ڤنرڠن
Definition and Particulate Nature of Matter
Matter is defined as anything that has mass and occupies space. Everything in the universe, from the air breathed to the water drunk, is composed of matter. To understand the properties of matter, scientists use the particle model, which states that all matter is made up of tiny, discrete particles such as atoms, molecules, or ions. These particles are not static; they possess internal kinetic energy and are in constant, random motion.
Evidence for the particulate nature of matter comes from phenomena such as mixing substances and dissolving solids in liquids, where particles of one substance fit into the spaces between the particles of another. The spaces between particles vary depending on the state of matter. Furthermore, attractive forces exist between these particles, pulling them together. The balance between the kinetic energy of the particles, which tends to pull them apart, and the intermolecular forces of attraction, which hold them together, determines the physical state of the substance at any given temperature and pressure.
States of Matter and Particle Arrangement
Matter exists predominantly in three states: solid, liquid, and gas. Each state has distinct macroscopic characteristics that arise directly from the arrangement, separation, and motion of its constituent particles.
In a solid, particles are packed very closely together in a regular, repeating, three-dimensional lattice structure. The separation between particles is minimal. Because of strong forces of attraction between them, the particles cannot move freely from place to place. Instead, they vibrate fixedly about their mean positions. This rigid arrangement gives solids a fixed shape and a fixed volume that cannot be easily compressed.
In a liquid, particles are still close together, but they are arranged in a random, irregular manner rather than a fixed lattice. The separation between particles is slightly greater than in solids. The forces of attraction between liquid particles are strong enough to keep them together, giving liquids a fixed volume, but not strong enough to hold them in fixed positions. Particles in a liquid can slide past one another. Consequently, liquids do not have a fixed shape; they flow and take the shape of the container they occupy.
In a gas, particles are widely separated by large empty spaces. The forces of attraction between gas particles are negligible. The particles possess high kinetic energy and move rapidly in random directions, colliding with one another and with the walls of their container. Because of the vast empty spaces and weak attractive forces, gases have neither a fixed shape nor a fixed volume. They expand to completely fill any container they are placed in and are easily compressible.
Diffusion in Gases and Liquids
Diffusion is defined as the net movement of particles from a region of higher concentration to a region of lower concentration down a concentration gradient, resulting from their random thermal motion. Diffusion occurs in both gases and liquids.
In gases, diffusion happens relatively quickly because gas particles possess high kinetic energy, travel at high speeds, and are separated by large distances. When a bottle of perfume is opened in a room, gas molecules diffuse rapidly through the air until they are evenly distributed.
In liquids, diffusion occurs much more slowly than in gases. This is because liquid particles are packed much closer together and experience frequent collisions with neighboring particles, which impedes their net forward movement. When a crystal of potassium permanganate is dropped into a beaker of water, purple particles slowly spread out through the liquid until the entire solution attains a uniform purple colour. Diffusion does not occur in solids because the particles are locked in fixed lattice positions and cannot migrate through the structure.
Changes of State and Energy
A substance can change from one state of matter to another when thermal energy is added or removed, altering the balance between particle kinetic energy and intermolecular forces. These physical changes do not alter the chemical identity of the substance.
Melting is the transition of a substance from a solid to a liquid. As a solid absorbs heat energy, its particles vibrate faster, increasing their kinetic energy. Eventually, the particles gain enough energy to overcome the strong forces holding them in their fixed lattice positions, and the solid turns into a liquid.
Freezing (or solidification) is the reverse process, where a liquid changes into a solid as heat energy is removed. The particles lose kinetic energy, slow down, and the attractive forces pull them into a fixed, ordered lattice structure.
Evaporation is the process where a liquid changes into a gas at temperatures below its boiling point. Evaporation occurs only at the surface of the liquid. Particles at the surface that possess higher than average kinetic energy manage to break free from the attractive forces of the liquid and escape into the surrounding space as a gas.
Cooling by evaporation occurs because the particles that escape into the gas phase are the ones with the highest kinetic energy. The remaining particles in the liquid now have a lower average kinetic energy, which results in a drop in the temperature of the liquid and its surroundings. This is why sweating cools the human body.
Condensation is the transition of a gas into a liquid. When gas particles lose thermal energy, their speed decreases, and the intermolecular forces draw them close together to form a liquid.
Sublimation is the direct transition of a substance from a solid state to a gas state without passing through the liquid phase, or vice versa (often called deposition). Examples of substances that sublime include solid carbon dioxide (dry ice) and iodine. Sublimation occurs when surface particles of a solid absorb enough energy to break away entirely into the gas phase.
