Matter Study Guide

When substances undergo a change of state, such as melting, freezing, evaporating, condensing, or sublimating, the total amount of matter, or mass, remains constant. This fundamental principle is known as the Law of Conservation of Mass. During these physical changes, the particles of the substance simply rearrange themselves; they do not gain or lose any atoms. For example, when ice melts into water, the water molecules (H₂O) are still present, just in a more disordered arrangement. Similarly, when water evaporates, it becomes water vapour, but it is still H₂O. No new atoms are created, and no existing atoms are destroyed. Therefore, if you measure the mass of a substance before and after a change of state in a closed system, the mass will be the same. An open system, like water evaporating from an open beaker, might appear to 'lose' mass because the water vapour escapes into the air, but the mass is still conserved; it has just moved from the beaker to the atmosphere.

Atmospheric pressure is the force exerted by the column of air above a given point per unit area. Imagine a stack of books; the bottom book experiences the pressure from all the books above it, while a book higher up experiences pressure from fewer books. The Earth's atmosphere is like this stack of air molecules. At sea level, there is the entire column of atmosphere above you, meaning a very large mass of air exerting a downward force due to gravity. This results in relatively high atmospheric pressure. As you move higher above the Earth's surface, for example, to the top of a mountain, the column of air above you becomes shorter. Consequently, there is less air above you, which means a smaller mass of air exerting a gravitational force downwards. This reduction in the mass of the air column leads to a decrease in the atmospheric pressure. Additionally, air is compressible. At lower altitudes, the weight of the air above compresses the air molecules closer together, increasing the density of the air. This higher density also contributes to greater pressure. At higher altitudes, with less weight above, the air is less compressed, leading to lower density and thus lower pressure. This combined effect of a shorter, less dense column of air explains why atmospheric pressure decreases as altitude increases.