# Surface Tension – Sand Castles

Surface Tension – Sand Castles

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If you have ever tried to build a sand castle you know that there is a certain kind of wet sand that is perfect for it. If there is too much water in your bucket the mixture is too soupy. If you don’t have enough water in your bucket the sand won’t hold a shape and just crumbles. Why does the perfect mixture of sand and water work? Surface tension. Surface tension is the attraction that happens between water molecules. Water molecules are attracted to each other. The surface of water has an elastic quality because the molecules are hugging close together. This is why some insects can walk on water.

Water is made up of two kinds of atoms, hydrogen and oxygen. The name for the water molecule is H20. The water molecule has 2 hydrogen atoms and 1 oxygen atom. Water molecules are attracted to each other because hydrogen atoms and oxygen atoms are attracted to each other and hug close together really tight. This is called cohesion. The molecules hug so close together they don’t want to touch other molecules around them. That’s why a bubble or a drop of water is round and only rests a small part of itself on a surface when it lands.

When you add sand to water, the surface tension of the water forms little elastic bridges between the grains of sand. When the ratio of sand to water is just right these bridges are the perfect strength for building sand castles. In today’s experiment you will be able to watch these bridge at work and figure out the best recipe for building sand castles.

What You Need:

• 12 Dixie Cups
• Sand
• Water
• 25 Pennies
• 4 Large Plates
• Large Bowl
• Measuring Cups (1/4, 1/3, 1/2, 1)

You are going to test what ratio of sand to water is the best one for building a strong sand castle. Label each plate – label the first one 1/4 cup, the second one 1/3 cup, the third one 1/2 cup and the last one 1 cup.  For each trial you are going to use 1 cup of sand. The variable in this experiment is going to be the amount of water you add to the sand. For the first trial mix 1 cup sand and 1/4 cup water in the bowl. Fill three dixie cups with this mixture and turn them over to make small sand castles in the plate labelled 1/4 cup. Do the castles flatten or stay formed like the dixie cup? If any of them stay formed, stack pennies on top of the little castle one at a time until the little castle collapses. Write down how many pennies each little castle could hold.  Repeat this test using 1 cup sand and 1/3 cup water, 1 cup sand and 1/2 cup water and 1 cup sand and 1 cup water. Keep track of your results on a chart like this:

 Amount of Water #pennies trial #1 #pennies trial #2 #pennies trial #3 1/4 Cup 1/3 Cup 1/2 Cup 1 Cup

One cup of sand to 1/3 cup water is what worked for us!

It turns out that water molecules attract to each other and they ALSO attract to sand. If you have a good balance of sand to water…nice and sticky…then you get a strong sand castle. If there is too much sand the mixture is too dry and the castle crumbles. If there is too much water the mixture is too wet and oozes all over the place.

More IndyPL Experiments about Surface Tension:

Words to Know:

Mixture – A substance made by mixing two or more substances together.

Surface Tension – The film that forms on the surface of liquids caused by the attraction of the particles in the surface.

Cohesion – The attraction between like molecules; to stick together.

# Surface Tension – Soap Bubbles

Surface Tension – Soap Bubbles

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The surface layer of liquids has a thin elastic “skin” called surface tension. You can see surface tension at work when you see a drop of water – it creates a little “bead” of water, like a little dome. Surface tension is what makes the dome shape – the water doesn’t flatten out.

Water is made up of two kinds of atoms, hydrogen and oxygen. The name for the water molecule is H20. The water molecule has 2 hydrogen atoms and 1 oxygen atom. Water molecules are attracted to each other because hydrogen atoms and oxygen atoms are attracted to each other and hug close together really tight. This is called cohesion. The molecules hug so close together they don’t want to touch other molecules around them. That’s why a bubble is round and only rests a small part of itself on a surface when it lands.

When you blow air into soap bubble solution the liquid molecules want to attract to each other again so they wrap around the burst of air until they can attach to each other again – this is what makes the round bubble shape. The air inside the solution is pushing the molecules in the soap bubble solution apart but the attraction between the soap bubble solution molecules is so great, the bubble doesn’t pop – the molecules are hugging each other too tight.

To experiment with bubbles you need a good bubble recipe. Below are some simple recipes to try. Each of the recipes use water and dish soap. The “other” ingredient can be baking powder, corn syrup, glycerin (sold at the pharmacy) or sugar. We had the best luck with baking powder. The baking powder recipe made some HUGE bubbles.

