Ch 1 · Physical and Chemical Changes · IGCSE Chemistry 0620
Topic 1 of 4 in Chemical Reactions — IGCSE Chemistry 0620 — 38 lessons.
Introduction
Ice melting and a candle burning both look like "something happening", but only one of them makes a new substance. Deciding whether a change is physical or chemical is the first step in every practical observation you record, and it is the reason the rest of this module talks about reactions, rates and reversibility. The test is always the same: has a new substance been formed?
Key ideas — Physical changes
A physical change does not form new substances and is usually easy to reverse. The particles themselves are unchanged; only their arrangement, movement or spacing alters.
🧑🏫 Teacher's explanation
Imagine you have a box of LEGO bricks. A physical change is like taking a LEGO house and pulling it apart to make a LEGO car. You have changed the shape and the arrangement, but you still have the exact same bricks you started with. You haven't created any new plastic; you've just rearranged what was already there.
📖 Definitions
🔍 Worked example — Easy
A student dissolves 5.0 g of NaCl in 100 g of water. Is this a physical or chemical change? Justify your answer.
🔍 Worked example — Standard
Calculate the mass of water produced when 18 g of ice melts completely at 0circC.
🔍 Worked example — Exam-style
A sample of sulfur powder is mixed with iron filings. A magnet is passed over the mixture. Describe the type of change occurring and explain why the iron can be removed.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) State whether the process of dissolving the salt in water is a physical or chemical change. [1] (b) Describe how the student can recover the solid salt from the salt solution. [2] [3 marks]
Key ideas — Chemical changes
A chemical change forms new substances and is usually difficult to reverse. Bonds between atoms are broken and new bonds form, so the products have different properties from the reactants. A chemical change is another name for a chemical reaction.
🧑🏫 Teacher's explanation
Imagine you have a LEGO castle. A physical change is like taking the castle apart and building a LEGO car. You still have the exact same bricks; you've just rearranged them. A chemical change is like taking those same LEGO bricks and melting them down into a solid block of plastic. You can't easily get the bricks back; you've created something entirely new with different properties.
📖 Definitions
🔍 Worked example — Easy
Classify the following as a physical or chemical change: H_2O(l) arrow H_2O(g)
🔍 Worked example — Standard
Identify the type of change occurring when magnesium ribbon is placed in dilute hydrochloric acid: Mg(s) + 2HCl(aq) arrow MgCl_2(aq) + H_2(g)
🔍 Worked example — Exam-style
A student observes a reaction between two clear solutions. After mixing, a white cloudy solid forms and the test tube feels warm. Identify the two signs of a chemical change observed.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Classifying everyday changes
Be careful with dissolving. Sugar or salt dissolving in water is physical. Magnesium "dissolving" in acid is chemical, because gas is given off and a new compound (magnesium chloride) is left in the solution.
🧑🏫 Teacher's explanation
Imagine you have a Lego castle. If you pull the bricks apart to make a pile of individual bricks, you haven't changed what the bricks are; you've just changed their arrangement. This is a physical change. If you were to take those bricks and melt them down into a single solid plastic blob, you have changed the substance itself. This is a chemical change.
📖 Definitions
🔍 Worked example — Easy
Classify the following as physical or chemical: H_2O(l) arrow H_2O(g)
🔍 Worked example — Standard
A piece of magnesium ribbon is placed in dilute hydrochloric acid. The following reaction occurs: Mg(s) + 2HCl(aq) arrow MgCl_2(aq) + H_2(g). Classify this change.
🔍 Worked example — Exam-style
Iron filings and sulfur powder are mixed together in a dish. A student uses a magnet to pull the iron filings away from the sulfur. Is this a physical or chemical change? Justify your answer.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Experiment A: Sugar is dissolved in water. Experiment B: Magnesium metal reacts with dilute acid to produce hydrogen gas.
Worked example
Question. A student mixes iron filings with powdered sulfur in a test tube and then heats the mixture strongly. Before heating, a magnet lifts the iron out of the mixture. After heating, a dark grey solid remains that the magnet does not attract, and the tube glows even after the Bunsen burner is removed. (a) Explain why mixing the iron and sulfur is a physical change. [2] (b) Give two pieces of evidence that heating the mixture caused a chemical change. [2]
🔍 Worked example — Easy
Classify the following as physical or chemical: (i) Ice melting in a glass of water, (ii) Magnesium ribbon burning in air.
🔍 Worked example — Standard
A student observes a reaction where a solid precipitate forms at the bottom of a test tube. Is this a physical or chemical change? Explain.
🔍 Worked example — Exam-style
Iron filings and sulfur powder are mixed together. A magnet is passed over the mixture. The iron is attracted to the magnet, but the sulfur is not. Is this a physical or chemical change? Justify your answer using two pieces of evidence.
⚠️ Common errors
Common mistakes
Practice
Answers A chemical change forms new substances, a physical change does not [B1]; a chemical change is usually difficult to reverse, a physical change is usually easy to reverse [B1]. (a) physical [B1]; (b) chemical [B1]; (c) chemical [B1]; (d) physical [B1]. Chemical change [B1], because a new substance (anhydrous copper(II) sulfate) with a different colour is formed [B1]; the fact that it can be reversed and the temperature change on adding water show that it is a reversible chemical reaction, not a physical change [B1]. Crushing only changes the size of the pieces; the chalk is still calcium carbonate [B1]; with acid, fizzing shows carbon dioxide, a new substance, is produced [B1].