BioEd Online

Teacher Background

The cells and tissues that make up living organisms are mostly water. For example, water comprises about 90% of the weight of a tomato, 80% of the weight of an earthworm, 70% of the weight of a tree, and 70% of the weight of a human body.

In this activity, students will investigate the amounts of water in two different fruits and use measures of weight and volume. The activity also introduces students to the concept of drying (or removing water) as a means of preserving foods. Drying can be traced back to ancient times, and was an important method of food preservation used by American Indians and early settlers in North America. When foods are dried, most of the moisture is removed. Drying makes many grains, meats and vegetables much less suitable environments for the growth and reproduction of molds, bacteria and insects.

Dehydration also makes foods lighter, and easier to store and transport. Other methods for preserving food that involve dehydration include smoking—which is faster and more effective because the absorbed smoke is toxic to many microorganisms—and salting, which draws moisture out of the food items.

Objectives and Standards

Concepts

• Water is a major component of most foods.

Science, Health and Math Skills

• Making and recording observations

• Weighing

• Measuring volume

• Estimating

• Predicting

Materials and Setup

Teacher Materials (see Setup)

• Equal arm balance (1 per group if possible)

• Interlocking 1-cm/1-g cubes (weight for balance)

Materials per Student Group

• Beaker, 1,000-mL (or clear plastic cup calibrated in milliliters)

• Beaker, 250-mL (or clear cup calibrated in mL)

• Drinking straw (or 2 sheets of paper towel)

• Hand lenses (magnifiers)

• Juicer (see Setup)

• Plastic serrated knife

• Apple

• Orange

• Water

• Copy of “How Much Juice Is in an Orange” page

Setup

1. Make a juicer for each group of students by combining the top and bottom pieces of a 2-liter soft drink bottle (see PDF).

2. This activity will take at least two periods and may be extended to three.

3. Have students work in groups of four.

Procedure and Extensions

Time

Two 30-minute sessions

Session 1: How much liquid does an orange have?

1. While holding a bag of oranges in front of the class, ask, How much water do you think is in this bag? Lead a class discussion about the amount of water in an orange. Ask the students to predict the amount of water contained in one orange. Make sure they equate orange juice with water.

2. Show the students how to measure the volume of an orange by observing and measuring “how much space it takes up.” Fill a prepared beaker with 800 mL of water. Record the number of mL in the beaker on the board. Then place an orange into the water. Hold it down gently, so that the whole orange is submerged. Ask, Did the water level go up or down? How much? Why? To help students understand the concepts of displacement and volume, talk about what happens to the water when someone gets into a bathtub.

3. On the board, subtract the original volume of water in the container from the volume in the container after the orange was added. Calculate and record the difference. Ask, What does the difference represent? (A standard juice orange will displace about 140–150 mL and will yield 40–50 mL of squeezed juice.)

4. Have each group measure the volume of an orange by submergence, as you demonstrated. Ask the students to suggest ways to measure the amount of juice inside their oranges.

5. Show the students how to squeeze the juice out of an orange. Have them cut their oranges in half using serrated plastic knives. Use the top and bottom portions of soft drink bottles as “juicers” (see PDF), use purchased plastic juicers, or let students devise their own ways to squeeze out the juice. Have each group squeeze the juice out of one orange. Make sure the students save the remainders of their oranges.

6. Have each group measure the amount of juice obtained by pouring it into a 250-mL beaker. Ask, How can the remaining material be measured? If students suggest weighing, have them consider the conversions necessary to equate the weight information with their earlier measurement in mL. Have students place the remaining orange pieces without juice into the beaker prepared with of 800 mL water and read the new volume. Ask, Has the amount of water displaced changed? Why? What was the volume of the entire orange? What is the volume of the remaining “stuff”? What fraction of the orange was water? Have students record the values they obtained on the “How Much Juice Is in an Orange” observations sheet.

Session 2: How much liquid does an apple have?

1. Ask the class, Do you think other foods contain water? How about an apple? Encourage the students to predict whether apples and other fruits and vegetables contain water. Ask, How could we find out? Could we squeeze an apple?

2. Give each group of students an apple and a plastic serrated knife. Direct the students to weigh their apples, record the values and cut their apples into slices vertically (about 1/2 cm in thickness). Have students place the slices between two sheets of paper towel, or skewer the slices along a straw (see PDF for illustration). Then let the apples sit in a warm place for 3–5 days. (The amount of time will vary depending on the temperature; see next step.)

3. Have students weigh their sliced apples every day and record the weights (or mass) in grams. When the slices no longer show an appreciable change in weight from one day to the next, they have dried as much as will be possible. Have older students make a graph of the daily weights of their apple slices. Ask, What does the graph tell us about the weight of the apple?

4. Have students in each group subtract the final weight of the slices from the starting weight of the apple. The difference will be the weight of the water lost from the apple during the experiment.

Extensions

• Students also may want to compare weight (mass) differences between raisins and grapes, dehydrated potatoes slices (packaged potato casserole mixes) and fresh potato slices, banana chips and fresh slices of banana, beef jerky and strips of raw beef, or dried peas and fresh peas.
• Approximately 70%, or 7/10, of the human body consists of water. Have students use the following formula to calculate approximately how much of their own weight is water.
  
  Step 1: Your weight x 7 =
  Step 2: Value from Step 1 ÷ 10 = approximate amount of water in body.
  
  OR
  
  Step 1: Count out the number of snap-together math cubes equal to your weight (i.e., 45 lb = 45 cubes).
  Step 2: Separate the cubes into 10 equal groups.
  Step 3: Place 7 groups in one set and 3 in another.
  Step 4: The largest set represents the portion of your body that is water. 
   
• Have students estimate the volume of water in their bodies (1 lb of water represents approximately 2 cups).

Handouts and Downloads