IB MYP 4-5 Chemistry -Evidence of a chemical change- Study Notes - New Syllabus
IB MYP 4-5 Chemistry -Evidence of a chemical change- Study Notes
Key Concepts
- Evidence of a Chemical Change
Evidence of a Chemical Change
Evidence of a Chemical Change
A chemical change (or chemical reaction) occurs when one or more substances are transformed into new substances with different physical and chemical properties. These changes are usually difficult to reverse because they involve breaking and forming of chemical bonds.
Key Idea: In a chemical change, atoms are rearranged to form new substances, and these changes are accompanied by observable signs known as evidence of chemical change.
Characteristics of a Chemical Change
- New substances are formed.
- The change is usually irreversible.
- Energy changes (heat, light, sound) often occur.
- The mass of the system remains constant (Law of Conservation of Mass).
Common Evidence of a Chemical Change
| Evidence | Observation | Explanation | Example |
|---|---|---|---|
| 1. Change in Color | Reactants and products have different colors. | Indicates new substance formation. | \( \mathrm{Fe + CuSO_4 \rightarrow FeSO_4 + Cu} \) (blue → green as copper is displaced) |
| 2. Formation of a Gas | Bubbles or fizzing seen. | Gas evolves during reaction. | \( \mathrm{Zn + 2HCl \rightarrow ZnCl_2 + H_2↑} \) |
| 3. Formation of a Precipitate | An insoluble solid appears in solution. | Indicates new product formation. | \( \mathrm{AgNO_3 + NaCl \rightarrow AgCl↓ + NaNO_3} \) |
| 4. Change in Temperature | System becomes hotter or colder. | Energy absorbed or released. | \( \mathrm{NaOH + HCl \rightarrow NaCl + H_2O + heat} \) |
| 5. Production of Light or Sound | Light, spark, or explosion observed. | Indicates energy release. | Burning of magnesium: \( \mathrm{2Mg + O_2 \rightarrow 2MgO + light} \) |
| 6. Formation of Odor | New smell produced. | Volatile products formed. | Spoiled food emits unpleasant gases. |
Distinguishing Chemical Changes from Physical Changes
| Property | Physical Change | Chemical Change |
|---|---|---|
| New Substance Formed | No | Yes |
| Reversibility | Usually reversible | Usually irreversible |
| Energy Change | Small or none | Often large (heat, light) |
| Example | Melting of ice | Rusting of iron |
Example :
When vinegar (acetic acid) is added to baking soda (sodium bicarbonate), bubbles are observed. What does this indicate?
▶️ Answer / Explanation
Equation: \( \mathrm{NaHCO_3 + CH_3COOH \rightarrow CH_3COONa + H_2O + CO_2↑} \)
Observation: Bubbles (CO₂ gas) produced.
Evidence: Gas formation shows a chemical change occurred.
Final Answer: Gas evolution is clear evidence of a chemical reaction.
Example :
When two colorless solutions of barium chloride and sodium sulfate are mixed, a white solid appears. Identify the evidence of chemical change and write the balanced equation.
▶️ Answer / Explanation
Equation: \( \mathrm{BaCl_2(aq) + Na_2SO_4(aq) \rightarrow 2NaCl(aq) + BaSO_4↓} \)
Observation: White precipitate of \( \mathrm{BaSO_4} \) forms.
Evidence: Formation of an insoluble solid indicates a new product — chemical change.
Final Answer: The reaction shows precipitate formation — definite evidence of a chemical reaction.
Example :
Iron reacts slowly with moist air to form rust. Explain how this process proves a chemical change has occurred, and describe the evidence involved.
▶️ Answer / Explanation
Equation (simplified): \( \mathrm{4Fe + 3O_2 + 6H_2O \rightarrow 4Fe(OH)_3} \)
Step 1: A reddish-brown solid (rust) forms on the iron surface.
Step 2: New substance with different color and properties appears — rust is not metallic and flaky.
Step 3: The process is irreversible and involves oxygen and water reacting with iron.
Final Answer: Rusting shows evidence of chemical change — color change, new substance formation, and irreversibility.