Solids, Liquids, Gases and the Water Cycle

Your pencil and the air around it are both matter. Matter has mass and takes up space. Mass tells you how much matter something has. A scale measures mass in grams or kilograms. The space something takes up is its volume.

Matter has three main states: solid, liquid, and gas. You can tell these states apart by what they do.

Look for a shape of its own

Put a wooden block on a tray, then inside a bowl. The block keeps its shape in both places. It also keeps its volume. The bowl does not make the block spread across its bottom. These are clues that the wood is a solid.

Bricks, coins, and ice cubes are solids too. You can stack solid blocks because each block holds its shape. Solid wood and bricks help a house stand up. Solid shoes keep a shape around your feet and protect them.

Soft things can be solid

A solid does not have to feel hard. A sweater stays in a heap when you put it down. It does not slowly spread into a puddle. Modeling clay stays in the shape you give it.

Try thinking about a rubber band in three steps:

1. Before you pull it, the band has its own shape.
2. Your pull stretches the band into a longer shape.
3. When you let go, it springs back.

The pull changed its shape. It did not turn the rubber into a liquid. Some solids spring back; others, such as clay, keep the new shape.

Look at each piece

Tear paper into scraps. Each scrap still has a shape of its own. Breaking a solid makes smaller solids.

Salt, sugar, and sand are made of many tiny solid grains. A magnifying glass helps you see each grain's shape. The grains can move past one another when you pour them. The grains themselves stay solid.

Pour water into a cup and watch where it goes. It flows down and fills the bottom. Water is a liquid, like milk, cooking oil, and juice.

Same amount, new shape

A liquid takes the shape of the container around it. It keeps its volume when you move it without losing any.

Imagine you measure 200 mL of juice. Here is how to check what happens when you pour it:

1. Pour the juice from a measuring cup into a wide bowl.
2. Notice that the juice now looks wider and shallower.
3. Pour all of it back into the measuring cup.
4. Read the level again. You still have 200 mL.

The low level in the bowl does not mean less juice. The same amount has spread across a wider bottom. Spilling, drinking, or evaporation can change the amount; changing the container does not.

You measure liquid volume in milliliters, written mL, or liters, written L. One liter is 1,000 milliliters.

A pool with a level top

Water poured onto a tray spreads into a pool. When it stops moving, its top is flat and level. It does not build a pointed pile like sand. Gently tilt a half-full clear cup. The cup tilts, but the still water's top stays level.

A liquid fills the lower part of its container. It does not spread through all the space above it as a gas does.

Slow flow still counts

Honey takes longer to pour than water. Watch it for long enough, and you see it flow. It settles into the shape of the jar. Thick liquids, such as syrup and shampoo, are still liquids.

Liquids are also very hard to squash into less space. In a plastic syringe with no needle, trapped water hardly squashes. Trapped air squashes much more easily.

Flow makes liquids useful. Water travels through pipes, and paint spreads across paper.

You cannot see most gases, but you can find out what they do. Feel wind on your face. Watch a beach ball grow as you blow air inside. Air is matter: it takes up space and has mass.

Find the air in a cup

An "empty" cup already holds air. Imagine pushing it upside down into a bowl of water:

1. The air stays trapped inside the cup.
2. It takes up space, keeping water out of that space.
3. Tip the cup. Air escapes as bubbles, and water enters.

The bubbles give you evidence of air you could not see before. A bottle glugs as you pour because air must enter to replace the water.

A gas spreads out to fill its container. It has no fixed shape or fixed volume. Air fills a round balloon or a long balloon. When gas escapes, it spreads into the surrounding air.

Squash it and weigh it

Trap air in a plastic syringe with no needle. Cover the opening and push the plunger. The air fits into less space and pushes back. Air pumped into tires and balls makes them firm. Trapped air also helps an air mattress hold you up.

Gases have mass. Near sea level at room temperature, one liter of air has a mass of about 1.2 g. Pumping more air into a basketball adds a few grams. A sensitive scale can show that extra mass. Being invisible does not make air weigh nothing.

