Chemical Measurements

The context of the study of chemistry depends upon several conservation principles which give us an overall understanding of chemical processes.

To characterize atoms of a given elements, a number of measured properties are included with the elements on the periodic table. Some of the measured properties are the ionization energy, electronegativity and the electron affinity.

Conservation Principles

• The law of conservation of mass: there is no detectable change in the total mass during a chemical reaction.
  • This important conceptual step in the understanding of chemistry was discovered by Lavoisier.
    这一对理解化学至关重要的概念性步骤是由拉瓦锡发现的。
  质量守恒定律：化学反应中总质量没有可检测的变化。
• The law of definite proportions: pure compounds always contain the same elements in the same proportion by mass.
  • This principle was discovered by French chemist J. L. Proust in quantitative experiments in 1799.
    这一原理是由法国化学家J. L.普鲁斯特在1799年进行的定量实验中发现的。
  定比例定律：纯化合物总是以相同的比例（按质量）含有相同的元素。

Both of these principles provided groundwork for the postulates of John Dalton in 1803 concerning the nature of atoms.

Dalton's Atomic Theory

In 1803, John Dalton proposed the following postulates which underly modern chemistry:

1. Elements are formed from small indivisible particles called atoms which are identical for a given element but different for any other element.
   元素由称为原子的小不可分割粒子构成，相同元素的原子彼此相同，不同元素的原子则不同。
2. Chemical compounds are formed by the combining of a definite number of atoms of each type of atom to make one molecule of the formed compound.
   化学化合物是由不同类型的原子以固定数量结合形成一种分子。

Dalton's efforts to confirm his postulates led to the law of multiple proportions: whenever two elements combine to form more than one compound, there is a ratio of small whole numbers between the various masses of one element that combine with a constant mass of the other element in the compounds.

States of Matter

Matter in the solid state can be characterized by density, by elastic properties, by electrical and magnetic properties, by thermal properties and by composition. Part of its characterization will be the description of its habit of formation, such as its crystal structure and symmetry or the lack thereof.

Matter in the liquid state can also be characterized by density. There are other characteristic fluid properties.

In most practical cases, matter in the gaseous state can be described by the ideal gas law. The dynamic properties of gases are described by kinetic theory. Mixtures of gases can be characterized by the partial pressures of the constituents.

It is a common practice to refer to plasma as the fourth state of matter. Though not so common on the Earth, the plasma state can be considered to be the most abundant state of visible matter since the stars are predominantly in the plasma state.

Solutions can be described by the concentration of the solutes, which is usually expressed in terms of the molarity of the solution.