Physical and chemical changes

1. Overview

Physical and chemical changes describe how matter behaves when energy is added or removed, or when different substances interact. Understanding these differences is fundamental to chemistry as it allows scientists to distinguish between simple changes in state and the complex reactions that create entirely new materials.

Key Definitions

• Physical Change: A change in which no new chemical substances are formed; it usually involves a change in state or appearance and is often easily reversible.
• Chemical Change: A process involving a chemical reaction that results in the formation of one or more new substances with different properties.
• Reactants: The starting substances in a chemical reaction.
• Products: The new substances formed as a result of a chemical reaction.
• Exothermic: A process that releases thermal energy to the surroundings.
• Endothermic: A process that absorbs thermal energy from the surroundings.

Core Content

Characteristics of Physical Changes

• No new substances are created.
• The change is usually reversible (e.g., ice can be melted into water and then refrozen).
• The chemical properties of the substance remain the same.
• Energy changes are typically small.
• Examples: Changes of state (melting, boiling, freezing), dissolving a solute (like sugar) in a solvent, or mixing two substances without them reacting.

Example: Melting Ice

• Word Equation: Ice → Water
• Symbol Equation: H₂O (s) → H₂O (l)

Characteristics of Chemical Changes

• New chemical substances are formed.
• The change is usually irreversible (or very difficult to reverse).
• The products have different properties from the reactants.
• Often accompanied by significant energy changes (heat or light).
• Signs of a chemical change:
  • Color changes.
  • Effervescence (bubbles of gas produced).
  • Formation of a precipitate (a solid forming from two liquids).
  • Temperature change (getting hotter or colder).

Example: Reaction between Magnesium and Hydrochloric Acid

• Word Equation: Magnesium + Hydrochloric acid → Magnesium chloride + Hydrogen
• Symbol Equation: Mg (s) + 2HCl (aq) → MgCl₂ (aq) + H₂ (g)

Summary Comparison Table

Feature	Physical Change	Chemical Change
New substance formed?	No	Yes
Reversibility	Usually easy to reverse	Usually hard to reverse
Energy change	Small	Often large
Mass	Total mass is conserved	Total mass is conserved

Extended Content (Extended Only)

There are no specific Supplement objectives for Topic 6.1 in the current syllabus.

Key Equations

1. Combustion of Methane (Chemical Change)

• Methane + Oxygen → Carbon dioxide + Water
• CH₄ (g) + 2O₂ (g) → CO₂ (g) + 2H₂O (l)

2. Thermal Decomposition of Calcium Carbonate (Chemical Change)

• Calcium carbonate → Calcium oxide + Carbon dioxide
• CaCO₃ (s) → CaO (s) + CO₂ (g)

3. Condensation of Steam (Physical Change)

• Water (gas) → Water (liquid)
• H₂O (g) → H₂O (l)

Common Mistakes to Avoid

• ❌ Wrong: Boiling water is a chemical change because bubbles are formed.
• ✓ Right: Boiling is a physical change (change of state). The bubbles are steam (H₂O gas), not a new chemical substance like Hydrogen gas.
• ❌ Wrong: Dissolving salt in water is a chemical change because the salt "disappears."
• ✓ Right: Dissolving is a physical change. The salt and water can be separated by evaporation, and no new substance is formed.
• ❌ Wrong: Forgetting to balance the atoms on both sides of a symbol equation.
• ✓ Right: Ensure the number of atoms for each element is equal on the reactant and product sides (e.g., 2HCl is needed to balance the two Cl atoms in MgCl₂).

Exam Tips

• Identify the Context: If a question mentions "heating a substance and then cooling it back to its original form," it is describing a physical change.
• Command Words: If asked to "describe the differences," ensure you compare specific features like reversibility and the formation of new substances.
• Observation Questions: If you are asked for evidence of a chemical reaction, mention specific observations like "fizzing/effervescence" or "the test tube feels hot," rather than just saying "a gas was made."
• State Symbols: Always check if the question asks for state symbols. Even if it doesn't, including correct state symbols (s, l, g, aq) shows a high level of chemical understanding.

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Exam-Style Questions

Practice these original exam-style questions to test your understanding. Each question mirrors the style, structure, and mark allocation of real Cambridge 0620 Theory papers.

