauze with a piece of the aluminum foil that is set out on the reagent bench.  Place about pea-sized amounts of each substance onto the foil covered gauze. Make sure there is plenty of space between each sample on the foil.  Place the gauze on the ring stand and ring apparatus that is in the fume hood.   Light the burner and place the heat under each sample to see if any change occurs. Record any evidence of change in table 1.

Using the data that you recorded in table 1, determine the identity of the unknown sample and answer the questions that follow the table.

Waste: Rinse spot plates in the sink.  Clean spot plates with tap water then deionized water. When aluminum foil has cooled. Wad it up and throw it in the trash.

Table 1: Determining the identity of an Unknown substance

Appearance (1) Reaction with water  (2) Reaction with universal indicator (3) Reaction with vinegar  (4) Reaction with heat   (5)
Sodium Chloride
Sugar (Sucrose)
Baking Powder
Baking Soda

(Sodium Bicarbonate)

Citric Acid
White Flour
Calcium Carbonate
Unknown #

Unknown #     ____________________

Based on you observations in the above table identify the unknown _____________________

Explain the evidence for your answer to the above question:

Questions:

1.  Explain the difference between a chemical and physical change? (You can use examples from this lab in your explanation.)

2. What was it that you did today that helped you see the difference between a chemical and physical change?

3. What happened when vinegar was added to the powders?

Notes

Jeff Bradbury is a professor of chemistry at Cerritos College in Norwalk and is community college director for CSTA; Patricia Buchanan is the Cal Grip Grant Project Assistant at Cerritos College.  The original idea for this activity came from the Los Angeles County Office of Education 15 years ago, which the authors modified.

Written by California Science Teachers Association

California Science Teachers Association

CSTA represents science educators statewide—in every science discipline at every grade level, Kindergarten through University.

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