Enamel is formed by epithelial cells called ameloblasts. Just before a tooth erupts from the gums, the ameloblasts are broken down, removing enamel's ability to regenerate or repair itself.

Fluorocarbons such as freon-1 (CFCl₃) and freon-12 (CF₂Cl₂₎ emitted as propellants in aerosol spray cans, refrigerators, fire fighting reagents, etc. are stable compounds and chemically inert. They do not react with any substance with which they come in contact and thus float through the atmosphere unchanged and eventually enter the stratosphere. There they absorb UV radiations and break down liberating free atomic chlorine which causes decomposition of ozone. This results in the depletion of the ozone layer.

Fluorine is the most electronegative element (electronegativity of on Pauling scale) will in any case (except in fluorine gas) have an oxidation state of -1. So that's the reason why oxygen has a +2 oxidation state in OF₂.

When two ice cubes are pressed over each other, they unite to form one cube, Hydrogen bonding is responsible for it.

A hydrogen bond is an intermolecular force (IMF) that forms a special type of dipole-dipole attraction when a hydrogen atom bonded to a strongly electronegative atom exists in the vicinity of another electronegative atom with a lone pair of electrons. Intermolecular forces (IMFs) occur between molecules. Other examples include ordinary dipole-dipole interactions and dispersion forces. Hydrogen bonds are generally stronger than ordinary dipole-dipole and dispersion forces, but weaker than true covalent and ionic bonds.

Since the weight of the bag with water is equal to the weight of the water displaced, the spring balance reading is zero.

The spring balance is a type of balance (scales). It consists of a spring, one end of which is fixed and the other end is hung by a hook, whose weight is to be determined. It works on Hooke's law.

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Plasma Membrane is the outermost boundary of the cell. It is also known as the cell membrane. It is the membrane found in all cells that separate the interior of the cell from the outside environment. A cell wall is attached to the plasma membrane on its outside surface in bacterial and plant cells.