Exam-Style Question 1 — Short Answer [5 marks]

Question:

Ethanol ($C_2H_5OH$) can be produced by fermentation of glucose ($C_6H_{12}O_6$). The equation for the reaction is:

$C_6H_{12}O_6(aq) \rightarrow 2C_2H_5OH(aq) + 2CO_2(g)$

(a) State two observations that would indicate a chemical change is occurring during this reaction. [2]

(b) Give one large-scale use of ethanol. [1]

(c) Explain why the fermentation of glucose is considered a chemical change, not a physical change. [2]

Worked Solution:

(a)

1. Bubbles of gas are produced. Bubbles of gas are produced.

2. The mixture may change colour. The mixture may change colour.

How to earn full marks:

• 1 mark for stating bubbles (or gas) are produced.
• 1 mark for stating the mixture changes colour (or any valid observation, e.g. temperature change).

(b)

1. Fuel for vehicles. Fuel for vehicles.

How to earn full marks:

• 1 mark for stating fuel (for vehicles, etc.) OR as a solvent OR in alcoholic drinks.

(c)

1. New substances are formed. New substances are formed.

2. The reaction is not easily reversible. The reaction is not easily reversible.

How to earn full marks:

• 1 mark for stating new substances are formed.
• 1 mark for stating the reaction is not easily reversible OR energy is transferred from the reacting mixture to the surroundings.

Common Pitfall: Students sometimes confuse observations with inferences. For example, stating "a new substance is formed" is an inference, not a direct observation. Focus on what you can actually see, hear, or feel during the reaction.

Exam-Style Question 2 — Short Answer [6 marks]

Question:

A student investigates the reaction between zinc and hydrochloric acid. The reaction produces hydrogen gas.

$Zn(s) + 2HCl(aq) \rightarrow ZnCl_2(aq) + H_2(g)$

(a) State one observation, other than the zinc disappearing, that indicates a chemical reaction is occurring. [1]

(b) The student filters the reaction mixture after the reaction has finished. State what would be observed in the filtrate. [1]

(c) Explain, in terms of bond breaking and bond making, why this reaction is exothermic. [4]

Worked Solution:

(a)

1. Bubbles of gas are produced. Bubbles of gas are produced.

How to earn full marks:

• 1 mark for stating bubbles (or gas) are produced.

(b)

1. The filtrate is colourless. The filtrate is colourless.

How to earn full marks:

• 1 mark for stating the filtrate is colourless.

(c)

1. Energy is required to break bonds in the reactants (Zn and HCl). Energy is required to break bonds in the reactants (Zn and HCl).

2. Energy is released when new bonds are formed in the products ($ZnCl_2$ and $H_2$). Energy is released when new bonds are formed in the products ($ZnCl_2$ and $H_2$).

3. The energy released when forming bonds is greater than the energy required to break bonds. The energy released when forming bonds is greater than the energy required to break bonds.

4. Therefore, the reaction releases energy to the surroundings as heat, resulting in a temperature increase. Therefore, the reaction releases energy to the surroundings as heat, resulting in a temperature increase.

How to earn full marks:

• 1 mark for stating energy is required to break bonds.
• 1 mark for stating energy is released when bonds are formed.
• 1 mark for stating that more energy is released than required.
• 1 mark for stating that the reaction releases energy (as heat) to the surroundings, resulting in a temperature increase.

Common Pitfall: When explaining exothermic reactions, remember to explicitly state that more energy is released during bond formation than is absorbed during bond breaking. Also, linking this to a temperature increase in the surroundings helps solidify the concept.

Exam-Style Question 3 — Extended Response [8 marks]

Question:

A student investigated the change of state of stearic acid from solid to liquid. The student heated solid stearic acid in a test tube in a water bath and recorded the temperature every minute. The data is shown below:

Time (minutes)	Temperature (°C)
0	20
1	30
2	40
3	50
4	60
5	70
6	70
7	70
8	70
9	75
10	80

(a) Define the term physical change. [1]

(b) On the grid below, plot the temperature of stearic acid against time. Draw a line of best fit. [3]

(c) Use the graph to determine the melting point of stearic acid. [1]

(d) Explain, in terms of the arrangement and movement of particles, what happens to the stearic acid as it melts. [3]

Worked Solution:

(a)

1. A change that does not involve the formation of new substances. A change that does not involve the formation of new substances.

How to earn full marks:

• 1 mark for stating a change that does not form new substances.

(b)

1. Correctly labelled axes with appropriate scales. Correctly labelled axes with appropriate scales.

2. All points plotted correctly. All points plotted correctly.

3. Line of best fit drawn. Line of best fit drawn.

How to earn full marks:

• 1 mark for correctly labelled axes with appropriate scales.
• 1 mark for plotting all points correctly.
• 1 mark for drawing a line of best fit (can be curved or straight sections).

(c)

1. The melting point is $\boxed{70 \text{ }^\circ C}$. The melting point is 70 °C.

How to earn full marks:

• 1 mark for stating the melting point is 70 °C.

