Elements are the simplest complete chemical substances. Matter can be broken down into two categories: pure substances and mixtures. Pure substances are further broken down into elements and compounds. Mixtures are physically combined structures that can be separated back into their original components.

Salt water can be separated into its parts. You can let the water evaporate, and you will have just the salt left. Salt water is a solution because it has these two characteristics: it has the same concentration of each of its parts throughout the solution, and it can be separated by some physical process.

A homogeneous mixture is a mixture in which the composition is uniform throughout the mixture. The salt water described above is homogeneous because the dissolved salt is evenly distributed throughout the entire salt water sample. One characteristic of mixtures is that they can be separated into their components.

In a NaCl solution (salt-water), the solvent is water. A solute is the component in a solution in the lesser amount. In a NaCl solution, the salt is the solute. An aqueous solution is a solution in which water is the solvent.

If it is pure, the substance is either an element or a compound. If a substance can be separated into its elements, it is a compound. If a substance is not chemically pure, it is either a heterogeneous mixture or a homogeneous mixture.

Examples of elements include carbon, oxygen, hydrogen, gold, silver and iron.

There are four types of compounds, depending on how the constituent atoms are held together: molecules held together by covalent bonds. ionic compounds held together by ionic bonds.

H2O

O2

Oxidane

Nitrogen tribromide Nitrogen

Arsenic sulfide

Methylidyne radical

Thus, benzene is represented by the empirical formula CH, which indicates that a typical sample of the compound contains one atom of carbon (C) to one atom of hydrogen (H). Water is represented by the empirical formula H2O, denoting that the substance contains two atoms of hydrogen (H2) for every atom of oxygen (O).

Ethene

Dicarbon monoxide

P₂O₅

tricyclo[3.3.1.13,7]tetraphosphoxane 1,3,5,7-tetraoxide

* Exposure to Ethylene can cause headache, dizziness, fatigue, lightheadedness, confusion and unconsciousness. * Ethylene is a HIGHLY FLAMMABLE and REACTIVE chemical and a DANGEROUS FIRE and EXPLOSION HAZARD.

Ethylene has been found not harmful or toxic to humans in the concentrations found in ripening rooms (100-150 ppm). In fact, ethylene was used medically as a anesthetic in concentrations significantly greater than that found in a ripening room.

Around 2–50 µL ethylene·kg–1 fresh mate- rial·h–1 should be released during the initial ripening phases and can be measured with the method proposed.