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Knowing melting and boiling points is not enough. Every set of information is very important because it tells us about the behaviour of the substance. For example, the behaviour of carbon dioxide will be different from the behaviour of oxygen, although oxygen is a reactant of carbon dioxide yet it there is a difference.
To check the behaviour of chemicals under temperature is one of the core fundamentals. The best way to find how the state of matter behaves under temperature is through melting and boiling curves. Some students think both are the same and that is incorrect. They might look the same but the difference is in the concept of the curves and that is the most important part for you to learn the concept behind the heating and cooling curves. In this resource, we will teach you how to make such curves.
Melting and Boiling Points
In the case of the heating curve, you will see these two common points. Imagine an ice cube, the ice cube will first melt and then it will get a boil. The melting point is the point at which the solid start to convert into liquid. The arrangement of atoms will shift from vibration to free movement. The ice cube requires a specific amount of heat energy that will cause atoms to start moving. That heat energy will be provided by a heating source and it will be measured in temperature.
When the ice cube has turned to 100% liquid and you still keep heating the liquid, the liquid particles will start to absorb the heat energy until a time will come when the motion of the atoms will become more freely that they won't have any resistance like they use to have in a liquid state due to fixed boundaries, just like gas. That will be the point when the liquid state is changing to the gas state. We call it the boiling point because the liquid started to boil.
Condensation and Freezing Points
These two points are common in a cooling curve. The cooling curve is the curve that shows how the substance is cooling from gas to solid. The water vapours (that you made above by heating the ice cube) is now to be cooled. Basically, you are removing heat energy from those molecules. You allowed the gas to cool at room temperature. The temperature decreases, why? Because the vapours contain excess energy. They will start to release that energy hence cooling will occur. A point will come when the gas will start to turn liquid. That point is called the condensation point. At this point, the gas atoms will have less energy and this will cause them to move in a fixed boundary.
Let's say you put that liquid into the freezer for more cooling. The particles will lose more energy and a time will come that the atoms will stop moving and started to form into a pack. The atoms will start vibrating instead of moving like in solids. The point at which the liquid will turn into a solid is called the freezing point.
How to Make a Heating Curve
To make the heating curve, you will require the melting and boiling point of the substance. Follow the below steps to make a heating curve of a substance. To make things easy, we will assume that the ice cube is been heated to make vapours. The ice cube temperature is at -10ºC. The melting point of the ice is 0ºC and the boiling point of water is 100ºC. Now you have all the information, you can start with the heating curve.
- Mark the initial, melting and boiling points of the water. If you are provided with times of melting and boiling then plot the melting and boiling points according to the time.
- Join the points and make sure you don't leave points behind.
- The graph would be like a zig-zag. Mark the axis of the graph.
Graph Interpretation
From -10ºC to 0ºC, the temperature rises because of no state change. When 0ºC is reached, the ice will start to melt and that will make the temperature constant for some time because heat energy is been utilized for changing the state. The point at which the solid started turning to liquid is called the melting point. Once the solid is 100% converted to liquid, the next step will be temperature rising up to the boiling point. The heat energy will be gained by the particles until a point when particles will be about to escape into the atmosphere. That is the point when the liquid will turn gas. When 100ºC is reached, the temperature will become constant again because heat energy is been converted to kinetic energy allowing particles to turn into vapour by moving freely without any restrictions. Above 100ºC, there will be no change of state.
How To Make a Cooling Curve
The cooling curve will be opposite to the heating curve. It is made for substances that are getting cooled. To make a cooling curve, follow the below steps. The vapours we made from the ice, we will collect it and then turn it back to the ice. We need to cool it from 100ºC to -10ºC. Before you even start plotting, make sure you have the condensation point and freezing point. Once you have all the information, then you can proceed to the cooling curve, following the below guidelines.
- Mark the initial, freezing and condensation points of the water. If you are provided with times of freezing and condensation then plot the melting and boiling points according to the time.
- Join the points and make sure you don't leave points behind.
- The graph would be like a zig-zag. Mark the axis of the graph.
Graph Interpretation
Let's say our gas is at 110ºC. You put the gas jar in the refrigerator. The refrigerator will start taking heat energy from the vapours and the temperature starts to decrease. From 110ºC to 100ºC, the temperature will steadily decrease. When 100ºC is reached, the temperature will become constant and the particles will start to come together. Particles will keep moving but in a fixed space just like in liquids. Basically, gas is turning back to liquid and 100ºC became the condensation point. Below 100ºC, the temperature will gradually decrease until 0ºC. When 0ºC is achieved, the particles will stop moving and will get closer. A state will come when particles will vibrate in a fixed position. It means the liquid is converting into solid and the point will be known as the freezing point. Below 0ºC, the water will remain solid for any temperature.
Was very educative and help. Gave me a broader understanding of the topic