Matter & Energy

Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.


Urea, colorless, crystalline compound with melting point 132.7° C (270.9° F), also known as carbamide. It is found abundantly in the urine of humans and other mammals (see Urinary System). In lesser quantities, it is present in the blood, liver, lymph, and serous fluids and is found in the excrement of fish and many other lower animals. Urea is produced mostly in the liver as the end product of protein metabolism. The nitrogen in urea, which constitutes most of the nitrogen in the urine, is produced mainly from food protein, but part comes from the breakdown of body cells. Urea is also present in various fungus molds as well as in the leaves and seeds of numerous legumes and cereals. The compound is soluble in both water and alcohol and is slightly soluble in ether. Urea is prepared synthetically by the Wöhler synthesis, which was devised in 1828 by the German chemist Friedrich Wöhler.

Because of its high nitrogen content, commercially prepared urea is used in the manufacture of agricultural fertilizers. Urea is also employed as a stabilizer in nitrocellulose explosives and is a basic constituent of synthetically prepared resins.