Matter, Properties of matter

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Matter, Properties of matter:

Matter, Properties of matter Mahesh Khatri Pharm.D

Matter :

Matter Normally exists in one of the three states: Solid Liquid and Gas There is no sharp demarcation between the various states and in many cases a substance may exist in any of the three states. For e.g. ice, water and vapor. Factors determining the state of matter are: Intermolecular forces Temperature Pressure

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Solids have the strongest intermolecular force of attraction and the gases the weakest. For e.g. any solid when heated changes into liquid form as it acquires enough energy to overcome the ordered structure of solids. On further increase of temperature, the liquid goes to gaseous state.

Change in state of matter:

Change in state of matter Matter is a collection of molecules, atoms or ions held together in close relation by intermolecular, intra-atomic or ionic forces respectively. The solid particles are arranged in order due to restriction to fixed positions of particles but as they acquire sufficient energy to disrupt the ordered arrangement, they pass to liquid state. On further increase of temperature, they pass into the gaseous state. Sometimes solids can go directly from the solid state to the liquid state known as sublimation .

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Enthalpy (heat content) of matter increases as it passes from one state to another i.e. enthalpy of gases>enthalpy of liquid> enthalpy of solid) whereas Entropy ( degree of molecular randomness) increases as it passes from a solid to a liquid and to gas.

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Latent heat: The heat which results in the change of matter without increasing the temperature is known as the latent heat. When a change in the state of matter of a material occurs, the temperature usually remains constant but heat is absorbed. The expression latent heat refers to the amount of energy released or absorbed by a chemical substance during a change of state without change in temperature, meaning a phase transition.

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When this heat results in the change of state from solid to a liquid, it is known as latent heat of fusion. For e.g. heat required to change ice to water at 0 o C is latent heat of fusion. The latent heat of vaporization is the quantity of heat absorbed when a change of state from liquid to vapor occurs at its boiling point without changing the temperature of the material. For e.g. the heat required to change water to vapor at 100 o C is the latent heat of vaporization.

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Specific latent heat It is the amount of energy required to convert 1Kg (or lb) of a substance from solid to liquid or vice-versa without a change in temperature is known as the specific latent heat of fusion for that substance. Likewise, the amount of energy required to convert 1Kg (or lb) of a substance from liquid to gas or vice-versa without a change in temperature is known as the specific latent heat of vaporization for that substance.

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The latent heat for a different mass of the substance can be calculated using the equation: Q= m x L Where, Q is the amount of energy released or absorbed during the change of phase of the substance (in kilojoules m is the mass of the substance in kg L is the specific latent heat for a particular substance[ kj -kg]; (L f to represent specific latent heat of fusion and L V to represent specific latent heat of vaporization)

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Vapor pressure , also known as equilibrium vapor pressure , is the pressure of a vapor at equilibrium with its non-vapor phases. All liquids have a tendency to evaporate to a gaseous form and all gases to condense back into their original form(solid or liquid). At any given temperature, for a particular substance, there is a pressure at which the gas of that substance is in dynamic equilibrium with its liquid or solid forms. This is the vapor pressure of that substance at that temperature. It reflects the tendency of particles to escape from the liquid or solids. A substance of a high vapor pressure at normal temperature is often referred to as volatile.

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Vapor pressure depends upon the temperature not on the amount of liquid or vapor as long as both are in equilibrium. As the temp., increases more of the liquid goes to vapor phase and vapor pressure is increased. Critical temperature : As the temp. is further increased, the density of the vapor increases while the liquid decreases. After sometime both the densities become equal and the phases cannot be distinguished. This temperature is known as critical temperature and above this temp, there is no liquid phase.

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Boiling point: The temperature at which the vapor pressure of a liquid equals the external or atmospheric pressure is known as its boiling point. At boiling point, all heat is absorbed and used to change the liquid to vapor state so there is no rise in temperature until all liquid is completely vaporized. Relationship between vapor pressure and boiling point: The higher the vapor pressure of a liquid at a given temperature , the lower the normal boiling point.

Sublimation :

Sublimation It is defined as the process of transformation of solids directly into vapor phase without passing into the intermediate liquid phase. For e.g. camphor, menthol, naphthalene etc. Apart from the substances going spontaneous sublimation, some substances can be forced to exhibit the phenomenon of sublimation by varying the temperature and pressure; the process is called freeze drying , used to dry heat labile substance.

