Chemistry First - Class 10 Telangana Board Chemistry

Chemical Changes and Chemical Equations

Understanding Change: Physical vs Chemical

A change refers to the process in which a substance or system transforms from an initial state to a final state. Changes can be observed by comparing the properties before and after the process. There are two main types of changes: physical and chemical.

• Physical Change: Only the physical properties (shape, size, state) change; the chemical composition remains the same.
• Chemical Change: The chemical composition of the substance changes, resulting in the formation of new substances.

Criteria to Distinguish Physical and Chemical Changes

Physical and…

Chemical Changes and Chemical Equations

Understanding Change: Physical vs Chemical

A change refers to the process in which a substance or system transforms from an initial state to a final state. Changes can be observed by comparing the properties before and after the process. There are two main types of changes: physical and chemical.

• Physical Change: Only the physical properties (shape, size, state) change; the chemical composition remains the same.
• Chemical Change: The chemical composition of the substance changes, resulting in the formation of new substances.

Criteria to Distinguish Physical and Chemical Changes

Physical and chemical changes can be differentiated based on the nature of the transformation and reversibility. Physical changes do not alter the chemical composition and are often reversible, while chemical changes result in new substances and are usually irreversible.

• Physical Change:
  • Change in shape, size, or state.
  • No new substance formed.
  • Chemical composition remains unchanged.
  • Often reversible (e.g., folding paper, melting wax).
• Chemical Change:
  • Change in chemical composition.
  • New substance(s) formed.
  • Usually irreversible (e.g., burning paper, rusting iron).

Examples of Physical and Chemical Changes

Several examples help clarify the distinction between physical and chemical changes. Observing the outcome and reversibility is key.

• Physical Change Examples:
  • Cutting a lemon: Only the shape changes, no new substance.
  • Dissolving sugar or salt in water: No new substance, can recover solute by evaporation.
  • Melting ice: State changes from solid to liquid, but composition (H₂O) remains.
• Chemical Change Examples:
  • Burning paper: Ash is a new substance with different composition.
  • Rusting iron: Iron (Fe) reacts with oxygen to form ferric oxide (Fe₂O₃).
  • Curdling milk: Milk turns to curd, new chemical composition.
  • Fermentation of grapes: Grape juice turns into wine, new substances formed.

Key Characteristics of Chemical Changes (Reactions)

Chemical changes, also called chemical reactions, have distinct characteristics. These features help identify when a chemical reaction has occurred.

• Formation of New Substance: Always results in new product(s) with different properties.
• Irreversibility: Most chemical changes cannot be easily reversed.
• Permanent Change: The change is lasting; original substance cannot be recovered by simple physical means.
• Energy Change: Often accompanied by absorption or release of energy (heat, light).
• Other Observable Signs:
  • Change in color.
  • Evolution of gas.
  • Formation of precipitate (solid from two liquids).
  • Change in temperature.

Chemical Reactions and Chemical Equations

A chemical reaction is the process in which one or more substances (reactants) are transformed into new substances (products). Chemical equations are symbolic representations of these reactions, making them concise and easy to understand.

• Reactants: Substances that undergo change.
• Products: New substances formed.
• Chemical Equation: Expresses the reaction in a symbolic form, showing reactants on the left and products on the right.

Example (as stated):

• Magnesium reacts with oxygen to form magnesium oxide:
  • Word equation: Magnesium + Oxygen → Magnesium oxide
  • Symbolic equation: $Mg + O_2 \rightarrow MgO$
• Iron reacts with oxygen and water to form rust:
  • Word equation: Iron + Oxygen + Water → Ferric oxide (rust)
  • Symbolic equation: $Fe + O_2 + H_2O \rightarrow Fe_2O_3$

Balanced and Unbalanced Chemical Equations

Chemical equations must represent the conservation of mass, meaning the number of atoms of each element is the same on both sides. If not, the equation is unbalanced.

• Unbalanced Equation: Number of atoms for each element is not equal on both sides.
• Balanced Equation: Number of atoms for each element is equal on both sides.

Example:

• Unbalanced: $Mg + O_2 \rightarrow MgO$
• Balanced: $2Mg + O_2 \rightarrow 2MgO$

Balancing ensures the law of conservation of mass is followed.

Conditions Necessary for Chemical Reactions

Certain conditions are required for chemical reactions to occur. Not all reactions happen spontaneously; some need specific triggers.

• Energy Supply: Most reactions require energy input (heat, light, electricity) to initiate.
• Physical Contact: Reactants must come into contact.
• No Requirement for Same State or Temperature: Reactants can be in different states or temperatures; only energy supply is essential.

Observable Changes During Chemical Reactions (FACTS)

Chemical reactions can be identified by specific observable changes, summarized by the acronym FACTS:

• F: Formation of precipitate (solid from liquids)
• A: Appearance of gas (evolution of gas)
• C: Change in color
• T: Change in temperature (heat absorbed or released)
• S: Change in state (solid, liquid, gas)

Examples:

• Mixing barium chloride and sodium sulfate forms a white precipitate.
• Reaction of zinc with hydrochloric acid releases hydrogen gas.
• Burning paper releases heat (exothermic reaction).
• Photosynthesis absorbs sunlight (endothermic reaction).

Exothermic and Endothermic Reactions

Chemical reactions involve energy changes, either releasing or absorbing heat.

• Exothermic Reaction: Heat is released to the surroundings (e.g., burning paper, respiration, rusting).
• Endothermic Reaction: Heat is absorbed from the surroundings (e.g., photosynthesis, heating metals).

Example:

• Calcium oxide reacts with water to form calcium hydroxide and releases heat:$$CaO + H_2O \rightarrow Ca(OH)_2 + \text{heat}$$

Practice Questions and Application

Understanding the nature of changes and chemical equations is essential for solving exam questions. Application-based questions may involve identifying the type of change, writing equations, or predicting products.

• Identify whether a process is a physical or chemical change based on the change in composition and reversibility.
• Write word and symbolic equations for given reactions.
• Balance chemical equations to satisfy the law of conservation of mass.
• Recognize observable signs (color change, gas evolution, precipitate formation, temperature change) to identify chemical reactions.

Summary Table: Physical vs Chemical Change

Key Takeaways for Exams

• Always check for new substance formation and irreversibility to identify chemical changes.
• Use observable signs (FACTS) to confirm chemical reactions.
• Write and balance chemical equations for all reactions.
• Remember that not all changes in state or appearance are chemical; check for change in composition.
• Practice distinguishing between exothermic and endothermic reactions based on heat flow.