Chemical reactions - Physical and chemical changes (3)
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1.
A student carries out an experiment where they heat a solid piece of copper wire. They observe that the wire changes colour and becomes duller. Identify whether this change is physical or chemical. Explain your answer, including referring to changes at a particle level.
This change is a chemical change.
When the copper wire is heated, it reacts with oxygen in the air to form copper oxide (CuO). This is evident by the change in colour from the shiny, reddish-brown copper to a dull, black oxide.
At a particle level, this involves the breaking of chemical bonds within the copper atoms and the formation of new chemical bonds between copper and oxygen atoms. The particles (atoms and molecules) are rearranged to form a new substance with different chemical properties. A physical change only involves changes in state or shape, without altering the chemical composition of the substance.
2.
A student mixes hydrochloric acid (HCl) with sodium hydroxide (NaOH). Write a balanced chemical equation for the reaction. State two observable signs that a chemical reaction has taken place.
The balanced chemical equation for the reaction between hydrochloric acid and sodium hydroxide is:
| HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l) |
Two observable signs that a chemical reaction has taken place are:
- Formation of a gas: The reaction produces hydrogen gas (H2), which is a colourless, flammable gas. You would see bubbles forming.
- Change of colour: The solution may change colour. For example, the hydrochloric acid is typically a clear solution, and the sodium hydroxide is also clear. The resulting solution (sodium chloride solution) is clear.
3.
Describe three differences between a physical change and a chemical change. For each difference, provide a specific example.
Here are three key differences between physical and chemical changes:
- Chemical Composition: In a chemical change, the chemical composition of the substance changes, resulting in the formation of new substances. In a physical change, the chemical composition remains the same.
Example: Burning wood (chemical change - forms ash, carbon dioxide, water) vs. melting ice (physical change - remains water).
- Reversibility: Many physical changes are easily reversible (e.g., melting, freezing, boiling, condensing). Chemical changes are often difficult or impossible to reverse without further chemical reactions.
Example: Melting ice can be reversed by cooling (physical) but burning wood cannot be easily reversed to regain the original wood (chemical).
- Energy Change: Chemical changes typically involve an energy change (either releasing energy - exothermic - or absorbing energy - endothermic). Physical changes usually involve a small energy change, or no significant energy change.
Example: Exploding dynamite (chemical change - releases a large amount of energy) vs. boiling water (physical change - absorbs heat to change state).