Meet some gases

Air is a mixture, mostly nitrogen and oxygen. You use oxygen when you breathe. Your breath contains more carbon dioxide when you breathe out than when you breathe in. Carbon dioxide also makes soda bubbles. Helium helps party balloons float. Water vapor is water in its gas state. Smells can spread through air as gases too.

The visible part of smoke is tiny solid bits and liquid drops carried in gases. The white mist above a hot drink is tiny liquid water drops. Water vapor itself is invisible.

Pouring alone will not tell you a material's state. Sand pours, and so does water. Look closely at the material and use more than one clue.

Compare what each state does

Liquids and gases both flow. Gas squashes easily. Most solids and liquids are much harder to squash, though holes filled with air can make a solid object squashy.

Grains and holes

Pour flour onto a dry plate. It makes a pile. Look closely: the flour is many tiny solid grains. The pile changes shape as grains move, but each grain stays solid.

A sponge has a solid frame with many holes. When you squeeze it, the frame bends. Air or water can leave its holes. Let go, and the frame springs back. Its squashiness does not make it a gas.

Soap foam contains liquid with gas bubbles inside. Soda also contains two states: liquid drink and gas bubbles. Name the state of each part.

Work through a mixed group

Suppose you have a wooden spoon, orange juice, salt, syrup, and a sponge.

• The spoon keeps its shape, so it is solid.
• The salt is solid because each grain holds its shape.
• The sponge has a bendy solid frame.
• The juice flows into its container's shape, so it is liquid.
• The syrup does that too, although it flows slowly.

Explain with a "because" sentence. "Salt is solid because its tiny grains each keep their shape" gives a reason someone can check.

One material, different states

Ice, liquid water, and water vapor are all water. Their states differ. Temperature helps decide the state you find. Chocolate may be solid in a room around 20 °C (68 °F), but runny in a hot car. Describe what a material is doing now; its name alone does not tell you its state.

Your hand can tell you something feels warm. A thermometer gives you a number for how hot or cold it is. That number is its temperature. Metal and wood in the same room can feel different, even at the same temperature.

Find the Celsius scale

Science temperatures use degrees Celsius, written °C. Say 20 °C as "twenty degrees Celsius." Many home thermometers and US weather reports use Fahrenheit, written °F. Check the label before reading a scale.

These temperatures help you picture the numbers:

In a liquid thermometer, colored liquid rises as it warms. It falls as it cools. Read the top of that liquid against the scale. A digital thermometer shows a number on a screen.

Work out one space first

Look at a scale labeled 20 and 30, with five equal spaces between them.

1. The numbered marks are 10 degrees apart.
2. Five spaces share those 10 degrees, so each is 2 degrees.
3. Two spaces above 20 gives 22, then 24 °C.

Do not assume every small space means one degree. On this scale, three spaces above 20 means 26 °C.

Go below zero

A minus sign means below zero. At −5 °C, you are five degrees below 0 °C. Going downward gives −1, −2, −3, and so on. Farther below zero means colder: −10 °C is colder than −5 °C. A home freezer is around −18 °C (0 °F).

To find a temperature change, count the degrees between readings. From 18 °C to 25 °C is a rise of 7 degrees. From 4 °C down to −3 °C is a fall of 7 degrees: four to reach zero, then three more below it.

A careful reading lets you compare temperatures. Use the same method each time, so a change in your method does not spoil the comparison.

Place, wait, read

For a liquid thermometer, follow these steps:

1. Hold its top end. Your fingers can warm the bulb.
2. Put the bulb into the water, clear of the cup's sides and bottom.
3. Wait until the colored liquid stops moving, usually about a minute.
4. Bring your eyes level with the top of the colored liquid.
5. Read the scale and write the temperature with °C.

Reading at once can give the thermometer's old temperature. Reading from above or below makes the scale harder to line up. Outdoors, measure air temperature in shade. Sunshine warms the thermometer itself.

Handle glass thermometers gently; tell an adult if one breaks. An adult heats and handles hot water. Watch boiling water from a safe distance, with your hands and face away from the vapor: both can burn.