(d)

1. In the solid state, particles are arranged in a regular pattern and vibrate about fixed positions. In the solid state, particles are arranged in a regular pattern and vibrate about fixed positions.

2. As the stearic acid melts, the particles gain kinetic energy and vibrate more vigorously. As the stearic acid melts, the particles gain kinetic energy and vibrate more vigorously.

3. The particles overcome the forces of attraction holding them in fixed positions, and the particles become more randomly arranged and can move more freely, allowing them to slide past each other. The particles overcome the forces of attraction holding them in fixed positions, and the particles become more randomly arranged and can move more freely, allowing them to slide past each other.

How to earn full marks:

• 1 mark for stating the particles are arranged in a regular pattern in the solid state and vibrate about fixed positions.
• 1 mark for stating the particles gain kinetic energy and vibrate more vigorously.
• 1 mark for stating the particles become more randomly arranged and can move more freely, allowing them to slide past each other.

Common Pitfall: When describing changes of state, it's important to be specific about the arrangement and movement of particles in each state. Don't just say "particles move faster"; explain how their arrangement changes and how this affects their movement relative to each other.

Exam-Style Question 4 — Extended Response [9 marks]

Question:

Cobalt(II) chloride crystals ($CoCl_2 \cdot 6H_2O$) are hydrated. When heated, they undergo a chemical change to form anhydrous cobalt(II) chloride ($CoCl_2$).

$CoCl_2 \cdot 6H_2O(s) \rightleftharpoons CoCl_2(s) + 6H_2O(g)$

The change is reversible.

(a) State the colour change observed when cobalt(II) chloride crystals are heated. [1]

(b) Describe a test to show that the gas produced is water. [2]

(c) Explain why heating cobalt(II) chloride crystals is considered a chemical change, not a physical change, even though the reverse reaction can occur. [3]

(d) The student heats $11.90 \text{ g}$ of cobalt(II) chloride crystals. After heating to constant mass, $6.50 \text{ g}$ of anhydrous cobalt(II) chloride remains. Calculate the mass of water lost and use this to determine the number of moles of water present in the original crystals. ($M_r$ of $H_2O = 18$, $M_r$ of $CoCl_2 = 130$) [3]

Worked Solution:

(a)

1. The colour changes from pink to blue. The colour changes from pink to blue.

How to earn full marks:

• 1 mark for stating the colour changes from pink to blue.

(b)

1. Test the gas with anhydrous copper(II) sulfate. Test the gas with anhydrous copper(II) sulfate.

2. The anhydrous copper(II) sulfate will turn blue. The anhydrous copper(II) sulfate will turn blue.

How to earn full marks:

• 1 mark for stating to test the gas with anhydrous copper(II) sulfate.
• 1 mark for stating the anhydrous copper(II) sulfate will turn blue.

(c)

1. The chemical composition of the cobalt(II) chloride changes. The chemical composition of the cobalt(II) chloride changes.

2. Water molecules are chemically bonded within the crystal structure. Water molecules are chemically bonded within the crystal structure.

3. Breaking these bonds requires energy, and forming these bonds releases energy. The properties of $CoCl_2 \cdot 6H_2O$ and $CoCl_2$ are different. Breaking these bonds requires energy, and forming these bonds releases energy. The properties of $CoCl_2 \cdot 6H_2O$ and $CoCl_2$ are different.

How to earn full marks:

• 1 mark for stating the chemical composition of cobalt(II) chloride changes.
• 1 mark for stating water molecules are chemically bonded within the crystal structure.
• 1 mark for stating breaking/forming bonds requires/releases energy AND/OR the properties of the reactants and products are different.

(d)

1. Mass of water lost = $11.90 \text{ g} - 6.50 \text{ g} = \boxed{5.40 \text{ g}}$ Mass of water lost = $11.90 \text{ g} - 6.50 \text{ g} = 5.40 \text{ g}$

2. Moles of water = $\frac{5.40}{18} = \boxed{0.30 \text{ mol}}$ Moles of water = $\frac{5.40}{18} = 0.30 \text{ mol}$

3. Moles of $CoCl_2$ = $\frac{6.50}{130} = \boxed{0.05 \text{ mol}}$ Moles of $CoCl_2$ = $\frac{6.50}{130} = 0.05 \text{ mol}$

How to earn full marks:

• 1 mark for calculating the mass of water lost as 5.40 g.
• 1 mark for calculating the moles of water as 0.30 mol.
• 1 mark for calculating the moles of $CoCl_2$ as 0.05 mol.

Common Pitfall: Remember that even if a reaction is reversible, it can still be a chemical change. The key is whether new substances with different chemical properties are formed. Also, be careful with units in calculations, especially when converting between mass and moles.