Science Project Idea:

Mix different formulas of bubble mix and test them to see which one makes the best bubbles. Use the same amount of water and the same amount of dish soap in at least three different buckets. Choose one “Other” ingredient and add it in different amounts to each of your trial buckets. To be fair, you should hold the bubble wand in front of a fan instead of trying to blow on it, that way you know that the amount of air being blown to make the bubble will be exactly the same. Test the three formulas several times and record your results on a chart. Decide before you begin what property you are looking for in the bubbles. Are you going to test which formula makes the biggest bubble, the bubble the last the longest without popping or the formula that makes the most bubbles?

Here are some books and websites that will help you understand and have fun with bubbles:

• Exploratoriuam: Bubbles
• Bubblesphere
• The Ultimate Bubble Book There are 3 fun activities in this book – Person Inside a Bubble (Page 41) Giant Bubbles (Page 48-51) Printing With Bubbles (Page 52-53)
• Google Preview: Science Experiments That Explode and Implode: Stink Bombs (bubbles that stink! pages 22-23)
• Prize Winning Science Projects for Curious Kids: The Bubble Olympids (pages 102-103)

More IndyPL Experiments about Surface Tension:

Words to Know:

Surface Tension – The film that forms on the surface of liquids caused by the attraction of the particles in the surface.
Cohesion – The attraction between like molecules; to stick together.

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# Surface Tension – Pepper Scatter

Surface Tension – Pepper Scatter

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The surface layer of liquids has a thin elastic “skin” called surface tension. You can see surface tension at work when you see a drop of water – it creates a little “bead” of water, like a little dome. Surface tension is what makes the dome shape – the water doesn’t flatten out. See the drops of water on that leaf? Water is made up of two kinds of atoms, hydrogen and oxygen. The name for the water molecule is H20. The water molecule has 2 hydrogen atoms and 1 oxygen atom. Water molecules are attracted to each other because hydrogen atoms and oxygen atoms are attracted to each other and hug close together really tight. They hug so close together they don’t want to touch other molecules around them. In the picture, you can see that the water drops have formed into balls so that they are touching the smallest amount of leaf possible.

Try this to see surface tension at work:

What You Need:

• bowl
• water
• pepper
• toothpicks
• dish soap

Fill the bowl with water. Sprinkle some pepper on top of the water – see how it just sits there on top of the water? The pepper is resting on that thin “skin” of surface tension. You can also try this with toothpicks – the toothpicks will also just sit on top of the water resting on the thin “skin” of surface tension.

No put a few drops of dish soap in the water with the pepper. What happens? Soap molecules are different than water molecules. Soap molecules DON’T stick together and they DON’T stick to the water molecule. One part of the soap molecule is attracted to water and the other part wants to push water away – that’s what makes the pepper, or the toothpicks, scatter. The soap breaks the surface tension that water has, it breaks those bonds between the water molecules.

More IndyPL Experiments about Surface Tension:

Science Experiments that Fly and and Move: Surface Magic (pages 6-7)

Words to Know:

Surface Tension The film that forms on the surface of liquids caused by the attraction of the particles in the surface.
Cohesion – The attraction between like molecules; to stick together.

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# Static Electricity: Salt & Pepper Separater

Static Electricity: Salt & Pepper Separater

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To understand electricity, you have to first understand the atom, the basic building block of all matter. Matter is all of the “stuff” in the universe. Matter is made up of atoms. Atoms have a nucleus, an inner core that is made up of neutrons which have no charge, and protons which have a positive charge. Atoms also have orbitals, particles that are located around the nucleus. These are called electrons and these have a negative charge.

If you had some salt and pepper and you mixed the two together, how long would it take you? Not long if you know how to do it.

What You Need:

• Salt
• Pepper
• Plate
• Balloon
• Measuring Spoon

Blow up the balloon and tie it closed. Pour 1 teaspoon salt and 1 teaspoon pepper onto the plate and stir it around. Rub the balloon back and forth quickly on the top of your head. Now hold the balloon close to the plate. What happens to the pepper flakes? What happens to the salt flakes?

Here are some websites that will help you understand how static electricity works:

When you rub the balloon on your hair the friction caused by the hair and the balloon rubbing against each other causes the electrons from your hair to transfer to the balloon. This gives the balloon a negative charge. When you held the balloon over the dish of pepper, the pepper stuck to the balloon because the pepper has protons and a positive charge. Just like with magnets…opposites attract.

