States of Matter and Particle Arrangement

Core concept

In gases, particles are far apart with very weak forces between them, allowing them to move freely and rapidly in all directions, filling any available space — this is why gases have neither a fixed shape nor a fixed volume. As a substance changes state (like ice melting to water, or water boiling to steam), only the particle arrangement and energy change; the particles themselves remain the same, meaning the chemical identity of the substance is unchanged during these physical state changes.