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Chemical Reactions - Physical and chemical changes

Grade 11IGCSEChemistry

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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A physical change is a type of change where the physical properties of a substance change, but its chemical identity remains the same. No new substances are formed. Examples include changes of state such as H2O(s)→H2O(l)H_2O(s) \rightarrow H_2O(l).

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A chemical change (chemical reaction) occurs when one or more substances are transformed into entirely new substances with different properties. This involves the breaking and forming of chemical bonds.

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Indicators of a chemical change include: temperature change (exothermic or endothermic), color change, evolution of a gas (CO2CO_2, H2H_2, etc.), and the formation of a precipitate.

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Physical changes are generally easy to reverse (e.g., freezing water that was just melted), whereas chemical changes are usually difficult to reverse (e.g., 2Mg(s)+O2(g)→2MgO(s)2Mg(s) + O_2(g) \rightarrow 2MgO(s)).

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Law of Conservation of Mass: In any chemical reaction, the total mass of the reactants is equal to the total mass of the products (mreactants=mproductsm_{reactants} = m_{products}).

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Energy changes: Chemical reactions are often accompanied by energy transfers. Exothermic reactions release energy to the surroundings (ΔH<0\Delta H < 0), while endothermic reactions absorb energy (ΔH>0\Delta H > 0).

📐Formulae

Reactants→Products\text{Reactants} \rightarrow \text{Products}

ΔH=Hproducts−Hreactants\Delta H = H_{\text{products}} - H_{\text{reactants}}

mtotal initial=mtotal finalm_{\text{total initial}} = m_{\text{total final}}

CH4(g)+2O2(g)→CO2(g)+2H2O(l)+EnergyCH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(l) + \text{Energy}

💡Examples

Problem 1:

Determine if the following process is a physical or chemical change: The heating of CuSO4⋅5H2OCuSO_4 \cdot 5H_2O (hydrated copper(II) sulfate) until it turns into white anhydrous CuSO4CuSO_4.

Solution:

Chemical Change.

Explanation:

Heating hydrated copper(II) sulfate causes the loss of water of crystallization, represented by the equation: CuSO4⋅5H2O(s)→CuSO4(s)+5H2O(g)CuSO_4 \cdot 5H_2O(s) \rightarrow CuSO_4(s) + 5H_2O(g). Because the chemical structure changes from a hydrated crystal to an anhydrous salt and a color change occurs (blue to white), it is a chemical change.

Problem 2:

Identify the type of change and write a balanced equation for the reaction between HClHCl and NaOHNaOH.

Solution:

Chemical Change; HCl(aq)+NaOH(aq)→NaCl(aq)+H2O(l)HCl(aq) + NaOH(aq) \rightarrow NaCl(aq) + H_2O(l).

Explanation:

This is a neutralization reaction where new substances (sodium chloride and water) are formed. There is also a measurable increase in temperature (exothermic), which is a sign of a chemical reaction.

Problem 3:

Is the sublimation of iodine (I2(s)→I2(g)I_2(s) \rightarrow I_2(g)) a physical or chemical change?

Solution:

Physical Change.

Explanation:

Sublimation is a change of state. The molecules of I2I_2 remain identical in both the solid and gas phases; only the intermolecular forces are overcome. No new chemical substance is created.

Physical and chemical changes - Revision Notes & Key Formulas | IGCSE Grade 11 Chemistry