Physical and Chemical Changes; Law

Physical and Chemical Changes; Law of Conservation of Matter (Mass) The objectives of the lab were met. Both chemical and physical changes were observed. In addition, the law of conservation of matter (mass) was tested and found to hold true for chemical reactions. Although, in some cases the total mass changed in some chemical reactions this change could be accounted for by either errors or the change of state in some of the matter from solid to gas that could not be weighed.

The definition of and the differences between physical and chemical changes were learned. A physical change is one in which a physical attribute of a substance changes but not the chemical composition of the substance. Physical changes include phase changes, changes in the texture of a substance, magnetizing of some metals, dissolving and evaporating of a substance in water, and heating of a substance. A chemical change occurs when a chemical reaction takes place.

For example, when a substance undergoes a chemical reaction and a different substance is formed from that original substance. The new substance often differs physically from the original substance. Chemical changes can be observed as some of the following: formation of gas when solids or liquids are mixed without addition of heat, formation of a precipitate when two solutions are mixed, color change, change in pH of a solution, or change in temperature especially when a flame appears.

All chemical changes involve a change in energy whereas physical changes do not. Also, the law of conservation of matter (mass) was learned. This law states that during a chemical reaction matter is not created or destroyed. Therefore, the total mass of all the starting materials will equal that of the end products. It was observed that melting wax consisted of a physical change. The wax changed states from a solid to a liquid. Whereas, burning a candle was an example of a chemical change.

This was evidenced by the evolution of gas (smoke) and the presence of a flame. Also, a chemical reaction was observed when mixing sodium carbonate solution with hydrochloric acid. This was indicated by the evolution of gas bubbles in the reaction and the formation of a precipitate. A change in total mass of the starting materials vs. The final products was noted upon weighing the materials before and after the reaction. This is not a contradiction in the law of conservation of matter (mass).

This can be explained by the loss of some matter as a gas, which could then not be weighed on a scale. Some errors in the experiments did occur. In experimenting with the concept of the law of conservation of matter (mass) some mass was lost when transferring liquids some liquid was spilled. No mass was actually destroyed through the chemical reaction. The loss of spilt liquid erroneously seemed to indicate a change in mass between the starting materials and end products.

Also, when taking precision measurements of mass down to the one milligram level some human error in measuring was observed. This could be due to not properly zeroing the scale.