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    Changes of State

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    Talib Al_Munawri
    Director-General
    Director-General

    Posts : 337
    Join date : 2009-07-03
    Age : 30
    Location : Sultanate of Oman

    Changes of State

    Post by Talib Al_Munawri on Fri Sep 11, 2009 11:00 pm

    Changes of State


    In general, matter (solid, liquid and gas) in one state can be changed into
    either of the other two states. Such transformations are called "phase changes"

    Change of state chemically:


    Chemical reactions involve the changing of the starting materials into new materials, and the changes that you see are the result of the appearance of the new materials.

    The new material might have a different phase than the starting materials. For example, if you create something new from two liquids and it happens to be a gas. There will be a new material with a new phase that wasn't there before. Also, you can observe chemical reactions by change in color

    When a chemical reaction takes place, usually the temperature will also change.
    It will either go up or down. Those are the kinds of changes that you might see
    in chemical reactions are caused by the creation of or loss of different kinds of materials.

    Eg., of chemical reaction: Decomposition reaction:

    Decomposition is a reaction in which a compound breaks down into simpler products.

    CaCO3(s) CaO(s) + CO2(g)

    2NH4NO3(s) 2N2(g) + 4H2O(g) + O2(g)

    The starting material is called as reactants and the new materials obtained
    after decomposition is called products. The arrows indicate the reaction
    is irreversible.




    Change of state physically:


    Water (H2O) is composed of water molecules, at room temperature; the molecules
    are packed closely together, and interact weakly. They do not stick together,
    and are able to slide past one another. This microscopic behavior of water molecules gives rise to the physical properties of water molecules, these
    water molecules do not form any rigid structure, water has no fixed shape,
    and adapts to the shape of any container in which it is placed because
    the molecules are very close to one another.

    If we make slight changes in the physical conditions, such as lowering the
    temperature, we will observe no abrupt changes in the properties of water.
    Cold water behaves little differently from lukewarm water. For instance,
    its compressibility changes slightly with temperature, but remains very low.


    However, if we reduce the temperature below a certain point, an abrupt and
    dramatic change occurs. At the microscopic level, the molecules suddenly
    align with one another to form a rigid hexagonal lattice, losing the ability
    to slide past one another. The system as a whole acquires rigidity, and can
    hold a definite shape. This is the solid phase of water, commonly known as ice.
    The transition from a liquid phase to a solid phase is called freezing, and it
    is a type of phenomenon known as a phase transition.


    Another phase transition, known as boiling, occurs if we raise the temperature
    of liquid water past a certain point. The water abruptly enters a gaseous phase,
    where it is called water vapor. In the gaseous phase, the molecules are spread
    far apart, and interact extremely weakly and so a gas has no fixed shape, but it
    can be compressed because there is enough space for the molecules to move closer
    to one another. Whereas a liquid placed in a container will form a puddle at the
    bottom of the container, a gas will expand to fill the container.




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    Energy Changes Accompanying Changes of State
    Each change of state is accompanied by a change in the energy of the system


    Whenever the change involves the disruption of intermolecular forces, energy must be supplied.

    The disruption of intermolecular forces accompanies the state going towards a less ordered state.

    As the strengths of the intermolecular forces increase, greater amounts of energy are required to overcome them during a change in state.

    Whenever the change involves the disruption of intermolecular forces, energy must be supplied.

    The disruption of intermolecular forces accompanies the state going towards a less ordered state.

    As the strengths of the intermolecular forces increase, greater amounts of energy are required to overcome them during a change in state
    The melting process for a solid is also referred to as fusion

    The enthalpy change associated with melting a solid is often called the heat of fusion (Hfus)

    Ice Hfus = 6.01 kJ/mol

    The heat needed for the vaporization of a liquid is called the heat of vaporization (Hvap)

    Water Hvap = 40.67 kJ/mol

    Less energy is needed to allow molecules to move past each other than to separate them totally.

      Current date/time is Wed Dec 13, 2017 4:07 pm