Describe the identifiable physical properties of substances and explain how changes in these properties can occur without changing the chemical nature of the substance.

Mass &
density are 2 properties of matter.
When water freezes it expands making it less dense yet it is still water.
When iron is heated it expands until it's a liquid; at that point it is less dense, yet it is still iron.

Identifiable physical properties of substances are characteristics that can be observed or measured without altering the chemical composition of the substance. These properties help us classify and differentiate one substance from another. Two common physical properties are mass and density.

Mass: Mass is a measure of the amount of matter in a substance. It is a fundamental property and remains constant, regardless of the physical or chemical changes a substance may undergo. Mass can be determined by using a balance or scale to compare the substance with a known mass.

Density: Density is the ratio of mass to volume and is a measure of how compact a substance is. It is calculated by dividing the mass of an object by its volume. Density is an intensive property, meaning it remains constant regardless of the quantity of the substance. Different substances have different densities, allowing us to identify and distinguish them.

Now, let's explore how changes in these properties can occur without changing the chemical nature of the substance.

When you mentioned water freezing, it is an example of a physical change. As water freezes, its temperature drops below its freezing point (0 degrees Celsius or 32 degrees Fahrenheit). During this process, the water molecules start forming a regular lattice structure, which leads to an increase in volume and, consequently, a decrease in density. Despite this change, the chemical composition of the substance remains the same; it is still H2O, just in a different physical state (from a liquid to a solid).

Similarly, when iron is heated, it undergoes a physical change. As the iron gets heated, it absorbs energy, causing its atoms to vibrate faster and move apart. This increase in atomic movement results in the expansion of the iron, increasing its volume and decreasing its density. Eventually, at its melting point (1538 degrees Celsius or 2800 degrees Fahrenheit), the iron transitions from a solid to a liquid state, with further expansion and density decrease. Nonetheless, it continues to be iron from a chemical perspective.

In both examples, the changes in density and volume are solely related to alterations in the physical state of the substance, rather than the chemical composition. The atoms or molecules within the substance remain the same, only their arrangement and movement change. Therefore, these physical changes do not modify the chemical nature of the substance.