CHEMISTRY IGCSE (0620) STUDY NOTES: PHYSICAL AND CHEMICAL CHANGES (6.1)
Hello future chemist! Welcome to one of the most fundamental topics in Chemistry: understanding change. Everything in the universe changes, but we need to know how it changes. This chapter teaches you how to distinguish between simple, reversible changes (like melting ice cream) and profound changes that create totally new substances (like lighting a match).
Mastering this distinction is key to succeeding in the rest of the chemical reactions section!
1. Understanding Physical Changes
A physical change is like rearranging the furniture in a room—it looks different, but the fundamental structure (the atoms and molecules) stays the same. No new chemical substance is formed.
Key Characteristics of a Physical Change
1. Identity Remains the Same: The chemical composition of the substance does not change. (Ice is H₂O, and liquid water is still H₂O.)
2. Easily Reversible: Often, you can easily reverse the change simply by changing the conditions (like temperature or pressure).
3. Small Energy Changes: These changes usually involve relatively small transfers of energy (for example, just enough energy to overcome weak intermolecular forces, not enough to break strong chemical bonds).
Common Examples of Physical Changes
- Changes of State: Melting, boiling, freezing, condensation, sublimation. When water boils, it changes from liquid to gas, but it is chemically still water.
- Dissolving: When salt dissolves in water, the salt crystals separate into ions, but they are still chemically salt ions (Na⁺ and Cl⁻). You can recover the salt by evaporating the water.
- Mixing: Mixing sand and sugar.
- Molding/Shaping: Bending metal or crushing a rock.
Analogy Aid: Imagine you have a Lego block. If you move it, heat it up a little, or dissolve it, it's still fundamentally a Lego block. That’s a Physical Change.
Quick Review: Physical Change
Goal: Change form or state.
Result: NO new chemical substance.
Reversibility: Usually reversible.
2. Understanding Chemical Changes (Chemical Reactions)
A chemical change, or chemical reaction, is far more dramatic! This is like taking the Lego block, breaking all the individual pieces apart, and building a completely new toy out of those fragments.
In a chemical change, the atoms rearrange, and new substances with different chemical and physical properties are formed.
Key Characteristics of a Chemical Change
1. New Substances Formed: The defining feature. The reactants (starting materials) are converted into products (new substances). (When methane burns, it becomes CO₂ and H₂O—very different from methane and oxygen!)
2. Difficult to Reverse: While some are reversible (see future sections!), most are not easily reversed under simple conditions.
3. Large Energy Changes: Significant amounts of energy are absorbed (endothermic) or released (exothermic) as chemical bonds are broken and new ones are formed.
How to Identify a Chemical Change: Observable Signs
You can tell a chemical reaction has happened by looking for these five signs:
- Permanent Colour Change: (e.g., copper turning green when it rusts).
- Energy Change (Heat or Light): The reaction container suddenly gets hot (exothermic, releasing heat) or cold (endothermic, absorbing heat). Light may also be produced, as in fireworks.
- Gas Production (Effervescence): Bubbles of a new gas are produced when two liquids or a solid and a liquid are mixed. (e.g., adding acid to a carbonate powder).
- Precipitate Formation: When two clear solutions are mixed, an insoluble solid (a precipitate) suddenly forms, making the mixture cloudy.
- Change in Odour: The smell changes significantly, often due to the formation of a new gaseous product. (e.g., milk souring).
Don't worry if this seems tricky at first! Seeing two of these signs usually guarantees a chemical change has occurred.
Examples of Chemical Changes
- Combustion (Burning): Wood burning to ash (irreversible, produces heat and light, new substances formed).
- Rusting: Iron reacting with oxygen and water to form hydrated iron(III) oxide (a colour change and slow chemical alteration).
- Baking a Cake: Chemical changes in the ingredients powered by heat.
Common Mistake to Avoid: Just because something bubbles doesn't mean it's chemical! Boiling water bubbles, but it’s a physical change (steam is still H₂O). You must ensure the bubbles are a new gas being formed, not just a substance changing state.
3. Physical vs. Chemical Changes: The Ultimate Comparison
Let’s summarize the fundamental differences so you can ace any comparison question!
Comparing the Two Types of Change
Type of Change: Physical
What happens? Only changes the form or state. No chemical bonds are broken or made.
New Substance Formed? Absolutely not.
Reversibility: Easy (e.g., melting/freezing).
Energy Change: Small (only affects weak intermolecular forces).
Type of Change: Chemical
What happens? Atoms are rearranged; chemical bonds are broken and formed.
New Substance Formed? Yes, substances with totally new properties (the products).
Reversibility: Difficult (often irreversible).
Energy Change: Large (involves breaking and making strong chemical bonds).
Did You Know?
Your ability to breathe relies on differentiating between these changes! When you inhale, oxygen dissolves into your blood (a physical change), but when that oxygen is used to burn glucose for energy inside your cells, that is a complex series of chemical changes (respiration).
Key Takeaway: The simplest and most important question to ask when identifying a change is: "Was a fundamentally new chemical substance created?" If the answer is yes, it's chemical. If the answer is no, it's physical.