IB MYP 4-5 Chemistry -Physical vs. chemical changes- Study Notes - New Syllabus
IB MYP 4-5 Chemistry -Physical vs. chemical changes- Study Notes
Key Concepts
- Physical vs. Chemical Changes
Physical vs. Chemical Changes
Physical vs. Chemical Changes
All changes in matter can be classified as either physical or chemical depending on whether the substance’s chemical identity is altered. A physical change affects only the form or state of matter, while a chemical change results in the formation of one or more new substances with different properties.
1. Physical Change
A physical change is a change in which no new substance is formed. Only the physical properties such as shape, size, state, color, or volume may change.
- The chemical composition remains the same.
- The change is often reversible.
- Energy change is usually small (e.g., heating or cooling).
Examples of Physical Changes:
- Melting of ice → water
- Evaporation of alcohol
- Dissolving sugar in water
- Breaking glass or cutting paper
- Boiling water to form steam
Key Point: In a physical change, no new substance is formed, and the process can usually be reversed by physical means (cooling, compressing, etc.).
2. Chemical Change
A chemical change (also called a chemical reaction) is a change in which one or more new substances are formed with different chemical properties and composition from the original substances.
- The new substances have their own distinct properties.
- Old bonds break and new bonds form between atoms.
- Usually involves noticeable energy changes (heat, light, or sound).
- Generally irreversible by simple physical means.
Examples of Chemical Changes:
- Rusting of iron
- Burning of wood or paper
- Baking a cake
- Formation of curd from milk
- Vinegar reacting with baking soda to release gas
Key Point: In a chemical change, a new substance is produced, and the change is often difficult or impossible to reverse physically.
Comparison Table: Physical vs. Chemical Changes
| Property | Physical Change | Chemical Change |
|---|---|---|
| New Substance Formed? | No new substance formed | New substance(s) formed |
| Composition | Chemical composition remains the same | Chemical composition changes |
| Energy Change | Usually small (e.g., heat for melting or boiling) | Usually large (heat, light, or sound released or absorbed) |
| Reversibility | Generally reversible | Generally irreversible |
| Example | Melting of ice, dissolving salt in water | Rusting of iron, burning of wood |
Indicators of a Chemical Change
Several signs help identify when a chemical change has occurred:
- Color change (e.g., iron rusting turns reddish-brown)
- Formation of gas (bubbles or fizzing)
- Temperature change (heat or cold released/absorbed)
- Formation of a solid (precipitate) from a liquid mixture
- Emission of light or odor
Key Idea
The main difference between physical and chemical changes lies in whether the identity of the substance changes or not:
- Physical change: Appearance or state changes, identity remains the same.
- Chemical change: Identity changes — new substances with new properties are formed.
Example:
Ice melts into water, and water can be frozen again into ice. Is this a physical or chemical change? Explain your answer.
▶️ Answer / Explanation
Step 1: The process involves a change of state — solid (ice) to liquid (water).
Step 2: The composition (H₂O) remains unchanged.
Step 3: The change is reversible by cooling.
Final Answer: It is a physical change because no new substance is formed, and the process is reversible.
Example :
When iron rusts, a reddish-brown substance forms on the surface. Is this a physical or chemical change? Justify your answer.
▶️ Answer / Explanation
Step 1: The rust (iron oxide) formed has a different composition from pure iron.
Step 2: New chemical bonds are formed between iron and oxygen.
Step 3: The process cannot be reversed easily by physical means.
Final Answer: Rusting is a chemical change because a new substance (iron oxide) is formed with different properties.
Example :
Both dissolving salt in water and burning magnesium in air involve energy changes. Why is the first a physical change and the second a chemical change?
▶️ Answer / Explanation
Step 1: Dissolving salt in water does not produce a new substance — salt can be recovered by evaporation; it only changes its physical form (ions dispersed in water).
Step 2: Burning magnesium forms a new compound, magnesium oxide, with properties different from magnesium metal and oxygen gas.
Step 3: Energy in the first case helps physical mixing, while in the second, it helps form new chemical bonds.
Final Answer: Dissolving salt is a physical change (no new substance), while burning magnesium is a chemical change (new compound formed with new properties).