CGS Seventh Grade Science

Saturday, October 31, 2015

Physical Change: Change in State of Matter

When a substance changes directly from a gas to a solid

(example: the forming of frost from water vapor)

or from a solid to a gas (examples: dry ice, solid air fresheners)

it is considered a change of state and is called sublimation.

This is still a physical change because the composition of the substance remains

the same.

Chemical Change: Formation of a Precipitate

When two solutions are combined, they may form a solid substance.

This solid substance is called a precipitate and indicates that a chemical change has occurred.

When carbon dioxide is combined with aqueous calcium hydroxide

(limewater), solid calcium carbonate (chalk) is formed as the precipitate. The

precipitate may be in the form of very small particles, appearing as cloudiness in the

solution or as a solid which settles to the bottom of the container.

Chemical Equation:

Ca(OH)2 + CO2 → CaCO3+ H2O

(Limewater + Carbon dioxide → Calcium carbonate + water)

Physical Changes:

Change in State of Matter

When a substance changes from one state of matter to another and the

composition remains the same.

While crushing sugar we demonstrated a

physical change, change in size and shape.

Litmus paper

• Has a special dye on it that changes colors in the presence of an acid or base.

• Blue litmus paper turns red in an acid, and stays blue in a base.

• Red litmus paper turns blue in a base, and stays red in an acid.

• Neutrals react with both types of litmus paper turning it violet.

pH paper:

Has a range of colors depending on the pH of the solution.

•The color of the paper is compared to the chart on the vial to determine the pH.