University of Virginia Physics Department Physical and Chemical Changes A Physical Science Activity

2003 Virginia SOLs

• PS.1
• PS.5

Objectives

Students will

• distinguish between a chemical and a physical change;
• use skills such as: observation, recording observations, and making inferences from observations.

Motivation for Learning

Discrepant Event

Materials

• 100 mL of granulated sugar
• Two 100 mL beakers
• Concentrated sulfuric acid

Procedure

1. Fill each beaker ½ full with sugar.
2. Add 40 mL of water to the first beaker and 40 ml of concentrated acid to the second. Stir and let them both sit.
3. Ask the students: "In which beaker do the reactants still have the same properties?" (The reaction should be carried out in a well ventilated area, under a hood or near a window.)

In beaker #1 the sugar and water are simply mixed together illustrating a physical change. The components of the mixture still retain their physical properties and could be separated back out of the mixture using those properties.

However, in the second beaker the sugar and the acid did not merely mix together. A chemical change took place, resulting in products that are completely different than the original sugar and acid. A gas (SO₂) is produced along with water vapor forcing a mass of charcoal to expand out of the beaker. The two key indications of the chemical reaction are the color change and the gas produced.

Background Information

If you break a piece of chalk, it loses its original size and shape. You have caused a change in some of its physical properties, but you have not changed the identity of the substance that makes up the chalk. This type of a change is called a physical change and it happens when substances freeze, boil, evaporate, or condense. These transformations may require energy, or may release energy, but the components that make up the substance do not change identity. An element, like iron, will change states if it is allowed to absorb enough energy, but it will still have physical properties that will identify it as iron.

Matter can also undergo transformations which result in a change in the identifying properties of the components that make up the substance. This type of change is called a chemical change. Fireworks exploding, matches burning, eggs rotting and cars rusting are all examples of chemical changes - changes where new materials are formed that were not there before. Most chemical changes can be identified by at least one of four clues which indicate a chemical change has occurred: a gas can be given off, a new color can appear, heat and light can be given off, or a precipitate can be formed. Each of these is evidence that a new substance has formed.

Student Activity

To print out the Student Copy only, click here.

Materials

Birthday candles	Safety glasses
Aluminum foil (15 cm X 15 cm)	Two test tubes (35 mL)
Modeling clay	Test tube rack
Small piece of paper	Test tube tongs
Watch glass (or other type of open glass dish)	Insulating pad
1 M hydrochloric acid*	Matches
Scoop	Table salt
Magnesium ribbon (1 cm long)	Dropper bottle of 0.1 M silver nitrate**

* To prepare 1 M solution of hydrochloric acid: add 36 grams of concentrated HCl to 1 liter of water.

* *To prepare .1 M silver nitrate solution: add 20.0 grams of silver nitrate to liter of water.

Procedure

Have students follow directions and complete the questions in the Data Sheet.

Data Sheet

To print out the Data Sheet only, click here.

Before beginning any part of this activity, first make observations of all initial materials. Part 1 Take a small piece of modeling clay and place it on the square of aluminum foil. Firmly place a candle in the clay so that it is well supported. Light the candle and allow it to burn while you continue with the rest of the investigation. Record your observations of the burning candle. Initial Obsevations:     Questions: 1. What did you observe as the candle burned?   2. What was left after the candle burned?   Part 2 Tear a piece of paper into small pieces and place them on the watch glass. Place the watch glass and the pieces of paper on the insulating pad. Light the pieces of paper with a match and allow them to burn completely. Record your observations of the burning paper. Initial Observations:     Questions: 3. What did you observe when the paper burned?   4. What was left after the paper burned?     Part 3 Add a small scoop of table salt to a test tube that has been ½ filled with tap water. Place your thumb over the test tube and shake until the salt has dissolved. Record your observations. Add 5 drops of silver nitrate to the salt solution and record your observations. Initial Observations:     Questions: 5. What did you observe when you added salt to the water in the test tube and shook it?   6. What did you observe after adding the silver nitrate to the solution?     Part 4 Place a small piece of magnesium ribbon in the test tube. Add 5 drops of hydrochloric acid to the test tube. Touch the bottom of the test tube and record your results. Initial Observations:     Questions: 7. What did you observe when the hydrochloric acid was added to the magnesium metal?

Answers to Data Sheet

1. Smoke was given off, candle got smaller, wax melted and dripped.
2. Melted wax.
3. Smoke given off and paper turned black.
4. Black ash.
5. The salt disappeared.
6. A white cloudy substance formed.
7. The acid started bubbling, the magnesium disappeared and the test tube got hot.

Extensions

To extend Part 3, make initial observations of the salt through a microscope. Then evaporate the water off and again look at the salt under a microscope to show that the salt was not altered by dissolving in the water.

Write out a recipe that involves cooking or baking. Identify each step in the recipe as either a chemical or a physical change.

Students with special needs

Click here for information on laboratories with students with special needs.

Assessment

(To print out the assessment only, click here).

1. Identify each of the following as either a chemical or a physical change.

You must give a reason for your answer.

a. Melting candle wax. _______________________________________________

b. Burning a candle. _________________________________________________

c. Tearing paper. ___________________________________________________

d. Burning paper. ___________________________________________________

e. Dissolving table salt. _______________________________________________

f. Mixing salt water and silver nitrate. _____________________________________

g. Cutting pieces of magnesium ribbon. ____________________________________

h. Mixing magnesium ribbon and HCl. _____________________________________

2. Describe two observations you might make when a physical change occurs.

3. Describe two observations you might make when a chemical change takes place.

Answers to Post-Lab Worksheet

1.

a. Physical, wax just gets smaller.

b. Chemical, candle changes to new substance, gas given off, heat given off.

c. Physical, paper gets smaller.

d. Chemical, paper changes.

e. Physical, salt is still present, can verify by tasting and evaporating.

f. Chemical, a new substance formed.

g. Physical, ribbon is made into smaller pieces.

h. Chemical, new substance is produced and heat given off.

 

2. The original substances stay the same, no energy changes, no gas given off and no color change.

3. New substances are produced, heat given off, color change and a gas given off.