A chemical compound is a pure substance formed when two or more different elements are chemically bonded together in a fixed, definite ratio. For example, water is always two hydrogen atoms bonded to one oxygen atom, giving it a unique structure that is distinct from the elements hydrogen and oxygen themselves. Unlike a mixture, which can be separated by simple physical means, a compound can only be broken down into its constituent elements through a chemical reaction. Understanding the inherent characteristics of a compound is necessary for its identification, determining its potential uses, and predicting how it will behave in different environments. This study of characteristics allows researchers and engineers to develop new materials, medicines, and technologies for various applications.
Distinguishing Physical and Chemical Properties
The characteristics of a compound are broadly divided into two categories based on how they are observed. Physical properties can be measured or observed without causing a change in the compound’s fundamental chemical identity. Observing the color of a substance or measuring its mass leaves the compound as the same substance it was before the observation.
Chemical properties, in contrast, can only be observed when the substance undergoes a chemical change, resulting in the formation of an entirely new substance. Determining a chemical property involves a reaction that alters the compound’s molecular structure, breaking old bonds and forming new ones. This distinction is based on whether the compound’s composition remains the same throughout the observation process.
Observable Characteristics (Physical Properties)
Physical properties are often further classified based on whether they depend on the amount of the compound present. Extensive properties, such as mass and volume, directly scale with the quantity of the substance being measured. If you double the amount of a compound, you double its mass and its volume.
Intensive properties are inherent to the compound itself and remain constant regardless of the sample size. These characteristics are particularly useful for identifying an unknown compound because they are unvarying. Examples of intensive properties include the temperature at which a solid turns into a liquid, known as the melting point, and the temperature at which a liquid becomes a gas, the boiling point.
Density is another intensive property, calculated as the ratio of a compound’s mass to its volume. Other easily observable intensive properties include a compound’s state of matter (solid, liquid, or gas) at a given temperature, its color, and its odor. Solubility, which is the ability of a substance to dissolve in a solvent like water, is also an intensive characteristic, showing how the compound interacts physically with its surroundings without changing its identity.
Predicting Transformation (Chemical Properties)
Chemical properties describe a compound’s potential to react or transform into a different material. They are determined by the compound’s internal structure and the arrangement of its atoms. Observing these properties always involves initiating a chemical reaction that yields a new compound with a different set of characteristics.
Flammability, a common chemical property, describes a compound’s ability to ignite and burn, which is a reaction with oxygen that converts the substance into new products like carbon dioxide and water. Reactivity is a broader term that describes how readily a compound participates in a chemical reaction with other substances, such as acids, bases, or water. For instance, a compound’s acid-base reactivity determines its potential to donate or accept protons.
Chemical stability refers to a compound’s resistance to decomposition or breakdown when exposed to factors like heat, light, or moisture. A highly stable compound will remain unchanged under a wide range of conditions, while an unstable one may spontaneously decompose. Toxicity is also classified as a chemical property, as its effect on a living organism is the result of a series of chemical reactions within the body that alter the compound’s structure.
How Chemical Bonding Dictates Properties
The type of chemical bond holding a compound together is the primary determinant of its properties. The two main bond types, ionic and covalent, result in vastly different macroscopic characteristics. Ionic compounds are formed by the electrostatic attraction between oppositely charged ions, which creates a strong, rigid crystal lattice structure.
These strong forces require a large amount of energy to overcome, giving ionic compounds characteristically high melting and boiling points. They also tend to be soluble in polar solvents like water, and while they do not conduct electricity as solids, they become excellent conductors when melted or dissolved because their ions are free to move.
Covalent compounds are formed by the sharing of electrons between atoms, creating discrete molecules. The forces within the molecule are strong, but the attractive forces between separate molecules are comparatively weak. This weakness explains why covalent compounds have low melting and boiling points and often exist as liquids or gases at room temperature. Because they consist of neutral molecules without free-moving charged particles, covalent compounds are poor electrical conductors in all states.