Eutectic mixtures:

Eutectic mixtures Two components system containing solid-liquid phases The mixture of certain substances like menthol, thymol, camphor, phenol, salol etc when mixed in a particular proportion tend to liquefy due to reduction in their melting points. Such mixtures are called eutectic mixtures.

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Applications: It has been widely used in pharmaceutical practice to improve the dissolution behaviour of certain drugs. Examples: aspirin- acetoaminophen (37% and 63% respectively) , urea- acetoaminophen (46% and 54% respectively), and griseofulvin-succinic acid(55% and 45% respectively) dissolve more rapidly than the drugs alone or their simple mixtures.

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Relative humidity: It is a term used to describe the amount of water vapor that exists in gaseous mixture of air and water. The relative humidity is the ratio of amount of water in the air at a specific temperature to the maximum amount of that the air could hold at that temperature, expressed as percentage(%). It can be expressed as; Relative humidity = Actual water vapor pressure x100 Saturated water vapor pressure

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Significance of relative humidity: Climate control: It refers to the control of temperature and relative humidity for human comfort, health and safety and for the technical requirements of machines and processes, in buildings, vehicles and other enclosed spaces. So specific humidity levels are maintained by using humidifiers, dehumidifiers and associated control systems.

Liquid crystals:

Liquid crystals Liquid crystals are substances that exhibit a phase of matter that has properties between those of a conventional liquid and those of a solid crystal. A LC can flow like a liquid but its molecules may be oriented in a crystal like way There are different types of LC phases which are distinguished by optical properties like birefringence.

Types of liquid crystals:

Types of liquid crystals Nematic The nematic phase is the simplest form of liquid crystal and is the phase in which the crystal molecules have no orderly position and are free to move any which way. However, while they have no specific order, during this phase the molecules do tend to point in the same direction, which is what differentiates it from a pure liquid. Characterized by its thread-like appearance when looked at under a microscope. Use of nematic liquid crystal is common in telescope lenses as it allows for a clear image when researchers are confronted with atmospheric turbulence.

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Smectic The smectic phase of liquid crystal, which is defined as being equivalent to the slippery, thick residue found at the bottom of soap dishes, is characterized by a slight degree of translational order in the crystal molecules which is not found in the nematic phase. While keeping similar orientation and pointing in the same direction as the molecules in nematic liquid crystal do, in this phase the molecules tend to line themselves up into layers. While these layers as a whole move freely, movement within the layers is restricted; therefore, it creates a slightly more solid substance. Smectic liquid crystal has been found to have fast electro-optical response time and because of this is used, along with nematic liquid crystal, in producing liquid crystal display (LCD) screens.

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Cholesteric The cholesteric phase, also known as chiral nematic phase , is characterized by the molecules being aligned and at a slight angle to one another, stacked within very thin layers– it is the last phase before a substance becomes crystalline, or solid. This type of liquid crystal also has the characteristic of changing color when it is exposed to different temperatures. It is for this reason that cholesteric liquid crystal is used in common household items such as thermometers and mood rings.

Solids:

Solids Crystalline solids or crystals: Atoms, ions and molecules are arranged in a regularly repeated pattern. Examples are metals, alloys, rocks and minerals. They can be further classified according to their distinct forms as: Cubic form: Tetragonal form Hexagonal form Orthorhombic form Monoclinic form Trigonal form and triclinic form. They exhibit a definite shape ad an orderly arrangement of units. They have a sharp melting point.

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Amorphous solids: Unlike crystalline solids, the structural units in amorphous solids are arranged in random manner. They don’t have characteristic regularity. For e.g. glass, wood, plastics and ceramics. They are considered as supercooled liquids. They don’t have a sharp melting point and melt within some narrow range of temperature.

Polymorphism :

Polymorphism Polymorphism is the ability of a compound whether crystalline or amorphous to exist in more than one distinct form. The different crystalline/amorphous forms are known as polymorphs or polymorphic forms . Some examples are: Chloramphenicol palmitate : A,B,C forms Phenobarbitone : form I, form II and form III

Slide 27:

The polymorphs differ in their melting points, the highest melting species is the most stable form. They are designated Roman numerals in order of their stability. Form I has the highest melting point i.e. most stable. The remaining as unstable (which readily converts into stable form) or metastable (which converts to stable form slowly).

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The polymorphs are structurally same but differ in the following: Melting point Solubility Dissolution Bioabsorption Polymorphs can be prepared by manipulation of conditions of crystallization such as solvent, temperature and rate of cooling.

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