EXPERIMENT 3
Materials
A bunch of fresh celery
A glass filled with colored water (as in Experiment 2)
A sharp knife slice
Procedure
1. Remove another stalk of celery from the bunch and ask an adult to cut the base, as described in Experiment 2.
2. The adult should also carefully remove a thin slice halfway across the center of the celery stalk.
3. Place the stalk in the glass of colored water.
4. Move the glass to a place where it will not be disturbed.
5. Examine the celery stalk every few hours.
Results
The part of the celery stalk directly above the thin slice that was removed should not be red. If you look at a cross section of the stalk above the portion that was removed, you will see that one half is full of red dots. The other half will have no red dots. Because the capillaries were cut when a slice was removed from the stalk, colored water could not travel to the part of the stalk above the cut.
EXPERIMENT 4
Materials
Masking tape
A permanent-marking pen
A glass filled with water
Two stalks of celery with leaves
Procedure
1. Using the tape and marking pen, make labels for each glass. The empty glass should be labeled A, and the glass with water should be labeled B.
2. Ask an adult to slice off the ends of both stalks of celery.
3. Place one stalk in each glass with the leaves up.
4. Place both glasses in the refrigerator.
5. Examine them every 3 or 4 hours for a few days. Add more water to Glass B if necessary.
Results
The stalk in Glass B should look about the same as it did at the beginning of the experiment, but the stalk in Glass A should show signs of wilting. The water in Glass B replaced the water that the celery lost through evaporation. Since the fluid in the stalk in Glass A was not replaced, the stalk grew more and more limp.
Observation 2
Add water to a glass until it is half full. Look at the surface of the water around the inside edge of the glass (where the water and glass meet). Notice the way the water curves upward, a tiny bit, at the edge of the glass. This curving is called a meniscus.
The water curves upward because a force called adhesion causes molecules of water to be attracted to the glass. Molecules of water stick to the glass the same way that adhesive tape sticks to objects.
Adhesion is also at work within plant tissue. Water is attracted to the sides of the tubes that carry it throughout a plant's tissues. cohesion This to pull attraction water from is necessary the roots to for the stem.
CRYING IN THE KITCHEN
If you've ever sliced, diced, or peeled an onion, you know it can be a very unpleasant experience. When an onion is cut open, a chemical called propanethiol S-oxide is released. This gas reacts with the water in your eyes to form sulfuric acid. Your eyes burn in response to this strong acid. Your body's defense mechanism responds by producing tears that will flush away the acid.
Over the years, people have come up with a number of "home remedies" for this problem:
* You can refrigerate the onions before cutting them. This helps to keep the propanethiol S-oxide from evaporating rapidly.
* You can slice the onions under water. The water absorbs the vapor before it gets into the air.
* You can wear a skin-diving mask to prevent the gas from reaching your eyes.
All these methods will make slicing or peeling onions easier, but the only way to prevent tearing is to convince someone else to cut the onions for you.
POPCORN! GET YOUR FRESH
POPCORN HERE!
Popcorn, popcorn, you're so cool. When I think of you, I start to drool. Slathered with butter, you are supreme. When I see you, my eyes start to gleam.
Who doesn't like popcorn, especially when it's slathered with butter? It's perfect for munching at a movie, or just about any other time.
To be used as popcorn, corn kernels must have a moisture content of about 13 percent, a starchy white center, and a tough outer shell. The two major types of corn used for popping are "rice popcorn," which has kernels that are pointed at both ends, and "pearl popcorn," which has spherical, tightly packed kernels.
When the kernels reach a temperature of about 400˚ F (200˚ C), the moisture inside the kernels vaporizes and turns into steam (a gas). Because a gas takes up more space than a liquid, the pressure inside the kernel increases. When the pressure inside the kernel reaches a certain point, the tough outer shell breaks, and the kernel explodes. The result is the little fluffy morsels we call popcorn.
If you ever use Jiffy Pop™ at home, you will see steam rising from the pan when you pierce the aluminum-foil covering. Before the popcorn was heated, this moisture was inside the kernels. If you allowed the Jiffy Pop to cool down before removing the foil, the steam would condense and transform back to liquid water, and you would end up with soggy popcorn.
Archaeologists—scientists who study ancient cultures and sites—working in Utah and Peru have found popcorn that is more than 1,000 years old. No Jiffy Pop aluminum pans were found at the sites, however.
Chapter Three
Supermarket Staples
A staple is not necessarily the little U-shaped metal thing that holds pages together. The word "staple" is also used to describe an item that is in high demand. A supermarket staple is an item that is a key ingredient in many recipes. Examples include salt, flour, eggs, and sugar.
