Page 36 - ICSE Chemistry 8
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Freezing

                 Freezing is a process by which a substance changes from its liquid state to its solid state on cooling.
                 Freezing of water into ice is a physical change which could be natural or man-made and is fast, reversible
                 and desirable.

                 Melting

                 Mel  ng is a process by which a substance changes from its solid state to its liquid state on hea  ng.
                 Mel  ng of ice is a physical change which could be natural or man-made, and is fast and reversible.

                 Condensation

                 Condensa  on is a process by which a substance changes from its gaseous state to its liquid state on
                 cooling. The water vapour condense to form clouds. This change is natural, reversible and desirable.

                 Sublimation

                 Sublima  on is a process by which a substance changes directly from its solid state to its gaseous state
                 without changing into its liquid state. Naphthalene balls are kept in cupboards to protect woollen clothes
                 from insects as naphthalene sublimes at room temperature.

                           Activity 2.6


                 Aim: To show that sublima  on of camphor is a physical change                             cotton plug
                 Materials required: A funnel, camphor, co  on wool, an evapora  ng dish,
                                                                                                           solid camphor
                 a burner, a tripod stand, a wire gauze

                 Procedure
                   •  Take camphor in an evapora  ng dish.
                                                                                                                  camphor
                   •  Cover the dish by inver  ng the funnel as done in the fi gure.

                   •  Block the end of the funnel with co  on.
                                                                                                                 Bunsen
                   •  Now heat the camphor.                                                                      burner
                 Observa  on: On hea  ng, fumes of camphor rise in the funnel, which
                 then change back to solid camphor and se  le along the cooler part of
                 the funnel.

                 Conclusion: This shows that sublima  on of camphor is a physical change.



                 Chemical Changes
                                                                                          Thirst for Knowledge
                 Changes in which two substances react to form new substances
                 with diff erent proper  es are called chemical changes. They are       •  Chemical changes are the result
                 generally irreversible in nature.                                        of chemical reac  ons.

                 In chemical changes, molecules of the original substances undergo     •  All chemical reac  ons  involve
                                                                                          change in substances and
                 change to form molecules of the products, i.e., substances
                                                                                          energy.
                 undergo chemical transforma  on to form new substances.


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