Grade 8 Natural Sciences: The Particle Model of Matter and Density

The Particle Model of Matter and the Three Phases Of Matter

The particle model of matter is a scientific theory used to explain the properties and behaviors of different states of matter: solids, liquids, and gases. Every substance is composed of tiny particles, and the way these particles are arranged and behave determines the phase of the substance. To compare these phases, we examine the spaces between particles, the forces acting between particles, and the movement of the particles themselves.

In solids, the forces between particles are described as being very strong. This keeps the particles in a fixed, rigid structure where they are closely packed together with very little space between them. Consequently, the movement of particles in a solid is limited to vibrations in a fixed position. In liquids, the forces between particles are weaker than in solids, allowing for more space between the particles. This enables particles to move more freely by sliding over one another, although they still remain in contact. In gases, the forces between particles are extremely weak, and the spaces between them are very large. This allows gas particles to move rapidly and randomly in all directions, often colliding with each other or the walls of their container.

Properties and Characteristics of the Three States

Each state of matter possesses distinct physical properties based on its internal particle structure. Solids are characterized by a fixed shape and volume; they cannot be compressed easily because there is almost no empty space between the particles. Liquids have a fixed volume but take the shape of the container they are placed in. While particles in a liquid are close together, they are not in a fixed pattern, allowing the liquid to flow.

Gases have neither a fixed shape nor a fixed volume; they expand to fill whatever container they occupy. Because of the vast spaces between gas particles, gases are highly compressible. The speed and energy of particles in a gas are much higher than those in liquids or solids. When particles in any phase move, their kinetic energy is related to the temperature of the substance. The units used to measure these characteristics often transition between the macroscopic (large scale) and microscopic (particle scale).

Phase Changes and Transitions in Matter

Phase changes occur when energy, usually in the form of heat, is added to or removed from a substance. These changes are represented diagrammatically as transitions between solids, liquids, and gases.

Starting from a solid, the process of changing into a liquid is called melting (smelt). The specific temperature at which this occurs is the melting point. When a liquid changes into a solid, the process is known as freezing (vries), occurring at the freezing point. The transition from a liquid to a gas is called evaporation (verdamp) or boiling, which occurs at the boiling point (kookpunt). Conversely, when a gas loses energy and turns into a liquid, the process is called condensation (kondensasie).

Sublimation (sublimasie) is a unique phase change where a substance transitions directly from a solid to a gas, or a gas to a solid, without passing through the liquid phase. A well-known example of a substance that undergoes sublimation is dry ice, which is frozen carbon dioxide ($CO_2$).

Thermodynamics, Energy, and Intermolecular Forces

The behavior of particles during a phase change is dictated by the amount of energy available to overcome intermolecular forces. For example, if the boiling point of nitrogen gas is $-196^{\circ}C$, several inferences can be made about its physical nature. First, very little energy is required to change its phase from a liquid to a gas because the transition happens at such an extremely low temperature. Second, the forces between the nitrogen particles are very weak, as they cannot hold the particles together even at cold temperatures.

When a gas is cooled, its heat energy decreases, leading to a decrease in temperature. As the energy diminishes, the particles move more slowly. Eventually, the gas condenses into a liquid. During this transition, the intermolecular forces become stronger, and instead of flying freely, the particles begin to slide over one another. If cooling continues, the energy will drop further until the substance reaches its freezing point and becomes a solid.

Comparative Study of Substances and Phase Points

To determine the state of matter of various substances at room temperature, which is typically taken as $25^{\circ}C$, we compare the room temperature to the substance's melting and boiling points.

Consider the following data for substances P, Q, R, S, and T:

• Substance P: Melting Point $-117^{\circ}C$, Boiling Point $78.5^{\circ}C$.

• Substance Q: Melting Point $658^{\circ}C$, Boiling Point $2467^{\circ}C$.

• Substance R: Melting Point $-39^{\circ}C$, Boiling Point $357^{\circ}C$.

• Substance S: Melting Point $-182^{\circ}C$, Boiling Point $-164^{\circ}C$.

• Substance T: Melting Point $1530^{\circ}C$, Boiling Point $2735^{\circ}C$.

At a room temperature of $25^{\circ}C$:

• Substances Q and T are solids because their melting points are significantly higher than $25^{\circ}C$.

