Reversible & Irreversible Changes
Investigate which changes can be undone and which cannot, and learn methods for separating mixtures.
Reversible Changes
A reversible change is one that can be undone -- the original substances can be recovered. No new substance is formed.
Dissolving
Salt dissolves in water. You can evaporate the water to get the salt back. The salt was always there -- just invisible.
Mixing
Mix sand and iron filings. You can use a magnet to separate the iron filings from the sand.
Changing State
Melting ice into water is reversible. Freeze the water and you get ice again. Same substance, different state.
Irreversible Changes
An irreversible change creates a new substance and cannot be undone. These are chemical changes.
Burning
Wood burns to produce ash, smoke, and gases. You cannot turn ash back into wood. New substances are formed.
Cooking
A raw egg becomes a cooked egg when heated. The proteins change permanently. You cannot un-cook an egg.
Rusting
Iron + oxygen + water = rust (iron oxide). Rust is a different substance from iron. It cannot be reversed simply.
Inquiry Question: When you bake bread dough in an oven, is this reversible or irreversible? What evidence tells you?
Separating Mixtures
Since mixing is reversible, we can use different methods to separate the parts of a mixture. The method depends on the properties of the materials.
Filtering
Separates an insoluble solid from a liquid. The liquid passes through the filter paper; the solid stays behind.
Example: Separating sand from water.
Evaporating
Separates a dissolved solid from a liquid. Heat the solution; the water evaporates and the solid is left behind.
Example: Getting salt crystals from salt water.
Sieving
Separates solids of different sizes. Smaller particles fall through the holes; larger ones remain.
Example: Separating flour from rice.
Using a Magnet
Separates magnetic materials from non-magnetic ones.
Example: Iron filings from sand.
Fair Test Concept
When doing experiments about changes or separating mixtures, scientists use a fair test. This means:
- ●Change only one variable at a time (the thing you are testing)
- ●Keep all other conditions the same (controlled variables)
- ●Measure and record the results carefully
- ●Repeat the experiment to check your results are reliable
Key Vocabulary
Dissolving
When a solid mixes completely into a liquid, seeming to disappear (but it is still there).
Solution
A mixture where a solid has dissolved in a liquid (e.g. salt water).
Filtering
Passing a mixture through a filter to separate insoluble solids from liquids.
Evaporation
Heating a solution so the liquid turns to gas, leaving the dissolved solid behind.
Fair Test
An experiment where only one variable is changed while all others are kept the same.
Variable
A factor that can change in an experiment (e.g. temperature, amount of water).
Worked Examples
How would you separate sand from water?
Step 1: Sand does not dissolve in water -- it is insoluble.
Step 2: Use filtering. Pour the mixture through filter paper in a funnel.
Answer: The water passes through the filter, and the sand stays behind on the filter paper.
How would you get salt back from salt water?
Step 1: Salt has dissolved in the water (it is a solution). Filtering will NOT work -- the salt passes through too.
Step 2: Use evaporation. Heat the salt water gently.
Answer: The water evaporates as steam, leaving salt crystals behind.
Is baking a cake reversible or irreversible?
Step 1: You mix flour, eggs, sugar, and butter, then heat them in an oven.
Step 2: Can you get back the raw ingredients? No -- the heat caused a chemical change.
Answer: Irreversible. Baking creates a new substance (cake). You cannot un-bake a cake back into flour and eggs.
Knowledge Check
Select the correct answer for each question. Click "Check Answer" to see if you are right.
Question 1
Which of these is a reversible change?
Question 2
What method would you use to separate salt from water?
Question 3
In a fair test, how many variables should you change at a time?
Question 4
How would you separate iron filings from sand?
Question 5
Burning a candle produces wax drips, smoke, and soot. Is this reversible or irreversible?
Key Concepts Summary
- ●Reversible changes can be undone (dissolving, melting, mixing). No new substance is formed.
- ●Irreversible changes cannot be undone (burning, cooking, rusting). A new substance is created.
- ●Mixtures can be separated: filtering (insoluble solids), evaporating (dissolved solids), sieving (different sizes), magnets (magnetic materials).
- ●A fair test changes only one variable at a time while keeping everything else the same.