Unit 6 · Matter and Waves

Pick up a rock. It feels heavy, and it fills your hand. Now think about the air in the room. You cannot see it, and it does not feel heavy. Yet the rock and the air have something in common. Both are matter. Matter is anything that has mass and takes up space.

Mass is the amount of stuff in an object. A balance measures mass by comparing an object to known weights. A bowling ball has more mass than a tennis ball. A cup of water has more mass than an empty cup. Mass does not change when you carry an object to a new place.

Air is the tricky one. Push an empty cup upside down into a sink full of water. The water does not fill the cup. Something is already inside, pushing back. That something is air. Air takes up space. Later in this chapter you will see that air has mass too.

Tear a piece of paper in half. Tear it again, and again. Soon the pieces are too small to tear. Now imagine a tool a million times sharper. You could keep going until you reached the smallest pieces of the paper. Scientists call these tiny pieces particles. Every kind of matter is made of them.

Particles are so small that no eye can see one. No classroom microscope can either. A single drop of water holds more particles than there are people on Earth. Many times more. Because we cannot see them, scientists use a model. A model is a picture or idea that stands in for something real.

Here is a simple particle model. In a solid, the particles are packed tight and only jiggle in place. In a liquid, they stay close but slide past each other. In a gas, they fly apart and zoom around. Try this model on ice, water and steam. It is the same water, with the particles moving in three different ways.

A gas is matter whose particles spread out to fill any space they are given. Air is a mix of gases. Because you cannot see air, it is easy to believe it weighs nothing. A balance says otherwise. Here is a test you can do with a stick, some string and two balloons.

Blow up one balloon as big as you can and tie it. Hang it from one end of the stick, and hang a matching empty balloon from the other end. Slide the string until the stick hangs level. Now pop the full balloon with a pin. The end with the empty balloon drops, and the popped side rises. The air that escaped had mass.

The difference is small, because a balloon holds only a little air. A whole room is another story. The air in a classroom can have a mass of more than 100 kilograms, more than the mass of an adult. You walk through it every day without noticing.

Put some ice cubes in a sealed plastic bag and set it on a balance. Write down the mass. Leave the bag in a warm spot. An hour later the ice is a puddle inside the bag. Put it back on the balance. The reading is the same. Melting changed the ice from solid to liquid, but no particles left the bag.

This is what the particle model predicts. When ice melts, warmth makes its particles jiggle faster. They break out of their tight pattern and begin to slide past each other. The same particles are still there. They are just moving more freely. None are made and none are lost.

The rule works the other way too. Freeze the bag and the mass stays the same. Scientists call this idea conservation of mass. Conservation means the total stays the same. It is one of the most tested ideas in all of science, and it has passed every fair test.

Drop a sugar cube into warm water and stir. The cube shrinks and then vanishes. The water stays clear. Has the sugar stopped existing? Taste the water and you have your answer. It is sweet. The sugar dissolved. To dissolve means to break into particles so small they spread all through a liquid and cannot be seen.

The balance agrees with your tongue. Weigh a cup of water and a spoon of sugar together before you mix them. Weigh them again after the sugar dissolves. The mass is the same. Every particle of sugar is still in the cup, just spread out. Salt does the same thing. So does drink powder.

You can even get the sugar back. Pour a little of the sweet water onto a dark plate. Leave it in a warm, dry place for a few days. The water slowly changes into a gas and drifts away. The sugar cannot do that. It stays behind and builds up into tiny crystals. Dissolving hides the sugar, but it does not destroy it.

A mixture is two or more materials stirred together while each keeps its own particles. Sand and gravel make a mixture. So do salt and pepper, or trail mix. The pieces might be big or too small to see, but nothing new is made. With patience, you could sort them out again.

Weigh a cup of sand. Weigh a cup of pebbles. Add the two numbers to get a total. Now pour them together and weigh the mixture. The balance matches your total. It works with water and food coloring too. The color spreads everywhere, but the balance reads exactly the water plus the drops.

This is conservation of mass again. Mixing moves particles around, but it does not make particles appear or disappear. When a measurement seems to break this rule, scientists get curious. Almost always, some matter slipped away where nobody was looking. The next lesson shows how.

Pour vinegar into a cup of baking soda. It fizzes and foams. Weigh the cup before and after. This time the balance reading drops! Was matter destroyed? Look at the fizz. Bubbles rose and popped. A gas formed and floated out of the cup into the room. The missing mass left with that gas.

Try it again in a bottle with a balloon stretched over the top. Put the baking soda in the balloon and the vinegar in the bottle. Weigh it all. Then lift the balloon so the powder falls in. The balloon puffs up with the new gas. Weigh it again. Now the mass is the same. Nothing got out, so nothing was lost.

A container that lets nothing in or out is called a closed system. In a closed system, mass never changes, no matter what happens inside. When a puddle evaporates and disappears, the water became a gas and joined the air. The world’s water has not shrunk. It has only moved.