Science Experiments That Surprise and Delight Salt and Pepper Separator (page 10-11)

Words to Know:
Electricity – Energy that comes from charged particles like electrons or or protons.
Static Electricity – An electric charge that is caused by friction. It typically causes a spark or a crackling sound. It can also make light things move…like hair or dust or a feather.
Electron – The part of the atom that has a negative charge. Electrons are orbital, meaning they are not located in the atom’s nucleus.
Proton (positive) – The part of the atom that has a positive charge. It is located in the atom’s nucleus.
Neutron – The part of an atom that has no electric charge. It is located in the atom’s nucleus.  (Except hydrogen…that one kind of atom does not have any neutrons.)
Nucleus – The positively charged The central core of an atom. It is positively charged because that is where the proton’s are.

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# Supersaturated – Borax Crystals & Rock Candy

Supersaturated – Borax Crystals & Rock Candy

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Crystals are made when a substance has atoms or molecules that form in a very organized, repeating, 3D pattern. Usually when we think of crystals we think of some well-known gemstones like diamonds or rubies, but there are some very common crystals too. Sugar, ice, snowflakes, salt…all of these are crystals. You can make your own crystals grow.

You will need:

• Glass Jar
• Pencil or Pen
• String
• Pipe Cleaner
• Borax
• Pitcher
• Measuring Cup
• Tablespoon
• Hot Tap Water
• Piece of Yarn or Cotton String, about 6 inches long

You can make crystals using Borax – a detergent booster located in the soap section of the grocery store. We made a snowflake shape out of pipecleaners to see if we could make a snowflake crystal. Fill a pitcher with 3 cups hot tap water. (Not so hot that you can’t touch it!) Add 3 tablespoons of Borax for each cup of water (9 tablespoons!). Stir the mixture.

If all of the Borax dissolves, add a little more Borax and stir. Add Borax until the water can’t dissolve it anymore -  the mixture is saturated. That means the water is holding as much of the Borax as it can. In fact, this solution is supersaturated, that means the water is holding even more Borax than it normally would because the water has been heated. Now pour this supersaturated solution in the glass jar.

Make a shape out of the pipe cleaners and tie one end of the string to it. Tie the other end of the string to the middle of the pen. Hang the pipe cleaner shape down in the jar with the pen across the top of the jar to keep it from touching the bottom of the jar. Watch what happens in the jar over the next few weeks.

Here is what our crystals looked like after growing on the pipe cleaner snowflake for about 2 weeks. The secret to good crystals is having a supersaturated solution.

Science Project Idea: Grow three different borax crystal snowflakes. You need three glass jars that are exactly alike. Fill one with cold tap water and one with hot tap water. Get an adult to help you fill the last jar with boiling water. Now add Borax to each jar until the Borax will not dissolve anymore. The warmer the water, the more Borax will dissolve in the water. That’s because heating the water helps it become supersaturated. Now add the pipecleaner snowflakes and compare the crystals that grow over the next couple of weeks. Which jar has the most crystals? Which jar has the largest crystals?

This video shows you how to make rock candy – a cool crystal you an eat. To make rock candy you need to make a supersaturated solution. That means you have to heat up the water on the stove, so get a grown-up to help you with this one.

Here are some websites that give you ideas for making crystals. You can grow crystals using salt, sugar, baking soda and many other substances.

Here is a video that shows you some super fast crystal growing:

Here are some books that gave good directions for growing different kinds of crystals:

• Science Rocks! Crystal Creation Pipecleaner crystal snowflakes (pages 16-17)
• Prize Winning Science Fair Projects for Curious Kids – Crystal Creation (pages 81-82)
• Mixtures and Solutions – Solubility Try This! Growing Crystals (page 22-23)

Words to Know:
Crystal – A solid whose atoms or molecules are arranged in a 3-dimensional repeating pattern. Examples: A snowflake, a sugar crystal, a diamond.
Crystallized – The process of crystal growth or crystal formation.
Dissolve – To mix two substances together and have the molecules of one substance spread out between the molecules of the other substance.
Saturated - When a liquid is holding as much of a solid as it can. It has dissolved all of the solid it can hold.
Supersaturated – When a liquid is holding as much of a solid as it can…but then can dissolve a little more because it is heated.

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