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Physical and Chemical Changes

Key Questions

  • A chemical change is any change that results in the formation of new chemical substances with new properties.

    For example, hydrogen reacts with oxygen to form water. This is a chemical change.

    2H₂ + O₂ → H₂O

    Hydrogen and oxygen are both colourless gases, but water is a liquid at ordinary temperatures.

    EXAMPLES

    Which of the following are chemical changes? (a) Sugar dissolves in warm water. (b) A nail rusts. (c) A glass breaks. (d) A piece of paper burns. (e) Iron and sulfur form a shiny nonmagnetic grey substance on heating.

    Solutions:

    (a) Not a chemical change. The sugar and water are still present.
    (b) A chemical change. The reddish-brown rust is different from the iron.
    (c) Not a chemical change. The glass is just in smaller pieces.
    (d) A chemical change. The paper disappears. All that remains is a small amount of ash.
    (e) A chemical change. Sulfur is yellow, and iron is magnetic. The product is neither yellow nor magnetic.

    Here is video of a lab with a number of examples of both chemical and physical changes.

    video from: Noel Pauller

  • Physical changes don't destroy your sample or make it into something new.

    Physical changes change the shape, size or phase of a substance. Crumpling paper, getting a haircut, dissolving salt in water, melting wax, making ice cubes, chopping veggies for tossed salad, growing crystals from a supersaturated solution, grinding metal and slicing bread are all physical changes.

    During a physical change the elements and compounds stay as they are.

    Chemical changes make new substances. Cooking an egg rearranges it's proteins. Burning fuels, etching metal with an acid, fermentation, respiration in your cells, decomposition in the compost pile, making zinc iodide in the lab, and forming a silver mirror are all chemical changes.

    During a chemical change the elements and compounds get rearranged and recombined into something new.

  • Answer:

    A change in (matter or substance) which does not alter the chemical properties of the matter.

    Explanation:

    A physical change is a type of change in which the form of matter (substance) is altered but one substance is not transformed into another different substance. For example, if we carve a piece of wood into a baseball bat, it will still burn in a fire and float on water. It remains wood, so it is a physical change.

    (b) Crushing a can : after crushing can changes its shape, size but it still remain Aluminum so it is a physical change.

    Burning of wood is a chemical change because on burning wood changes to a totally different new products (ash, smoke and carbon dioxide).

    Here is video of a lab with a number of examples of both chemical and physical changes.

    video from: Noel Pauller

    Simple explanation with good results can be on this site http://chemistry.about.com/od/matter/a/10-Physical-Change-Examples.htm

Questions