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Changes Around Us: Physical and Chemical - Can Physical and Chemical Changes Occur in the Same Process?

Grade 7CBSE

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

🔑Concepts

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A Physical Change is a change in which no new substance is formed and the chemical composition remains the same. Examples include changes in state like Solid→LiquidSolid \rightarrow Liquid.

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A Chemical Change is a change in which one or more new substances are formed with different chemical properties. It is often represented as Reactants→ProductsReactants \rightarrow Products.

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In certain processes, both physical and chemical changes occur simultaneously. This means that while some matter is changing its state or appearance, other matter is undergoing a chemical reaction to form new substances.

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The most common example is the burning of a candle. The melting of wax is a physical change, while the burning of the wax vapor is a chemical change.

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Another example is the digestion of food. The mechanical breaking down of food by teeth is a physical change, whereas the breakdown of complex molecules by enzymes and acid (HClHCl) is a chemical change.

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In an LPG cylinder, when the gas comes out of the cylinder, it changes from liquid to gas (Physical Change: Liquid→GasLiquid \rightarrow Gas). When it burns in the stove, it undergoes combustion (Chemical Change).

📐Formulae

Wax(Fuel)+O2(Oxygen)→CO2+H2O+Heat+LightWax (Fuel) + O_{2} (Oxygen) \rightarrow CO_{2} + H_{2}O + \text{Heat} + \text{Light}

Reactants→Chemical ReactionProductsReactants \xrightarrow{\text{Chemical Reaction}} Products

Solid Wax→HeatLiquid Wax (Physical Change)\text{Solid Wax} \xrightarrow{\text{Heat}} \text{Liquid Wax (Physical Change)}

💡Examples

Problem 1:

Describe the two types of changes occurring when a candle burns.

Solution:

  1. Physical Change: The solid wax melts into liquid wax due to heat. This is a change of state: Solid→LiquidSolid \rightarrow Liquid.
  2. Chemical Change: The liquid wax wicks up and turns into vapor, which reacts with oxygen to produce Carbon Dioxide (CO2CO_{2}), water vapor (H2OH_{2}O), soot, and energy.

Explanation:

Melting is physical because the liquid wax can be solidified back into solid wax. Burning is chemical because new substances like CO2CO_{2} are formed and the process cannot be reversed to get the original wax back.

Problem 2:

Is the consumption of food in our body a physical or chemical change?

Solution:

It is both a physical and chemical change.

Explanation:

The chewing of food into smaller pieces (mastication) is a physical change as it only changes the size. The action of saliva and stomach acids breaking down carbohydrates and proteins into simpler substances like glucose and amino acids is a chemical change because new substances are formed.