Every substance, whether solid, liquid, or gas, is made up of tiny particles (atoms or molecules) that are always in motion. These particles are far too small to see individually, but their behavior determines everything about how a substance looks, feels, and behaves. Here’s what science has confirmed about the particles in any substance.
Particles Are Always Moving
The single most important fact about particles is that they never stop moving. In every state of matter, particles have kinetic energy, which means they’re vibrating, sliding, or flying around at all times. The only theoretical exception is absolute zero (0 on the Kelvin scale, or about −273°C), where classical physics predicts all particle motion would stop. No substance has ever been cooled to true absolute zero.
This constant motion isn’t just a theory. In 1827, botanist Robert Brown noticed that tiny grains suspended in water jiggled around randomly and never settled into stillness. At first, people assumed the movement came from living organisms, but Brown showed that even inorganic particles behaved the same way. Decades later, Albert Einstein explained that this “Brownian motion” was caused by the grain being constantly bombarded by invisible water molecules. When Jean Baptiste Perrin confirmed Einstein’s math experimentally, it provided undeniable proof that atoms and molecules exist and are always in motion. Perrin won the Nobel Prize in Physics for this work in 1926.
How Particles Behave in Solids, Liquids, and Gases
The state of a substance depends on how its particles move relative to one another.
- Solids: Particles vibrate in place but don’t move from their positions. They’re locked into a rigid arrangement, which is why solids hold their shape.
- Liquids: Particles vibrate, move about, and slide past each other. This gives liquids the ability to flow and take the shape of their container while still maintaining a consistent volume.
- Gases: Particles move freely at high speeds in all directions. They spread out to fill whatever container they’re in, and most of the space a gas occupies is actually empty.
There’s also a fourth state of matter called plasma. In plasma, particles have so much energy that electrons break free from their atoms entirely. This creates a mix of positively charged ions and free-floating electrons. Plasma behaves very differently from a regular gas because these charged particles interact through electrical forces, creating waves and instabilities. Stars, lightning bolts, and neon signs all contain plasma.
Temperature Controls How Fast Particles Move
Temperature is directly tied to particle motion. Specifically, the temperature of a substance reflects the average kinetic energy of its particles. Higher temperature means particles are moving faster on average. Lower temperature means they’re slower.
This relationship is proportional: double the temperature (measured in Kelvin) and you double the average kinetic energy of the particles. That’s why heating a solid enough will eventually turn it into a liquid, and then a gas. The particles gain enough energy to break free from their fixed positions, then enough to overcome the attractive forces holding them near each other entirely.
Attractive Forces Hold Particles Together
Particles in a substance are attracted to each other. The strength of these attractive forces, compared to how much kinetic energy the particles have, determines the state of matter. When the attractive forces are stronger than the particles’ kinetic energy, the substance stays as a liquid or solid. When kinetic energy wins out, the particles fly apart and you get a gas.
Different substances have different strengths of attraction between their particles. Water molecules, for example, attract each other quite strongly, which is why water stays liquid over a wide temperature range. Oxygen molecules attract each other much more weakly, which is why oxygen is a gas at room temperature. The type and strength of these forces between particles explain why different substances melt and boil at such different temperatures.
Particle Collisions Create Pressure
When gas particles hit the walls of their container, each collision exerts a small force. Add up billions of these tiny collisions happening every second and you get measurable pressure. This is why a balloon stays inflated and why tires feel firm.
Anything that increases the number or force of collisions increases pressure. Pumping more gas into a container means more particles hitting the walls more often. Heating the gas makes particles move faster, so each collision is harder. Both raise the pressure. These collisions between particles are considered perfectly elastic, meaning no kinetic energy is lost when particles bounce off each other or off container walls. The total energy in the system stays constant unless something external adds or removes heat.
Particles Don’t Change Size When Heated
When you heat a substance and it expands, it’s natural to assume the particles themselves are getting bigger. They’re not. What actually happens is that faster-moving particles maintain a greater average distance from their neighbors. The space between particles increases, not the size of the particles. This is true for thermal expansion in solids, liquids, and gases alike. A heated metal rail, for instance, gets longer because its atoms vibrate more vigorously and push slightly farther apart, not because each iron atom has grown.
Most of a Gas Is Empty Space
One of the more counterintuitive truths about particles is how little space they actually take up, especially in gases. Gas molecules are tiny compared to the distances between them. The vast majority of the volume a gas occupies is simply empty space. Between collisions, gas particles travel in straight lines and exert no forces on one another. This is why gases are so easy to compress: you’re mainly just squeezing out empty space.
Liquids and solids have much less empty space. Their particles are close together, which is why you can’t easily compress water or squeeze a rock into a smaller volume. The particles are already packed about as tightly as their repulsive forces at close range allow.