Make a table

Give each column a name and unit. You can then put just numbers in its boxes. Here are example readings from stirred ice and water in a warm room:

In this example, the last ice melts at 15 minutes. The readings rise after that. Your own experiment may take a different time, so record what you measure.

Turn the table into a line graph

Put time along the bottom and temperature up the side. Label both with units. Choose equal spaces for equal amounts. Plot each table row as one point: for example, 20 minutes across and 3 °C up. Join points in time order.

A flat line means the temperature stayed the same. A rising line shows warming; a falling line shows cooling. A flat line does not mean nothing happened: ice may still be melting.

For a fair comparison, use the same thermometer, waiting time, and reading method. Repeat measurements. If a reading is very different from the rest, check it again before using it.

An ice pop left in warm air becomes runny. It has changed state from solid to liquid. Heating adds heat energy, also called thermal energy. Cooling takes heat energy away.

Two opposite changes

Melting is a solid changing into a liquid as it takes in heat. Freezing, also called solidifying, is a liquid turning solid as it loses heat.

Freezing does not always feel icy. Melted wax turns solid as it cools, even while it feels warm. Melted chocolate can turn solid in a cool room. Each material has its own temperature for becoming solid.

Compare melting points

A melting point is the temperature at which a solid melts. Pure ice melts at 0 °C (32 °F). Pure water freezes at that same temperature.

Different materials need different temperatures. Chocolate starts melting around 34 °C (93 °F), though different chocolates melt over slightly different ranges. Your body is warmer than this, so chocolate can melt in your hand. Iron needs about 1,538 °C (about 2,800 °F), reached in a furnace. Melting temperature helps you compare materials.

Use a temperature to find the state

Here is how to think about water under ordinary conditions:

• At −5 °C, it is below its freezing point: solid ice.
• At 12 °C, it is above its melting point: liquid water.
• At 0 °C, ice and liquid water can be together.

Do not choose just one state at exactly 0 °C. A glass containing both ice and water shows why both can be there.

Why melting ice stays at zero

Imagine stirring ice and water while heat enters from the room. Ice keeps melting, but the mixture stays around 0 °C. The incoming heat is being used for the change of state. After the last ice melts, further heating warms the liquid water.

The ice has become water; it has not vanished. Heat enters from warmer surroundings to melt it. Saying "cold came out" misses the cause: it is the heat going in that changes the solid into liquid.

Water can feel very hot before it boils. Boiling begins when vapor bubbles keep forming throughout the liquid. The temperature where this happens is its boiling point.

Pure water boils at 100 °C (212 °F) at sea level. Water at 50 °C is hot, but still well below that boiling point.

Follow a bubble

Before boiling, tiny bubbles may gather on a pot's sides. These are mostly air leaving the warming water. Boiling looks different:

1. Larger bubbles keep forming inside the liquid.
2. They rise through the water.
3. They burst at the surface and release water vapor.

The boiling bubbles contain water as a gas. They are not little pockets of heat. Heat causes the change; water vapor is the material inside the bubbles.

What more heat does

Once water boils in an open pot at sea level, its temperature stays about 100 °C. More heat turns liquid into vapor faster. You get more bubbling, and the water level falls faster.

Suppose an adult turns up the heat under already boiling water. The reading does not keep rising just because the stove setting rises. The extra heat is being used to change more water into gas.

The white cloud is liquid

Water vapor itself is invisible. Near a kettle spout, there may be a clear gap before the white mist. As the vapor mixes with cooler air, tiny liquid drops form. Those drops make the white cloud people call "steam."

The clear gas and the visible mist are different states of water. The gas cools and becomes liquid drops.

Boiling high in the mountains

Water's boiling point is lower in high places. In Denver, Colorado, about 1 mile (1.6 km) above sea level, water boils around 95 °C (203 °F). Boiling water there is cooler than boiling water at the coast. Some foods need longer cooking in high places because of that lower temperature.

Freeze melted ice and you get ice again. A reversible change can be undone. Changes of state are reversible: the material stays the same even when its state changes.

