⚛ A chemical change is usually difficult to reverse.
⚛ A physical change involves a change in the physical state or appearance of the substance.
⚛ A physical change is usually easy to reverse.
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A physical change
Heating solid water (ice) so that it melts into liquid water is a physical change because it:
Some common examples of physical changes are given in the table below:
| Physical Change | Example |
|---|---|
| Melting a Solid (change of state) |
Ice, solid water, can be heated to form liquid water. (no new substance(s) formed) |
| Freezing a liquid (change of state) |
Liquid water can be frozen to form solid water, ice. (no new substance(s) formed) |
| Boiling a liquid (change of state) |
Liquid water can be heated to form water vapour, a gas. (no new substance(s) formed) |
| Condensing a gas (change of state) |
Water vapour can be cooled to form liquid water. (no new substance(s) formed) |
| Dissolving a solid | Solid sodium chloride can be dissolved in liquid water to form sodium chloride solution. (no new substance(s) formed) |
| Evaporating a solution | Sodium chloride solution can be gently heated to remove the water as water vapour leaving the solid sodium chloride behind. (no new substance(s) formed) |
| Grinding a solid | Pieces of calcium carbonate can be ground down to form smaller pieces of calcium carbonate. (no new substance(s) formed) |
| Changing the shape | A piece of copper wire can be hammered out into a flat sheet of copper. (no new substance(s) formed) |
A chemical change
Burning wood is a chemical change because
In order to determine whether a chemical change has taken place, we need to know if at least 1 new substance has been formed.
There are a number of observations that we can make to help us decide when a chemical change has taken place and a new substance has been formed.
These observations include:
Some examples of the observations that help you decide if a chemical change has taken place are given in the table below:
| Observation | Example of a Chemical Change |
|---|---|
| A gas is evolved. | When magnesium reacts with hydrochloric acid a new substance, hydrogen gas, is produced.
You can observe bubbles of gas forming on the magnesium and rising to the surface of the acid. |
| An odour is produced. | When hydrochloric acid is added to iron sulfide, the pungent odour of "rotten eggs" is produced due to the formation of the new substance hydrogen sulfide gas. |
| A new solid is formed. | When colourless silver nitrate solution is added to colourless sodium chloride solution, a new white solid of silver chloride is produced. |
| There is a colour change. | When metallic iron reacts with oxygen in the presence of water, a new red coloured solid called rust is formed. |
| There is a change in the temperature of the system. (not caused by an outside agency) |
When sodium metal reacts with water the temperature of the water increases substantially due to the chemical reaction between the sodium and the water. |
| A solid disappears. (that is not due to a physical change) |
When solid sodium hydroxide is added to hydrochloric acid, the solid disappears due to a chemical reaction between the sodium hydroxide and the hydrochloric acid. |
Refer to the StoPGoPS model for problem solving.
Question 1:10 mL of a colourless solution labelled sulfuric acid is added to 10 mL of a colourless solution labelled barium nitrate in a test tube.
The solution turns cloudy and finally settles to the bottom of the test tube as a white solid.
Is this an example of a physcial change or of a chemical change?
Solution:
What is the question asking you to find out?
Determine whether a physical change or a chemical change has taken place.
What information have you been given in the question (what do you know)?
Sulfuric acid solution is colourless.
Barium nitrate solution is colourless.
The 2 solutions mixed together produced a white solid.
What is the relationship between what you know and what you need to find out?
Physical change: easy to reverse because no new substance(s) are formed.
Chemical change: difficult to reverse because new substance(s) are formed.
Answer the question.
A new substance, a white solid, has formed so this is a chemical change.
Is your answer plausible?
Consider whether the description given could be a physical change.
A physical change could involve a change state (liquid to solid) but only if the system was cooled.
The question did not mention any change in temperature so it seems unlikely that the change in state was due to a physical change.
Similarly, if this were a physical change in which a liquid turned to a solid, you would not expect the solid to be a different colour to the liquid, that is, if the liquid is colourless you expect the solid to be colourless instead of white.
The question is more likely to be describing a chemical change than a physical change.
State the solution to the question.
This is an example of a chemical change.
Question 2:A small amount of a dark solid is placed in the bottom of a sealed flask.
The flask is heated very gently.
The solid disappears and the flask fills with a purple gas.
On cooling, the purple gas disappears and the dark solid re-appears at the bottom of the flask.
Has a chemical change taken place?
Solution:
What is the question asking you to find out?
Decide if a chemical change has taken place.
What information have you been given in the question (what do you know)?
Sealed vessel: no gas or solid can enter nor leave the flask.
Dark solid forms a purple gas when heated.
Purple gas forms a dark solid when cooled.
What is the relationship between what you know and what you need to find out?
Physical change: easy to reverse because no new substance(s) are formed.
Chemical change: difficult to reverse because new substance(s) are formed.
Answer the question.
The change was easy to reverse. Just by cooling the gas it re-formed the solid.
This is most likely to be a physical change.
Is your answer plausible?
Consider whether a chemical change could have taken place.
A solid disappeared and a gas appeared, but this could be the result of a physical change called sublimation in which a solid changes into a gas (without becoming a liquid first).
Has there been a change in colour? The solid is described as dark, but, a densely packed purple solid can appear dark rather than purple.
Given that the change was so easy to reverse, it is far more likely to be a physical change than a chemical change.
State the solution to the question.
No. A chemical change has not taken place.