PHYSICAL PROPERTIES

Form of matter

· Heat of vaporization: - It is the amount of energy needed for transformation of a boiling liquid into vapour. For example, 540 cal. Of heat is required to vaporize 1gm of water at 100̊ c and 1 atmospheric pressure.

·      Latent heat of fusion: - It is the amount of energy released (in the form of heat) when a liquid freezes. When 1 gm of water freezes, 80 cal. of heat is released.

·      Melting temperature: - it is the amount of energy required for transformation of solid into liquid state.

·      Sublimation: - It is process in which some solids transform directly to a gas phase.

 INTER ATOMIC BONDS

The atom are held together by some force. These interatomic forces that hold the atom together are cohesive forces.

Inter atomic bond may be classified as:-

1.   Primary bond

2.   Secondary bond

o Primary bond :-

1.    These are chemical in nature:-

a.  Ionic

b.  Covalent

c.   Metallic

a)    Ionic bond :-

§  They are simple chemical bond

§  Resulting from mutual attraction of negative and positive charge.

b)   Covalent bond :-

§  In many chemical compound, 2 valence electron are shared,.

§  The H₂ molecule is an example of covalent bonding.

c)Metallic bond:-

§  The chief characteristics of metal is its ability to conduct electricity (due to mobility of the so called free electron).

§  The outer shield valence electrons can be removed easily, leaving the balance electron tied to the nucleus, thus forming a positive ion.

§  The free electron they are able to move in the metal space lattice forming electron cloud or gas

§  The electrostatic attraction between this electron cloud and the positive ions in the lattice bonds the metal atoms together as a solid.

o   Secondary bond:-

a)  This weaker bond may be said to be more physical than chemical

b) This is due to formation of dipole.

c)   In a symmetric atom (e.g. inert gas) a fluctuating dipole is formed, i.e. within an atom there is accumulation of electron in one half leading to negative polarity and on other half a positive polarity. This attracts other similar dipoles. A permanent dipole ids formed within asymmetric molecules, e.g. water molecule.

GLASS TRANSITION TEMPERATURE

·      The temperature at which there is an abrupt

Increase in the thermal expansion coefficient,

Indicating increased molecular mobility is called the glass transition temperature and it is characteristic of the particular glassy structure.

·      When impression is heated in a hot water bath the material looses its hardness at approximately 39^o C. however, at this stage, it is still not plastic and soft enough for making an impression. This temperature at which the material looses its hardness and brittleness on heating or forms a rigid mass upon cooling is referred to as glass transition temperature.

v  The force of attraction between like molecules is called cohesion. The force attraction between unlike molecule is adhesion.

v  The material or film added to produce the adhesion is known as the adhesive, and the material to which it is applied is called the adherend.

v  The capacity for the adhesion is directly related to the surface energy of the material. Because of the higher surface energy of many restorative materials compared with that of the tooth, there is a greater tendency for the surface and margins of the restoration to accumulate debris with resultant high incidence of marginal caries.

v  Hue :-

It is the specific colour produced by a specific wavelength length of light. It describes the dominant colour of an object, for example, red, green or blue. (Refers to the basic colour of an object.)

v  Value (Brilliance) :-

It is the lightness and darkness of an object. Lighter shaded object are with highest value and dark shaded objects are with lower value.

v  Saturation (Chroma) :-

It is the amount of colour per unit area of an object. Chroma represents the degree of a particular hue. For example, some teeth appear more yellow than others.

A particular colour may be dull or more vivid, this difference in colour intensity or strength is called Chroma.

Higher the chroma the more intense and mature is the colour.

Always associated with hue and value.

v  Translucency :-

It is the property of an object that permits the passage of light through it but does not give any distinguishable image.

v  Metamerism :-

Objects that appear to be colour matched under one type of light may appear different under another light source. This phenomenon is called Metamerism.

v  Fluorescence: -

The energy that a tooth absorbs is converted into light with longer wavelengths, and becomes a light source. This phenomenon is called fluorescence. Fluorescence make a definite contribution to the brightness and vital appearance of a human tooth.