Undo a change

Melting and freezing undo each other. Evaporation or boiling turns liquid into gas; condensation turns the gas back into liquid. These are physical changes. The same water can be ice, liquid, or vapor.

Baking a cake is different. Baking makes new materials. Cooling the cake does not turn it back into separate flour, eggs, and sugar. Burning wood and frying an egg also make new materials.

Check the mass

Imagine sealing ice in a bag and putting it on a scale. The bag and its contents have a mass of 60 g.

1. Leave the bag sealed while the ice melts.
2. Put the same bag back on the scale.
3. The reading is still 60 g. No matter entered or left.

Changing state does not remove mass. In an open dish, water can evaporate and escape into the room. The dish then has less mass because some water has left it. That water still exists in the air.

Same mass can take more space

Water takes up more space when it freezes. About 100 mL of liquid water becomes 109 mL of ice. No extra water was made: the same water now fills more space.

This is why freezing water can split a full bottle or burst a pipe. Water needs extra room as it turns to ice. Many other materials take up a little less space when they become solid; water's increase is unusual.

Ice floats because it has less mass than the same volume of liquid water. A cupful of ice has less water matter than a cupful of liquid water. You can see ice cubes floating in a drink. Ice also forms on the top of a pond.

People choose a material's state to help it do a job. A liquid can flow into a mold. A solid can hold the new shape after cooling.

Make a chocolate shape

An adult can help you see these three stages:

1. Warm chocolate gently until it melts.
2. Pour the liquid into a shaped mold. It flows into the corners.
3. Cool it until it is solid. Take it out: it keeps the mold's shape.

The chocolate changed shape and state, but stayed chocolate. Cooling matters as much as heating: liquid chocolate would flow out if you removed it too soon.

Candle makers use melted wax in a similar way. Crayons begin as colored wax that flows into molds and cools. Some plastics can also be melted, molded, and cooled to make useful objects.

Use old material again

Recycling can use changes of state. Aluminum cans are melted in a hot furnace. Aluminum melts at about 660 °C (about 1,220 °F). The liquid metal can be shaped and cooled into solid metal again. Old glass is also heated until it can flow and be reshaped.

A melting point helps people choose a material for a job. A metal cooking pot can stay solid while food heats inside it. A pot made of wax would lose its shape on the stove.

Keep a picnic cool

Ice takes in heat as it melts. In a cooler, some of that heat comes from the food and drinks around it. This helps keep them cold while the ice changes into liquid water.

You use other state changes at home too. Juice freezes into ice pops. Butter melts in a pan. Water boils when an adult cooks pasta.

In nature, melting snow feeds streams. Freezing weather turns pond water to ice. People also use evaporation: wet laundry dries as water enters the air, and shallow salt ponds leave salt behind as their water evaporates.

A puddle can dry on a cool day. Its water becomes water vapor, an invisible gas that mixes into the air. This change is evaporation.

Evaporation happens at a liquid's surface. It can happen far below the boiling point. Your wet hair and clothes dry without reaching 100 °C.

Find how much evaporated

Imagine marking 100 mL on a dish of water. Three days later, 70 mL remains. Nothing spilled, and no one added or removed liquid water.

The amount that left the dish is the difference between the readings:

So 30 mL of liquid water has evaporated. The water has moved into the air as gas. It has not turned into nothing.

Compare evaporation and boiling

Both changes make water vapor. A quiet puddle drying is evaporation; water bubbling all through a boiling pot is boiling.

Cover a dish with plastic wrap. Vapor stays inside and can form drops underneath the wrap. Some drops return to the pool. The water level drops much less than in an uncovered dish.

What stays behind?

When salty water evaporates, the salt stays behind. After the water leaves a shallow saucer of salt water, you can see salt crystals. The water that entered the air did not carry that salt with it.

Evaporation also takes heat from its surroundings. As sweat evaporates, it takes heat from your skin and helps cool you. Water evaporating from your skin after swimming can make you feel cold, especially in moving air.

