Category: chemistry

List: Substances that undergoes sublimation

These are some common substances that undergo sublimation upon heating under the normal atmospheric pressure condition.

  • dry ice (solid carbon dioxide)
  • iodine crystals
  • napthalene (mothballs)
  • camphor
  • *ammonium chloride
  • *ammonium chloride doesn’t actually undergo sublimation upon heating. It decomposes and forms ammonia gas and hydrogen chloride gas. When they are in contact with cooler surfaces, they recombine to form back ammonium chloride. This gives a false impression that it has undergone sublimation.

    -decomposition of ammonium chloride-

    NH4Cl → NH3 + HCl

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    Q&A: Condensation

    What is condensation and what are the conditions required for condensation to take place ?

    Condensation is a process where a gas changes state into the liquid state. This happens when the warmer gas is in contact with a cooler surface. The gas condenses on the surface and forms the liquid state.

    Examples
    -wait for further updates-

    Nutshell: Alloy vs Pure Metals

    Why are alloys preferred over pure metals?

    Key points:

    – stronger
    – harder
    – better resistance to corrosion

    Description:
    20121106-234900.jpg
    The diagram above shows the regular and orderly arrangements of the particles in pure metal. When a forces are applied on both sides, the upper and bottom layer slides over each other easily. That is the reason pure metals are not strong.

    20121107-074620.jpg
    The diagram below shows the disorderly arrangements in an alloy due to difference in sizes of the 2 metal particles. In this arrangement the layers of atoms cannot slide over each other easily. This makes alloy harder and stronger than pure metals.

    Nutshell: Cooling and Heating Graphs

    Cooling Graph: A temperature graph showing an increase in temperature over a period of time.
    20121106-074345.jpg
    – example of a cooling graph –

    Heating Graph: A temperature graph showing a decrease in temperature over a period of time.
    20121106-112441.jpg
    – example of a heating graph –

    The segments which show constant temperature means that a change in state is taking place.

    The possible state changes in a cooling graph are condensation and freezing.

    The possible state changes in a heatin graph are melting and boiling.

    No Temperature Change = State Change
    You can identify the portions which is undergoing state change by looking at which portion is experiencing no temperature change.

    Boiling Point
    is the temperature at which a liquid changes into its gaseous state.

    Melting point is the temperature at which a solid changes into its liquid state.