Physical and chemical changesIB MYP Chemistry: Revision notes
Section 1
Two kinds of change
In a physical change no new substance is made. The substance may change shape, state or size, but it is still the same substance. Examples: melting ice, dissolving sugar in water, breaking a glass.
In a chemical change (a chemical reaction) one or more new substances are made, with different properties. Examples: burning wood, rusting iron, baking a cake.
The key question is always: has a new substance been made?
Section 2
Evidence of a chemical change
Signs that a chemical change may have happened:
- A new substance is formed, for example a different material or smell
- A colour change
- A gas is produced (fizzing or bubbles)
- A temperature change (the mixture gets hotter or colder without being heated or cooled)
- A precipitate forms: an insoluble solid appears when two solutions are mixed
Worked example: magnesium is added to acid. It fizzes (gas), disappears (new substance) and the tube warms (temperature change). These are strong evidence of a chemical change.
One sign is not always proof. Some physical changes, such as dissolving certain solids, also change the temperature. Look for several pieces of evidence, especially a new substance.
Section 3
Reversibility
Most physical changes are easy to reverse. Melted wax can be frozen again, and dissolved salt can be recovered by evaporating the water.
Most chemical changes are difficult to reverse. You cannot turn burnt wood back into wood.
Some chemical changes can be reversed. For example, heating blue copper(II) sulfate crystals makes a white powder and water. Adding water to the white powder makes the blue crystals again, and the tube warms. This is a reversible reaction.
Being reversible is a clue, but the test is still whether a new substance was made.
Do not say that all chemical changes are irreversible. Say that most are difficult to reverse.
Section 4
Deciding which type of change
Use this routine:
- Look for a new substance. If one is made, it is chemical.
- List the evidence: colour change, gas, precipitate, temperature change, new smell.
- Ask whether the change can easily be reversed, for example by melting, freezing or evaporating.
Example: a candle. Melting and solidifying of wax are physical. Burning is chemical because carbon dioxide and water are made and heat and light are given out.
Must Know
- Physical change: no new substance, usually easy to reverse
- Chemical change: a new substance is made, usually hard to reverse
- Evidence: new substance, colour change, gas, temperature change, precipitate
- A temperature change alone is not proof of a chemical change
- A few chemical changes are reversible
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Physical and chemical changes
- Students in Nairobi observe four changes in a kitchen. Butter melts in a warm pan. Sugar dissolves in hot tea. Bread turns brown and crisp in a toaster. A cake mixture rises, turns golden brown and gives off a new smell in the oven.Give two pieces of evidence from the description that baking the cake involves a chemical change.2 marks
- A student in Cairo adds a 3 cm strip of magnesium ribbon to dilute hydrochloric acid in a test tube. The ribbon fizzes and gradually disappears, and the temperature of the mixture rises from 21 °C to 33 °C.Explain why the changes in the test tube are chemical rather than physical.2 marks
- Students in Manila heat 2.00 g of blue crystals of hydrated copper(II) sulfate in a test tube. The crystals turn white, droplets of liquid form on the cooler upper part of the tube, and the mass of solid left is 1.28 g. After the tube has cooled, the students add a few drops of water to the white powder. It turns blue again and the tube feels warm.Describe three pieces of evidence from the investigation that show chemical changes took place.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).