The Sun warms puddles and helps their water evaporate faster. It does not suck the liquid upward. The liquid changes into gas at its surface.

The rate of evaporation means how fast a liquid evaporates. You can change that rate by changing the conditions around the water.

Warmer water evaporates faster. Moving air helps it evaporate faster too. Spreading water across a wide surface exposes more water to the air. That is why an open towel dries faster than a crumpled one. Damp air slows drying because it already contains a lot of water vapor.

Plan a fair test

A fair test changes one thing while keeping other things the same. To test warmth, an adult prepares two identical, sealed water bags. One holds warm tap water; the other holds room-temperature water. Lay both flat, side by side.

1. Cut two equal squares from the same paper towel.
2. Put 10 drops of room-temperature water on each square.
3. At the same time, place one square on each bag.
4. Keep their size, position, and airflow alike.
5. Check both every five minutes and record wet or dry.

The sealed bags warm the towels without adding escaping water from inside. Measure the bags' water temperatures before sealing. These show how warm the heating water starts; the towels themselves may be cooler.

Before testing, make a prediction with a reason: "I think the towel on the warm bag will dry first because warmth speeds evaporation."

Read the results

Here are example results. One bag starts at 45 °C; the other starts at 20 °C. The warmer towel is first seen dry at 10 minutes. The cooler towel is first seen dry at 25 minutes. That is 15 minutes later. The result supports the prediction.

The cooler towel dried too. Cooler water still evaporates; it takes longer.

To test moving air, keep warmth and wetness the same and change only airflow. To test surface area, use equal water amounts in a wide dish and a narrow one under matching conditions. If you change both warmth and dish width, you cannot tell which change caused faster drying.

Take a sealed drink can out of the refrigerator. Drops soon appear on its outside. That water came from the air around the can.

Condensation happens when a gas cools and turns into liquid. Water vapor can condense on a cold surface or as the air itself cools.

Follow the water onto a can

1. The air around the can contains invisible water vapor.
2. Vapor near the cold metal cools.
3. It turns into tiny liquid drops on the outside.
4. Drops join to make bigger drops you can see.

The can does not need to leak for this to happen. A cold spoon can become wet too, and it has no drink inside. This helps you work out where the water came from.

See the same change elsewhere

A bathroom mirror gets covered in tiny drops after a shower. Water vapor in warm, damp air cools at the mirror. Your breath can also fog a cold window because it contains water vapor.

Dew forms as liquid drops on cool grass or spider webs. Fog is tiny liquid drops floating in air near the ground. Clouds can contain tiny liquid drops, tiny ice crystals, or both.

You can see the drops in fog, but not the water vapor they came from. The white mist above a hot drink is also liquid drops. Calling visible mist "water vapor" mixes up the gas with the liquid made when it cools.

Put the two directions together

Evaporation changes liquid water into water vapor and takes in heat. Condensation changes water vapor back into liquid and gives out heat. They undo each other.

A covered pool of water can show both changes. Water evaporates from the pool, then condenses on a cooler cover. The drops can fall back. The water moves between a visible liquid and an invisible gas without becoming a new material.

Water travels between oceans, air, and land again and again. This is the water cycle. It moves the same water around Earth; it does not make new water.

From water to clouds

The Sun supplies heat. Water evaporates from oceans, lakes, rivers, and wet ground. Most water vapor in the air comes from the oceans.

Plants add vapor too. Roots take in water from soil, and water escapes through tiny holes in leaves. This is transpiration.

Warm, damp air can rise and cool. Its water vapor condenses into tiny drops. Large numbers of drops make a cloud. Cold clouds may also contain ice crystals. Water vapor can turn straight into solid ice as crystals grow.

Water falls and collects

Cloud drops join and grow. When they become too heavy to stay up, they fall. Precipitation is water falling from clouds as rain, snow, sleet, or hail. A cloud does not need to become "full" or get squeezed.

Snowflakes grow from ice crystals as vapor turns straight into ice. Hail grows when liquid drops freeze onto pieces of ice inside storm clouds. Snow growth can be gas to solid; freezing onto hail is liquid to solid.

