Physical versus Chemical Changes
In the beginning of their chemistry classes, students learn how to distinguish physical vs chemical changes. Physical changes are changes to affect the substance without changing its identity. Chemical changes, on the other hand, change the identity of the substance. Therefore, anytime we think there is a reaction, it will be a chemical change since reactions turn reactants into products.
What ways are there to determine physical changes? Any change of shape or phase such as turning liquid into gas is a physical change. Chemical changes can be classified by the production of gas, bubbles, and changes of color.
Lets now look at some examples of physical versus chemical changes.
Souring of milk - chemical (reaction)
melting of ice - physical (phase change)
drawing iron into wires - physical (shape change)
rusting of iron - chemical (reaction of iron with oxygen)
tarnishing of copper - chemical (reaction)
evaporation of alcohol - physical (phase change)
digestion of food- chemical (reaction)
Let's look at a regents example
Some properties of the element sodium are listed below. • is a soft, silver-colored metal • melts at a temperature of 371 K • oxidizes easily in the presence of air
• forms compounds with nonmetallic elements in nature • forms sodium chloride in the presence of chlorine gas
Identify one chemical property of sodium from this list.
Let's find a chemical property here. Melting is a phase change and is therefore, physical. Oxidizes easily in the presence of air. Oxidation is a reaction and therefore a chemical property. Forming compounds with nonmetallic elements is also a chemical property as well as the formation of sodium chloride.
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