Sugar is used to make brownies, cookies, lemonade, and many other tasty treats. A pinch of salt can be added to just about any recipe for a little extra flavor. When you add these substances according to the directions in a recipe, they seem to just melt away and disappear.
Going, Going , Gone
Lemonade is a solution made of lemon juice, water, and sugar. A solution is a mixture in which all of the ingredients are uniformly distributed. Hot cocoa is a solution made of chocolate powder and warm milk. Oil-and-vinegar salad dressing is not a solution because the ingredients do not mix together. They form separate layers.
When sugar is added to lemonade or iced tea, it will disappear when you stir the solution. If you love sweet things, you might want to add a lot of sugar to these drinks. Is it possible to add too much sugar? Could you add more sugar to a cup of hot cocoa than to a glass of lemonade? Try Experiment 5 to find out.
EXPERIMENT 5
Materials
Two measuring cups
Cold water
Hot water
A thermometer
Salt
Measuring spoons
A spoon
Procedure
1. Pour 1 cup (240 mL) of cold water into one measuring cup and 1 cup (240 mL) of hot water into the other measuring cup. Be sure to hold each measuring cup at eye level as you add water. If you measure from any other angle you will not get a true reading because of the thickness of the glass.
2. Measure the temperature of the water in each measuring cup. Record your results.
3. Add 1 tablespoon (15 mL) of salt to the cup of cold water, and stir vigorously. Does all of the salt disappear (dissolve in the water)?
4. Add 1 tablespoon (15 mL) of salt to the hot water, and stir vigorously. Does the salt seem to dissolve more quickly in the hot water or in the cold water?
5. Keep on adding salt to each cup of water, ½ tablespoon (7.5 mL) at a time. How much salt can dissolve in each measuring cup? Record your results.
Results
At a certain point, the water will not be able to dissolve any more salt. Scientists would say that the water is saturated with salt. After the water reaches its saturation point, salt will start to sink to the bottom of the cup.
You should notice that the warm water can dissolve more salt that the cold water. That's because temperature and a liquid's ability to dissolve another substance are related. The hotter th
e liquid, the more it can dissolve.
If you keep watching the cup of warm water, you will see that, as the water cools, some of the salt comes out of the solution and become a solid again.
Observation 3
Pour exactly 1 cup (240 mL) of warm water in a measuring cup. Stir salt into the water, one tablespoonful at a time. How many tablespoons do you have to add before the volume starts to increase?
Note that as the salt dissolves in the water, the total volume of the water does not increase very much—if at all. This is because the salt occupies the empty spaces between water molecules. As a result, the water weighs more without taking up more space. (I wish I could do that.)
IT'S A DELICATE BALANCE
A solution's freezing point, boiling point, and density can all be affected by the type and amount of material dissolved in it.
First, let's take a look at how a solution's freezing point is affected by the substances dissolved in it. When antifreeze is added to the water that runs through your car's cooling system, it lowers the water's freezing point. As a result, your car can operate in very cold weather.
The same scientific principle can be demonstrated using salt—a supermarket staple. Try Experiment 6.
EXPERIMENT 6
Materials
Three identical hard plastic drinking cups
A measuring cup
Tap water
A marking pen
Masking tape
Measuring spoons
Salt
Two spoons
A freezer
A thermometer (optional)
Procedure
1. Rinse and dry the plastic cups.
2. Using the measuring cup, add exactly 4 ounces (120 mL) of water to each plastic cup.
3. Using the pen and masking tape, label the plastic cups A, B, and C.
4. Set aside the cup labeled A.
5. Add exactly ⅛ teaspoon (0.6 mL) of salt to the plastic cup marked B.
6. Add exactly ¼ teaspoon (1.2 mL) of salt to the plastic cup labeled C.
7. Stir Cup B with a spoon until no more salt will dissolve. Stir Cup C with a different spoon until no more salt will dissolve.
8. Place all three cups in the freezer compartment of your refrigerator.
9. Check the cups after 90 minutes, and then every 15 minutes. Note the condition of the water in each cup. Which cup freezes first? Which freezes last?
10. If you have a thermometer, measure the temperature of the water or ice in each cup each time you check it. What is the lowest temperature in each cup before the water in it freezes?
Results
The ratio of water to salt was different in each of the three cups. You should have observed that each of the three water samples froze at different times. The saltless water froze first, and the water with the most salt froze last. A solution's freezing point can be affected by the amount of material dissolved in it.