• Substances P and R are liquids because $25^{\circ}C$ falls between their melting and boiling points.

• Substance S is a gas because its boiling point ($-164^{\circ}C$) is much lower than $25^{\circ}C$.

Furthermore, the strength of forces between particles can be inferred from these points. Substance T has the strongest intermolecular forces because it requires the most energy ($1530^{\circ}C$) just to melt. Substance S has the weakest intermolecular forces, as it remains a gas even at $-164^{\circ}C$.

Temperature versus Time Graph Analysis

Graphs representing temperature over time provide a visual record of how a substance absorbs energy. When a solid is heated, the graph shows a rising line as the temperature increases (Section A). When the substance reaches its melting point, the temperature remains constant even though heat is still being added; this is represented by a horizontal plateau (Section B). During this time, the energy is used to break the forces holding the solid particles together rather than increasing the temperature.

Once the substance has entirely melted into a liquid, the temperature begins to rise again (Section C). When the liquid reaches its boiling point, another plateau appears (Section D). For a specific substance reaching the boiling point at $85^{\circ}C$, the temperature will stay at $85^{\circ}C$ until all the liquid has turned into gas. In the provided data, this process was observed over a duration of $160$ minutes.

Mass, Volume, and Measurement Apparatus

All matter possesses mass and occupies volume. Mass is an indication of the quantity of matter a substance contains and is measured in grams ($g$) or kilograms ($kg$). A critical conversion factor is: $1000\,g = 1\,kg$ To determine mass, scientists use instruments such as a triple beam balance (driebalkskaal), a mass meter, or an electronic scale.

Volume refers to the amount of space a substance occupies. Common units for volume include cubic meters ($m^3$), cubic centimeters ($cm^3$), and cubic millimeters ($mm^3$). For liquids, volume is often measured using a measuring cylinder (maatsilinder), a burette (buret), or a pipette (pipet).

The volume of an irregular object can be determined using the displacement method. This involves lowering the object into a measuring cylinder filled with water. The amount by which the water level rises is exactly equal to the volume of the object.

Density: Principles and Calculations

Density is defined as the amount of mass contained within a specific volume of a substance. It is a measurement of how tightly the "stuff" or matter is packed together. Substances with a large mass per cubic centimeter ($cm^3$) have a high density, such as a lead brick, whereas substances like bread or a sponge have a low density. The formula for density is: $\text{Density} = \frac{\text{Mass}}{\text{Volume}}$ Density is typically expressed in units of grams per cubic centimeter ($g/cm^3$) or $g\,cm^{-3}$.

The density of a substance depends on three primary factors: the nature of the particles (the size and type of particles), the size of the spaces between the particles, and the strength of the forces between them. Generally, solids are more dense than liquids, and liquids are more dense than gases. This is because particles in a solid are packed closely together, meaning there is more mass in a given volume.

Floating, Sinking, and the Anomaly of Water

Density plays a crucial role in determining whether an object will float or sink in a liquid. A substance with a lower density will float on a liquid with a higher density. Pure water has a density of approximately $1\,g/cm^3$. Any object with a density less than $1\,g/cm^3$ will float in water, while objects with a density greater than $1\,g/cm^3$ will sink.

A significant exception to the general rule of density is water. Usually, the solid phase of a substance is denser than its liquid phase. However, ice (solid water) is actually less dense than liquid water. This occurs because water molecules form a specific crystalline structure when freezing that creates larger spaces between the particles than those in the liquid state. Consequently, ice floats on water.

Practical Application: Calculating Brick Density

To calculate the density of a standard brick with the dimensions of $23\,cm$ (length) $\times 8.5\,cm$ (width) $\times 7\,cm$ (height), we must first determine the volume: $\text{Volume} = L \times B \times H$ $\text{Volume} = 23\,cm \times 8.5\,cm \times 7\,cm = 1368.5\,cm^3$

If the mass of the brick is $3000\,g$ (which is equivalent to $3\,kg$), we apply the density formula: $\text{Density} = \frac{3000\,g}{1368.5\,cm^3} \approx 2.19\,g/cm^3$ Since $2.19\,g/cm^3$ is greater than the density of water ($1\,g/cm^3$), the brick will sink.