Two white powders sit in cups. One is sugar and one is salt. They look alike. How can you tell them apart without tasting? You test their properties. A property is a feature you can observe or measure. Color, shine, hardness and how a material acts near a magnet are all properties.

Hardness is how well a material resists being scratched. Try to scratch a penny with your fingernail. You cannot. Try to scratch a piece of chalk. You can. Steel scratches glass, and diamond scratches steel. Scientists rank minerals by hardness on a scale from 1 to 10, with diamond at the top.

Magnetism is another quick test. Hold a magnet near a paper clip and the clip jumps. Hold it near an aluminum can and nothing happens. Iron and steel are pulled by magnets; most other metals are not. A magnet is a fast way to sort a pile of mixed metal scraps.

Some materials let electricity flow through them. Copper wire does; a rubber eraser does not. This property is called conductivity. Metals are good conductors of electricity. Many are also good conductors of heat. That is why a metal spoon in hot soup gets hot fast, and a wooden spoon does not.

Solubility is whether a material dissolves in a liquid. Salt and sugar are soluble in water. Sand and chalk are not. The amount matters too. Water can hold a lot of sugar but much less baking soda. Testing how much dissolves is one safe way to tell white powders apart.

Whether something floats in water is a property as well. Wood floats; a steel nail sinks. This depends on how much mass is packed into a given space. Put several properties together and you have a fingerprint for a material. Detectives, doctors and engineers all identify materials this way.

Stir salt into water and it disappears. Yet nothing new has been made. The salt particles and the water particles simply sit side by side. This is a mixture. You can prove it by getting the salt back. Boil the water away, and salt crystals are left in the pan. They taste like salt and look like salt.

A substance is one pure kind of matter with its own set of properties. Salt is a substance. Water is a substance. Salt water is a mixture of two substances. Its properties are a blend. It is wet like water and salty like salt. That is a sign that nothing new formed.

Ocean water is a huge natural mixture. People near the sea have collected salt from it for thousands of years by letting the sun dry shallow pools. Closer to home, when winter road salt washes into a river, the salt is still salt. Mixing did not change what it is. You could separate it out again.

Now mix baking soda and vinegar. This time the change is different. The mix fizzes, bubbles pour out, and the cup turns cold. When the fizzing stops, the baking soda is gone, and drying the liquid will not bring it back. Something new was made. The bubbles are carbon dioxide, the same gas that makes soda pop fizz.

When mixing makes new substances, we call it a chemical change. The particles rearrange into different groups. The new substances have their own properties. Baking soda is a powder that does not fizz by itself. Carbon dioxide is a gas that can put out a candle flame. New properties mean new substances.

Clues of a chemical change are bubbles, a new color, a new smell, or a change in temperature. Rust forming on a bike is a chemical change. So is bread rising in the oven. Not every clue is certain, so scientists test what was made. And in a closed container, the total mass still stays the same, even here.

Tie one end of a jump rope to a doorknob. Pull it tight and snap your wrist up and down. A bump races along the rope to the door. The rope did not travel. Each piece of rope moved up and down, then came back. The bump is a wave. A wave is a pattern that moves through something.

Drop a pebble in a still pond. Rings spread outward. Now watch a leaf floating nearby. As the rings pass, the leaf bobs up and down. It does not race away with the rings. Water waves move the water up and down, and the pattern rolls on. The water stays close to where it was.

What travels with the wave is energy. Energy is the ability to make things move or change. A big wave on Lake Michigan can toss a boat and push sand up the beach. The water in that wave did not come all the way across the lake. The energy in the wave did.

Waves come in sizes. Snap the jump rope gently and a small bump travels. Snap it hard and a tall bump travels. The height of the wave above its resting line is its amplitude. Bigger amplitude means more energy. Ocean waves with a big amplitude are the ones that knock you over.

Now shake the rope up and down over and over. You see a chain of bumps and dips. The top of each bump is a crest. The distance from one crest to the next crest is the wavelength. Shake faster and the crests crowd together, so the wavelength gets shorter. Shake slower and it stretches out.

You can describe any wave with these two measurements. Scientists draw a wave as a wavy line. Then they label the amplitude and the wavelength. This site has an oscilloscope. There you can change each one and watch what happens. Try making a wave that is tall and long. Then make one that is short and small.

Waves do work. Put a toy boat in a bathtub and slap the water at the other end. The boat rocks. Slap harder, with a bigger amplitude, and it rocks more. The energy of your slap traveled through the water and reached the boat. That is how waves make objects move.

Sound is a wave too. It travels through air as a pattern of pushes, a vibration that spreads out. Stand next to a big drum when someone strikes it. You feel the thump in your chest. Hold a sheet of paper near a loud speaker and it flutters. The air carried a wave to the paper and made it move.

Big waves can do big things. Storm waves on Lake Michigan have thrown chunks of ice and stones onto the Chicago lakefront. Earthquake waves shake the ground and can crack roads. Engineers who build harbors and bridges have to plan for the energy a wave brings.

