What is a Chemical change?
A chemical change forms a new substance and this process is usually irreversible. Varying from each chemical reaction, the substance either absorbs or gives off energy.
What is a Physical change?
During a physical change, unlike the chemical change, there is no release or absorption of energy. There will not be any new substances formed, usually it's a change of state or shape, and the process is easily reversed by physical means.
You will need:
Copper (II) carbonate
Sodium chloride
2 Test tubes
Bunsen burner
Procedure:
1. Scoop one spatula of Copper (II) carbonate into a dry test tube
2. If there is not observable change, heat strongly by placing the bottom of the test tube at the hottest part of the flame
3. After heating, place the test tube in a test tube rack and leave it to cool. Observe any changes in color or state that might take place on cooling
4. Record your observation in the table below
5. Repeat steps 1 to 4 with sodium chloride
It is true that in a physical change the substance, in this case sodium chloride, changes in state and color. However, by cooling the substance, it solidifies again. This means that the process is reversible. Hence it is a physical change.
Whilst in a chemical change, the process is irreversible. The substance, copper (II) carbonate, had changed completely from a green powder solid to a black solid. After cooling, the process could not be reversed too.
Therefore, we can conclude that chemical change, unlike physical change, is irreversible.
A chemical change forms a new substance and this process is usually irreversible. Varying from each chemical reaction, the substance either absorbs or gives off energy.
What is a Physical change?
During a physical change, unlike the chemical change, there is no release or absorption of energy. There will not be any new substances formed, usually it's a change of state or shape, and the process is easily reversed by physical means.
The Experiment
To prove that in a chemical change, unlike a physical change, the process is irreversible, we shall conduct an experiment.You will need:
Copper (II) carbonate
Sodium chloride
2 Test tubes
Bunsen burner
Procedure:
1. Scoop one spatula of Copper (II) carbonate into a dry test tube
2. If there is not observable change, heat strongly by placing the bottom of the test tube at the hottest part of the flame
3. After heating, place the test tube in a test tube rack and leave it to cool. Observe any changes in color or state that might take place on cooling
4. Record your observation in the table below
5. Repeat steps 1 to 4 with sodium chloride
Observations
 |
| The table (filled in) |
Reflections
From what we just observed, sodium chloride just underwent melting whilst copper (II) carbonate underwent thermal decomposition. I will cover more on the chemical changes in my later posts.It is true that in a physical change the substance, in this case sodium chloride, changes in state and color. However, by cooling the substance, it solidifies again. This means that the process is reversible. Hence it is a physical change.
Whilst in a chemical change, the process is irreversible. The substance, copper (II) carbonate, had changed completely from a green powder solid to a black solid. After cooling, the process could not be reversed too.
Therefore, we can conclude that chemical change, unlike physical change, is irreversible.
***
The Other Experiment
Now, to prove that a chemical change releases or absorbs energy (in this case only release), we shall conduct an experiment with magnesium.
You will need:
A magnesium ribbon
A pair of metal tongs (not thongs)
A bunsen burner
Procedure:
1. Hold the strip of magnesium ribbon using a pair of metal tongs and heat it directly to the flame
2. DO NOT stare directly at the flame, you may become blind
3. Let it cool for about 5 minutes and observe any changes that might have take place
4. Record all your observations in the table below
Observations
 |
| The table (filled in) |
Reflections
The magnesium ribbon had indeed given off light energy. Furthermore, this can also confirm that in a chemical change the process is irreversible and a new substance is formed. The initially silver solid strip, after heating, had become a bright white light. This light is the evidence that energy is given off.
But as observed in the previous experiment, the sodium chloride did not release any form of energy, be it light energy or heat energy.
Hence, we can conclude that chemical changes release energy.
