The physical properties of a pure substance can be used to identify the substance and distinguish it from other pure substances. Boiling temperature is one such physical property. Boiling is characterized by the formation of vapor bubbles within the liquid phase as a substance changes from a liquid to a gas. Melting describes the phase change as a solid changes to a liquid. Understanding phase changes as a characteristic property of matter and determining what happens to temperature while a phase change occurs are objectives for this unit.
PHASES OF MATTER
Physical properties – mass, volume density, odor, color, hardness etc, are properties that can be observed without changing the identity or essence of the substance. Yet the same substance can have a different appearance. Think of water, a substance that is fairly common, and that you as a living thing, cannot do without. Ice, liquid water, and steam or water vapor all have a different appearance, but they are all the same substance. As you know, a water molecule consists of hydrogen and oxygen. That does not change whether water appears as solid ice, liquid water, or gaseous vapor. These states are different phases. The main phases of matter are solid, liquid, and gas, and matter can exist in any of these phases depending upon other factors. What are those factors that determine the phase of matter in which a substance exists? The primary ones that we will experiment with include temperature and pressure. Phases of matter are technically “energy states of matter”. Matter exists in a particular phase depending upon how fast the particles that make them up are moving and far apart they are from each other.
KINETIC THEORY OF MATTER
Science uses a technical vocabulary to describe phenomena. Despite the “technical / intimidating” sound of the word “kinetic”, the kinetic theory is actually pretty simple. It is an explanation of how particles in matter behave. Because of that, you can accurately predict what is happening to the particles of a solid, liquid, or gas. The word “kinetic” means motion. The kinetic theory is as follows:
1. All matter is composed of small particles (atoms & molecules)
2. These particles are in constant, random motion.
3. These particles are colliding with each other and the walls of the container.
Applying this theory, you can describe the motion of particles in a solid, liquid, and gas. To visualize the kinetic theory think of each particle as a tennis ball. As a gas, the tennis balls are bouncing off the walls of the room. As a liquid, they are rolling around each other in a small container. As a solid, they remain fixed and vibrating in a tennis canister. Applying energy, in other words, increasing the temperature, makes the tennis balls move even faster. This continues until something else happens.
As mentioned earlier, matter can exist as either a solid, liquid, or gas at different times depending upon other factors. Hydrogen hydroxide – a technical term for the substance you know as water, exists as a solid – ice, as a liquid – water, and as a gas – steam or vapor. The phase of matter of water is determined by the amount of energy applied to the substance. Temperature is not a measure of the amount of energy but it is a sufficient indicator of energy. The higher the temperature, the more energy there is. Either adding energy (increasing the temperature) or taking energy away (decreasing the temperature) causes matter to change from one phase to another.
For an example, let’s start with a block of ice in your freezer. Like all solids, the particles in a block of ice remain fixed and are vibrating back and forth. You may also notice that a block of ice is cold, a characteristic of a low energy state. Take the ice out and sit it on the kitchen counter. As the ice warms to room temperature, the particles in the solid ice vibrate even faster until they break free of their fixed positions and flow around each other. You can’t actually witness this since the particles themselves are way too small, but you can observe solid ice changing into liquid water – the phase change called melting.
Now collect the water, put it in a pot and heat it over the stove. The particles in the liquid move faster and faster as its heated (temperature increases) until they are moving so fast that they need to break free from being close to each other spread out to move even faster. In this case, the individual water particles break away from each other and bounce off the containers, the ceiling, and each moving at rates up to 500 m / s and colliding with other things about 10 billion times each second. Again, the action of individual particles is invisible to you but you will see the pot bubble and vapor emerge from the liquid. Keep the pot on the stove for enough time and you’ll eventually lose all the water. This phase change is called boiling.
These phase changes are examples of physical changes that do not alter the physical properties of the substance. The size and shape of the substances are being changed but the identity or essence of the substance remains the same. The phase changes of matter are melting, freezing (solidifying), boiling (vaporizing), condensing, and sublimation.
The opposite phase change is called condensation. Gaseous water in the atmosphere condensing on objects is called dew.
Some substances skip the liquid phase and can change from a solid directly to a gas. These substances go through the process of sublimation. Dry ice is a substance that is sublime. Dry ice is a solid form of carbon dioxide that changes directly to a gaseous form of carbon dioxide as it absorbs energy. Dry ice is itself, very cold, and can be used to keep other items cold. Because dry ice goes through sublimation, it is considered sublime. Water in the solid form of snow (tiny ice crystals) also is sublime. Think of a snow bank in winter becoming smaller over time. Even without melting, a snow bank would carry out sublimation and eventually disappear.
Data Table: Melting Points and Boiling Points of Substances
HEAT & TEMPERATURE
As previously noted, heat and temperature play an important role in phase changes. Heat is a form of energy, so as you heat an object, you are giving that object more energy. One of your first tasks in the lab will be to determine what specifically happens to the temperature of a substance as it is heated and undergoes a phase change. To investigate this, you will heat substances, make observations, and record the temperature at regular intervals. You will summarize your data on graph that details what happens to temperature as phase changes occur. That graph is also called a phase change curve or a heating curve of a substance. Make sure you are able to describe the shape of a phase change curve.
Apply the kinetic theory of matter to help analyze a phase change curve. Do you remember that all particles are moving? As heat is added, those particles move faster and faster. Temperature is a measure that actually indicates how fast those particles are moving, so as the particles’ speed increases, so does the temperature. Technically speaking, temperature measures the average kinetic energy of a substance.
While a substance undergoes a phase change, the additional heat energy causes the particles to break away from each other instead of move faster. This additional heat energy is called latent energy. As a result, the temperature remains the same as you observe a substance changing from one phase to another. The temperature stays constant until all of the particles in the substance have changed into the new phase. Once they all have changed phase, the additional heat energy is used to make the particles’ speed increase and the temperature will rise once again. When you graph the temperature of a heated substance on a phase change curve, the part of the curve that shows no temperature change is called the plateau. The point at which a phase change curve plateaus is use to determine the boiling point or melting / freezing point of a substance. In this class, you will conduct phase changes with substances such as water, isopropanol (isopropyl alcohol), and moth ball flakes.