Sit in a closet with the door shut and the light off. Wait. No matter how long you wait, you cannot see the shelf in front of you. Your eyes work fine. There is just no light. Now open the door a crack. Light slips in, hits the shelf, and reflects toward your eyes. Now you can see it.

To reflect means to bounce off. Light leaves a source like the sun, a lamp or a flashlight. It travels in straight lines until it hits something. Some of it bounces off. If the bounced light reaches your eyes, you see the object. The eye does not send anything out. It only catches light coming in.

For a long time many people believed the opposite. They thought the eyes sent out beams that felt objects. About a thousand years ago a scientist named Ibn al-Haytham argued from experiments that this could not be true. Looking at the sun hurts because light is coming in, not going out.

Shine a flashlight through a dusty room. The beam is a straight bar of light. Light does not bend around corners on its own. That is why you cannot see around a corner, and why a wall blocks a lamp. When an object blocks light, a dark patch forms behind it. That patch is a shadow.

A mirror is a very smooth, shiny surface. Light bounces off it at the same angle it came in, like a ball off a wall. Because the bounce is so neat, the light forms a clear picture. A rough surface, like paper, scatters light in every direction. You can see paper, but you cannot see yourself in it.

You can send a beam around a corner with mirrors. Point a flashlight at a mirror at the right angle and the beam turns. Add a second mirror and it turns again. A periscope uses two mirrors so a person in a submarine can see above the water. Light only goes straight, but mirrors can redirect it.

Put a straw in a glass of water and look from the side. The straw looks broken at the water line. It is not. Light from the underwater part of the straw bends as it leaves the water and enters the air. Your eye follows the bent light and sees the straw in the wrong place. This bending is called refraction.

Light bends because it slows down when it enters water or glass. In water, light moves at about three-quarters of its speed in air. The change in speed turns the beam. Think of a shopping cart. One wheel rolls onto grass while the other stays on the sidewalk. The cart swings toward the grass. Light does the same at the edge of the water.

A lens is a curved piece of glass or plastic that uses refraction on purpose. A lens that bulges in the middle bends light toward a point. That is how a magnifying glass makes small things look big. It is also how the microscope and the telescope on this site work. Your own eye has a lens, too.

A code is an agreement about what patterns mean. In the 1830s and 1840s, Samuel Morse and Alfred Vail built a code out of short and long electric pulses. A short pulse is a dot. A long one is a dash. The letter S is three dots. The letter O is three dashes. Together, they spell the emergency call SOS.

In 1844, Morse sent the first public message over a wire from Washington to Baltimore. The pulses arrived almost at once. Before that, a letter took days. Within a few years, telegraph wires reached Chicago, and news could cross Illinois in minutes. A signal is any pattern sent to carry a message.

Morse code works with anything that can be on or off. A flashlight can blink it. A whistle can chirp it. Ships still use signal lamps to flash it between decks. The wave that carries it does not matter, as long as the sender and the receiver agree on the code.

Long before wires, people sent messages with sound. In parts of West Africa, drummers used special drums to send news from village to village. A trained listener far away could hear the rhythm and understand it. Rhythm is a pattern of beats and pauses in time. The sound wave carried the pattern through the air.

A drum message has parts you can change. You can hit hard or soft, which changes the amplitude of the sound wave. You can hit fast or slow. You can leave long gaps or short ones. Each choice is a piece of the pattern. Put the pieces together and you have a signal.

Try it with a partner and the drum machine on this site, or a desk and your hands. Agree on three patterns: one for yes, one for no, one for help. Then turn your backs and send messages. If the patterns are too alike, mistakes happen. Good codes use patterns that are easy to tell apart.

A phone or a computer sends everything as binary. Binary uses only two signals, written as 1 and 0. A 1 can be a flash of light, a pulse of electricity or a burst of radio. A 0 is a gap. Each 1 or 0 is called a bit. Bits are the smallest pieces of a digital message.

How can two signals say anything? By making patterns. With one bit you can make two patterns: 1 or 0. With two bits you get four: 00, 01, 10, 11. Every extra bit doubles the count. Eight bits make 256 patterns, enough for every letter, number and mark on a keyboard. The letter A, for example, is 01000001.

Every photo you have ever sent was a long string of bits. Some traveled as flashes of light down a glass thread thinner than a hair. Some traveled as radio waves from a Wi-Fi router. Some ran as electric pulses along copper wires. Different waves, same code: on and off, over and over, very fast.

Every wave gets weaker as it travels. Shout across a field and your voice fades. Along the way, other sounds mix in. Scientists call this unwanted mix noise. A soft, smooth signal like a voice is hard to fix once noise gets in. You cannot tell which wobbles were the message.

A digital signal is different. The receiver only has to decide one thing: was that on or off? Even a weak, fuzzy flash is clearly not a gap. So a receiver can read the bits, make a fresh, strong copy, and send it on. Do this every few miles and the message crosses an ocean as clean as it started.

That is why a text from Chicago reaches a friend in another country without a single wrong letter. It is also why Jamal’s flashlight worked across a noisy gym. Rosa never had to hear his voice. She only had to see light or no light. Simple patterns are hard to break.