Physical and Chemical Changes
Matter and Energy
Physical and Chemical Changes
Background
Matter has physical and chemical properties that determine the types of changes that can occur.
In the digestive process, organisms break down the food (matter) using both physical and
chemical processes. The goal of digestion is to get food to a form that can be absorbed by the
cell. This means you literally are what you eat.
Physical Changes
Physical changes include breaking a substance into smaller size
particles or changing the phase (state of matter: solid, liquid, and
gas) of the substance. Physical changes do not alter the identity
(chemical composition) of the substance. Examples of physical
changes during digestion include teeth cutting/crushing food into
pieces, churning food in the stomach, and the pushing of the food
from the stomach to the small intestine. Evidence of a physical
change in matter includes a change in shape, size, or state of
matter.
Chemical Changes
Chemical changes occur when a substance is transformed into a
different substance. There are several signs that a chemical
change has taken place. They include the production of a gas, a
color change, the formation of a precipitate (a solid forms when
liquids mix), and the production of heat or light. Examples of
chemical change during digestion includes the action of enzymes (amylase) of the mouth that
breaks down carbohydrates to sugar and the actions of digestive juices in the stomach
(hydrochloric acid) and small intestines that chemically change food particles into nutrients that
the body can use.
Starch
Food contains proteins, lipids (fats), and carbohydrates. A type of carbohydrate is called starch.
Starch is found in breads, crackers, and vegetables. Scientists use a solution of iodine and
alcohol, called Lugol’s, to test for the presence of starch. Iodine is normally an orange-red color,
but when it is placed on something containing starch, it will turn a blue-black color.
Please answer the Background questions in your Student Journal.
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Physical and Chemical Changes
Matter and Energy
Part I: Chemical Changes During Digestion
Saliva contains an enzyme called amylase that chemically breaks down starch to sugar. This
chemical change starts in your mouth as you chew and mix the food with your saliva.
Procedure for Saliva
1.  Obtain three small cups and label them #1, #2, and Starch with a marker.
2.  Fill a separate cup with approximately 125 mL of clean tap water. Do not measure using lab
equipment.
3.  One student should rinse their mouth and drink the tap water so their mouth is clean.
4.  Have that student spit into cups #1 and #2. The bottom of the cups should be covered with the
student’s saliva.
5.  Add 2 mL of starch solution to cups #1, #2, and Starch.
6.  Add three drops of the iodine solution to the cups labeled Starch and #1. Do not add iodine to
the cup labeled #2.
7.  Let all three cups stand without disturbing for 30 minutes. While waiting, complete the next
activity: Procedure for Acid.
8.  After 30 minutes, add three drops of iodine to cup #2.
9.  Place your final observations on the chart titled Procedure for Saliva in your Student Journal.
Procedure for Acid
Your stomach contains hydrochloric acid that helps to break down some foods. The lemon juice
that is used in this procedure is a weaker form of acid called citric acid.
1.  Cover the bottom of the 100 mL beaker with milk.
2.  Pour in an equal amount of lemon juice and mix with the spoon. Continue stirring for two
minutes, then make observations and record in chart titled Procedure for Acid in your Student
Journal.
3.  Clean up according to your teacher’s instructions.
4.  After completing the Procedure for Saliva and the Procedure for Acid, answer the questions in
your Student Journal.
Complete Part I: Chemical Changes during Digestion in your Student Journal.
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Physical and Chemical Changes
Matter and Energy
Part II: Physical Digestion and Surface Area
The physical breakdown of the food particles increases the surface area or exposed area for the
digestive chemicals to work on. As surface area increases, volume decrease, thus allowing faster
chemical digestion to take place. When digestion occurs efficiently, the cells in the body can absorb
glucose and other nutrients they may need in a timely manner.
In this investigation, sugar cubes will be used to model how physically-digested food can have
greater surface areas. The surface areas and volume will be calculated and then tested.
Doing the Math:
To find the surface area of a sugar cube:
1.  Find the area of each side (length x width) then multiply that number by 6 (the number of sides
of a cube.)
2.  To find the volume of the cube, multiply length x width x height.
3.  To find the surface area to volume ratio of a cube, divide the surface area by the volume.
Example:
• The surface area of a 2cm3 cube is 24 cm2.
• The volume of a cube with 2 cm sides is 2 x 2 x 2 (8 cm3).
• The ratio of surface area to volume is: 24 ÷ 8 (3 to 1).
Continue to the next page.
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Physical and Chemical Changes
Matter and Energy
Part II: Physical Digestion and Surface Area, continued
Procedure:
1.  Use your metric ruler to determine the volume and surface area of one sugar cube in
centimeters. Take care to measure to the nearest tenth of a centimeter and use centimeters
squared or cubed in your answers. Record your answers in the data table in your Student
Journal.
2.  Build a large, solid cube that is three cubes wide, three cubes long, and three sugar cubes tall.
Use your metric ruler to determine the volume and surface area of the larger cube in
centimeters. Record your answers in the data table in your Student Journal.
3.  Next, take the two extra sugar cubes and place them each in one of two 100 mL beakers.
Crush one sugar cube with the end of an unsharpened pencil.
4.  Add 25 mL of water to each beaker and start timing. You will time how long it takes for the sugar
cubes to dissolve in the water. When one cube is completely dissolved, or at 5 minutes,
whichever comes first, stop timing. You will have to estimate how much is left by using
percentages.
Complete Part II: Physical Digestion and Surface area in your Student Journal.
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Physical and Chemical Changes
Matter and Energy
Part III: Physical Changes for Digestion
Planning your investigation
Breaking a Cracker
You will develop an investigation to determine if physical or chemical change takes place when a
cracker is broken or broken and mixed with water. You will compare your results to a cracker that is
not broken or changed in any way.
Question of Inquiry:
With your class and teacher, discuss the Question of Inquiry and list the materials that you will
need to conduct your investigation.
Safety Precautions:
It is necessary to work carefully with the iodine, it can irritate skin and eyes. Wear safety goggles to
protect your eyes. Flush iodine from the skin or eyes with water and inform the teacher.
Procedure:
Follow the instructions in the following part: Implement Your Investigation.
Complete Part II: Plan Your Investigation in your Student Journal.
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Physical and Chemical Changes
Matter and Energy
Part III: Physical Changes for Digestion
Implement Your Investigation
Breaking a Cracker Procedure:
1.  Layer two paper towels and fold in half.
2.  Place one whole cracker on the paper towels.
3.  Place a second cracker on the paper towels and
break the cracker into 5-6 pieces.
4.  Place four drops of Lugol’s iodine on the broken
cracker, and four drops on the whole cracker.
5.  Record your observations in your Student Journal.
6.  Place one whole cracker in the plastic bag and break
it into 5-6 pieces.
7.  Add 20mL of water to the bag.
8.  Seal the bag, and mix the crackers and water
together by gently squeezing with your hands.
9.  Open the bag, and place four drops of the Lugol’s
iodine on the wet cracker.
10. Record your observations in your Student Journal.
Complete Part III: Implement Your Investigation in your Student Journal.
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