Observation 4
Fill two identical cups or mugs half full of water. Dissolve as much salt as possible in one cup. The other cup should contain only water. Take an egg out of your refrigerator and add it to the glass with salt-free water. What happens?
Take the egg out of the cup and place it in the cup with saltwater. Do you notice any difference? The egg should float in the saltwater, but not in the plain tap water. Can you explain why?
As you add salt to water, the water becomes denser. The more salt you add, the more dense the water becomes. If enough salt is added to water, it will actually be denser than an egg, so the egg will float.
A CLOSER LOOK AT DENSITY
An object's density is measured by dividing its mass by its volume, either in grams per cubic centimeter (g/cm3 or pounds per cubic foot (lbs./ft.3). Tap water has a density of 1 g/cm3. So any object with a density of less than 1 g/cm3—like an empty soda bottle—will float in water. An object with a density of more than 1 g/cm3—like a full soda bottle—will sink.
Density of Various Materials
Material Density (g/cm3)
Air 0.08
Aluminum 2.70
Gasoline 0.70
Gold 19.30
Ice 0.92
Iron 7.90
Lead 11.30
Limestone 3.20
Mercury 13.60
Pine 0.50
Seawater 1.30
Tap water 1.00
People have understood density for a very long time. More than 2,000 years ago, a Greek scientist named Archimedes made a startling discovery concerning density, weight, and water. As the story goes, Archimedes was taking a bath when he had a profound thought. He jumped out of the water and ran through town in a state of undress, yelling, "Eureka!" ("I have found it!")
What Archimedes had found was the answer to a problem that had been troubling him for many weeks. Because the king respected Archimedes' honesty and intelligence, he had asked Archimedes if there was any way to make sure that the local metalsmith had not cheated him.
The king had asked the metalsmith to make a new crown out of a large piece of gold that the king provided. To do this, the metalsmith had to melt down the gold and then shape it into a crown. The king was afraid that the metalsmith had mixed some of the gold with a less valuable metal and kept some of the pure gold for himself.
There was no way to tell if the crown was really made of pure gold by just looking at it or weighing it. But while Archimedes was taking a bath, he realized that he could determine whether the metalsmith was honest by measuring the density of the metal used for the crown and comparing this value to the density of pure gold.
To do this, Archimedes put the crown in a container full of water and measured the amount of water that spilled over the top of the container. This information told him the volume of the crown. He then divided the crown's weight by its volume. His calculations showed that the crown did not have the same density as pure gold. The metalsmith had tried to cheat the king by replacing some of the gold with a cheaper, less dense metal.
Challenge 1
Using what you learned in Observation 4, try to answer
* Why is it easier to do a handstand in a swimming pool than on dry land?
*Why do some things float and other things sink?
*Why does a helium-filled balloon rise in the air, while a balloon you inflate with your mouth sinks to the floor?
The key to answering these questions requires an understanding of buoyancy. An object's buoyancy is influenced by its density and its surroundings.
When an object is placed in water, the object displaces some of the water. In other words, the object pushes water out of its way. The water, in turn, pushes back against the object with a force equal to the weight of the water the object has displaced. If an object displaces more water than is comparable to its weight, it will float. If not, it will sink. In water, most pieces of wood, cork, ice, and sponge float—so do people. Objects such as rocks and coins sink.
As Experiment 7 shows, two objects with the same volume, but different weights, will have different buoyancies.
EXPERIMENT 7
Materials
Two identical, empty soda bottles with screw-on caps
Water
A sink
Procedure
1. Fill one bottle with water. Screw the cap on tightly.
2. Screw the cap tightly onto the empty bottle.
3. Fill a sink with water.
4. Put the bottles in the sink. What happens?
Results
The empty bottle floats, but the bottle full of water sinks. Even though the bottles have the same volume, the full bottle sinks because it weighs more and is more dense.
EXPERIMENT 8
Materials
Several lengths of string
Several objects that sink in water (a brick, a ceramic coffee cup, etc.)
Fisherman's scale (that measures fractions of an ounce)
Paper
A pencil
A sink
Procedure
1. Tie one end of each piece of string around one of the objects that sink in water.
2. Make a loop at the other end of each string, so that you can weigh the object with the fisherman's scale.
3. Weigh and record each object's weight.
4. Fill a sink with water.
5. Place each object in the sink and reweigh it while it is under water. (Make sure each object is completely submerged.)
6. Compare the dry and wet weights of each object.
Results
The objects weigh less when they are underwater. Even though they don't float, the water buoyed up each object. Based on your results, can you guess which of the objects is the most dense? The least dense? Think about that.
Science Lab in the Supermarket (Illustrated) Page 2