After falling, water can take different routes:

• Runoff flows over land into streams and rivers.
• Some water soaks down into the ground as groundwater.
• Lakes and oceans collect water that can evaporate again.
• Snow and ice can store water until they melt.

Link the changes

Try this route: "Ocean water evaporates, so it enters the air as vapor. The vapor cools and condenses into cloud drops. The drops grow and fall as rain. Rain flows into a river and back to the ocean."

Evaporation and condensation change water's state. Liquid runoff stays liquid as it travels downhill. Movement and state change are different things.

The cycle has no first step or finish. One drop may soon return to the sea. Another may stay underground for many years. You can begin explaining at any point and follow the arrows from there.

Earth keeps water in many places: oceans, ice, rivers, lakes, soil, air, and living things. Water below the ground is another large store. The water cycle moves water between these places.

Salt water and fresh water

About 97 out of every 100 liters of Earth's water are salty. Most of that salty water is in the oceans. Fresh water contains very little salt. Most fresh water is frozen in ice sheets and glaciers or stored underground. Rivers and lakes hold only a tiny share.

Rain can be fresh even when its water came from the salty ocean. Evaporation leaves the salt behind. Water vapor travels through the air and later forms fresh drops. Seawater needs its salt removed before people can drink it. Fresh water also needs checking and cleaning to be safe to drink.

Find two kinds of water store

Surface water lies on top of the land. You can see it in streams, ponds, rivers, and lakes.

Groundwater fills tiny spaces below ground, between soil grains and pieces of rock. Picture a wet sponge: water fills many small spaces. Groundwater is not all one huge underground lake.

People can reach groundwater through wells. They also collect water from rivers, lakes, and rainwater tanks. A reservoir stores water for use; many are lakes held back by dams.

Trace a drink backward

Imagine a town taking its drinking water from a reservoir.

1. Your faucet gets cleaned water through pipes.
2. A water treatment plant cleaned water from the reservoir.
3. A river carried rainwater into the reservoir.
4. Rain fell on nearby hills from clouds.
5. Some of the cloud water may once have evaporated far out at sea.

Other towns may use a different water source, such as wells. The water cycle refills rivers, lakes, and groundwater with fresh water. During a drought, little rain falls for a long time. Water stores can shrink, leaving less water for people, plants, and animals.

A diagram helps you show where water goes and why. Draw the Sun, ocean, land, clouds, and a river. Add plants and the ground so you can include more than one route.

Make each arrow mean something

Draw evaporation arrows from water into the air. Draw a transpiration arrow from leaves into the air. Condensation labels show vapor becoming cloud drops. Precipitation arrows point down from clouds. Runoff arrows follow the land downhill toward rivers and the sea. Add a route into the ground for groundwater.

The Sun gives energy, not water. Show its heat separately from the arrows that follow water. An arrow from the Sun labeled "evaporation" would start in the wrong place.

Make a small working model

Put about 100 mL of water and a drop of blue food coloring in a zip-top bag. Seal it and tape it securely to a sunny window.

Watch over several hours:

1. The Sun warms the pool at the bottom.
2. Some water evaporates into invisible vapor inside the bag.
3. Vapor cools at the plastic and forms clear drops.
4. Bigger drops run down and return to the pool.

The coloring stays in the pool as water evaporates. The clear drops help show that water left the colored liquid and returned elsewhere.

The sealed bag keeps its water inside. It shows evaporation, condensation, and water returning downward. Its running drops stand in for rain. Real rain falls from clouds, not down a plastic wall. The bag also leaves out rivers, wind, land, and plants. A model helps explain some parts of the real world without showing every part.

Explain causes along a route

Link steps with "because" and "so." For example: "Damp air is pushed up a mountain, so it cools. Water vapor forms cloud drops, which can grow and fall as rain." The far side of a mountain often gets less rain.

Temperature changes the route too. Hot weather speeds evaporation. In cold weather, snow can hold water on the land until warmer weather melts it. Your explanation should connect what the water does with what makes it happen.