Matter is the physical substance of the universe, defined as anything that has mass and takes up space (volume). Every object, from the air we breathe to the devices we use, is composed of matter. To fully describe matter, scientists look at two fundamental aspects: its physical form and its inherent chemical makeup. Understanding the arrangement of its particles and its composition is necessary for a complete characterization.
Physical States of Matter
The most common way to categorize matter is by its physical state, which is determined by the energy and arrangement of its constituent particles. The three states commonly observed are solid, liquid, and gas.
A solid maintains a fixed shape and volume because its particles are tightly packed and held in fixed positions. This dense arrangement means solids are nearly incompressible.
In a liquid, particles remain close together but have enough energy to move past one another. Liquids maintain a constant volume but take the shape of their container. They are less compressible than gases because the particles are still in close contact.
Gas particles possess more energy, moving rapidly and freely with large spaces between them. Gases do not have a fixed shape or volume, expanding to fill any container they occupy. This dispersed arrangement makes gases easily compressible. Beyond these three, a fourth state called plasma exists, which is a high-energy, ionized gas containing charged particles. Plasma is the most abundant state of ordinary matter in the universe, making up stars and lightning.
Classification by Composition
Describing matter by its composition involves examining its chemical makeup, sorting it into two main categories: pure substances and mixtures. Pure substances have a constant chemical composition and uniform properties. These are subdivided into elements and compounds.
Elements are the simplest pure substances and cannot be broken down into chemically simpler components. Each element consists of only one type of atom, such as gold or oxygen. Compounds are pure substances formed when two or more different elements are chemically bonded together in a fixed ratio. Water, for example, is a compound composed of two hydrogen atoms bonded to one oxygen atom, and it can only be separated back into its elements through a chemical change.
Mixtures are physical combinations of two or more substances present in varying amounts. Unlike compounds, the components of a mixture retain their individual chemical identities and can often be separated by physical means, such as evaporation. Mixtures are further classified based on how uniformly the components are distributed.
Homogeneous mixtures, often called solutions, have a composition that is uniform throughout. Salt dissolved in water is a common example, as the components are completely dispersed and visually indistinguishable. Heterogeneous mixtures, conversely, have a composition that varies from point to point, and the different components can typically be visually distinguished. Examples include a mixture of sand and water, or a salad dressing where the oil and water separate.
Observable Characteristics Used for Description
To characterize matter, scientists rely on observable characteristics, which are divided into physical and chemical properties. Physical properties are characteristics that can be observed or measured without changing the substance’s chemical identity.
Examples of physical properties include color, density, hardness, and the temperatures at which a substance changes state, such as its melting or boiling point. Measuring the density of a metal or observing its color does not change the metal into a new substance. Even when water freezes into ice, the process is a physical change because the substance remains water, merely in a different physical state.
Chemical properties describe a substance’s potential to undergo a reaction that results in the formation of a new substance. Observing a chemical property involves a change in the chemical composition of the matter. Examples include flammability, the ability to burn, and reactivity with acids or oxygen. The rusting of iron is an observation of a chemical property, as the iron combines with oxygen and water to form rust.