States of Matter: Solids, Liquids & Gases - Third Grade Science
Everything you can see, touch, or hold in your hands—and even the invisible air filling your lungs right now—is made of matter. Matter is the scientific name for all the physical "stuff" that exists in the entire universe. Scientists classify matter into three familiar everyday states: solids, liquids, and gases. By looking closely at how particles behave inside each state, you can understand why an ice cube holds its shape, water splashes, and steam floats into the sky.
What Is Matter?
In science, matter is defined as anything that has mass and takes up space. Mass means that it contains a certain amount of physical material, and taking up space means that it has volume. A giant boulder, a droplet of morning dew, and the helium gas filling a birthday balloon are all examples of matter because they all have mass and volume.
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| THE THREE STATES |
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| SOLID: Definite shape | Definite volume |
| LIQUID: Changes shape | Definite volume |
| GAS: Changes shape | Changes volume (Expands!) |
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The Particle Theory of Matter
All matter is constructed from unimaginably tiny building blocks called atoms and molecules, which scientists refer to simply as particles. These particles are far too small to see with a regular classroom microscope, but they are constantly in motion. The way these particles are packed together and how freely they move determines whether matter is a solid, a liquid, or a gas.
Solids: Tightly Packed and Firm
A solid is matter that has a definite shape and a definite volume. Definite means fixed, unchanging, and stable under normal conditions. If you take a wooden building block out of a toy chest and set it on a desk, it stays a cube; it does not puddle out or swell up to fill the room.
Particle Arrangement in a Solid:
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| (O) (O) (O) (O) | - Particles packed tightly in a grid
| (O) (O) (O) (O) | - Strong bonds hold them in place
| (O) (O) (O) (O) | - Particles only vibrate in place
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Particle Motion in Solids
Inside a solid, particles are packed tightly together in orderly rows. Strong attraction forces lock the particles in place so they cannot slide past one another. The particles are not completely still, however; they vibrate gently back and forth in their fixed positions, like children sitting quietly in assigned theater seats. Because the particles cannot move around, solids maintain a rigid, permanent shape.
Liquids: Flowing and Adaptable
A liquid is matter that has a definite volume but does not have a definite shape. Instead, a liquid takes the shape of whatever container holds it. If you pour 200 milliliters of orange juice from a tall glass cylinder into a wide, flat bowl, the juice changes its shape to match the bowl, but you still have exactly 200 milliliters of juice.
Particle Arrangement in a Liquid:
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| (O) (O) (O) | - Particles close together
| (O) (O) (O) | - Weaker bonds allow movement
| (O) (O) (O) | - Particles roll and slide past each other
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Particle Motion in Liquids
In a liquid, the particles are still close together, but the attraction holding them is weaker than in a solid. This allows liquid particles to slip, slide, and tumble past one another freely. Scientists call liquids fluids because of this ability to flow. Because particles can move around, a liquid spreads out across the bottom of any container it enters, yet stays held together at a fixed total volume.
Gases: Free, Expanding, and Invisible
A gas is matter that has neither a definite shape nor a definite volume. A gas expands to fill the entire shape and volume of any container it enters. If you open a bottle of fragrant lavender oil in the front of a room, the gas particles spread quickly through the air until people in the back corner can smell it.
Particle Arrangement in a Gas:
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| (O) | - Particles very far apart
| (O) | - Virtually no attraction between them
| (O) | - Zoom rapidly in all directions!
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Particle Motion in Gases
In a gas, the particles have a tremendous amount of energy and move at blistering speeds. They are separated by huge empty gaps of space, with virtually no attractive forces holding them together. Gas particles fly in straight lines until they collide with each other or bounce off the walls of their container. Because of the vast empty spaces between gas particles, gases can be easily compressed (squeezed) into smaller tanks, like scuba diving cylinders.
Comparing the Three States of Matter
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| Property | Solid | Liquid | Gas |
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| Shape | Definite (Fixed) | Takes container | Fills container |
| Volume | Definite (Fixed) | Definite (Fixed) | Expands/compresses|
| Particle Spacing | Tightly locked | Close together | Far apart |
| Particle Movement | Vibrates in spot | Slides past each | Zooms freely |
| Can it be poured? | No | Yes | Yes (spreads) |
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The Mystery of Powders and Grains
Sometimes students wonder: "If sand and flour can be poured into a bowl and take the shape of the bowl, are they liquids?" The answer is no! Sand is made of millions of tiny, individual solid grains of rock. If you place one single grain of sand under a magnifying lens, it has its own rigid, definite shape and volume. Powders are collections of many miniature solids, not liquids.
Chapter Practice Exercises
Question 1: Explain why water can easily change its shape when poured into different containers, while an ice cube carved from the same water keeps its shape.
Question 2: Describe what happens to the spacing and movement of gas particles when air is pumped into a rubber bicycle tire.
Question 3: A student claims that air is not matter because you cannot see it or pick it up in your hands. Write a scientific argument explaining why air is matter, including proof of mass and volume.
Question 4: Complete the comparison: How are the particles of a liquid similar to the particles of a solid, and how are they different?
Solutions and Explanations
Solution 1: An ice cube is a solid whose particles are tightly locked in a rigid crystalline structure, vibrating in place without moving around, allowing it to hold its fixed shape. Liquid water has looser attraction between its particles, allowing them to tumble and slide past one another, which lets the liquid flow and adapt to the shape of any container.
Solution 2: When air is pumped into a tire, billions of fast-moving gas particles are forced into a confined space. The particles are compressed closer together, colliding constantly with one another and hammering against the inner walls of the rubber tire, which creates air pressure that keeps the tire firm.
Solution 3: Air is matter because it meets both scientific criteria: it has mass and it takes up volume. When you blow air into an empty balloon, the balloon visibly expands, proving that the air takes up physical space (volume). If you place an inflated balloon on a sensitive digital scale, it weighs slightly more than an empty balloon, proving that air has measurable mass.
Solution 4: Liquid particles are similar to solid particles because they are both packed closely together, meaning both solids and liquids have a definite, fixed volume that cannot be easily compressed. They are different because solid particles are locked in fixed positions and only vibrate, whereas liquid particles can slide and tumble past each other, giving liquids the ability to flow.