Comprehensive Study Notes on Changes Around Us: Physical and Chemical

Introduction to Changes Around Us

  • Observations of Change: Common daily occurrences indicate various types of changes:

    • State of Matter: A cube of ice melting into liquid water over half an hour.
    • Temperature: A bottle of cold water losing its coldness and reaching room temperature.
    • Biological Growth: A rose bud developing into a full flower overnight.
    • Chemical Degradation: Brown spots appearing on a banana, which increase over time and are accompanied by a strong smell.

  • Categorizing Changes: Changes are observed through the five senses: sight, smell, touch, hearing, and taste. These changes can be categorized based on their properties and outcomes.

Identifying Changes in Daily Life

  • Activity 5.1: Observations and Reflections: Various changes involve alterations in size, shape, smell, or other properties:
    1. Melting ice cubes (change in state).
    2. Chopping vegetables (change in size and shape).
    3. Boiling water (change in state).
    4. Making popcorn from corn.
    5. Cutting a piece of paper (change in shape).
    6. Adding beetroot extract to water (change in color).
    7. Burning wood (change in composition).
    8. Drying wet clothes.
    9. Making small balls of dough (change in shape).
    10. Rolling small balls of dough into chapatis (change in shape).

Physical Changes: Altering Appearance without New Substances

  • Definition of Physical Change: Such changes in which only physical properties like shape, size, and state of substances change are called physical changes. In these cases, no new substance is formed.

  • Activity 5.2: Creating and Discussing Physical Changes:

    • Paper Folding: Creating objects like boats or planes by folding paper allows the paper to be unfolded back to its original sheet form (reversible).
    • Balloon Dynamics:
      • Inflating and deflating a balloon returns it to its original uninflated state.
      • Pricking an inflated balloon with a pin destroys its shape, but the material (rubber sheet) remains the same substance.
    • Crushing Chalk: Crushing a piece of chalk into powder changes its size and shape, but it remains chalk. While difficult to reform into the original stick, the material identity is unchanged.

  • Key Characteristic: In physical changes—such as those involving paper, rubber, or chalk—the material remains the same even if the appearance (shape or size) is modified.

Chemical Changes: Formation of New Substances

  • Definition of Chemical Change: Such changes, in which one or more new substances are formed, are called chemical changes. New substances are formed through a process called chemical reaction.

  • Activity 5.3: Exhaling into Lime Water:

    • Control (Tap Water): Blowing air through a straw into tap water (Glass Tumbler A) produces bubbles but results in no change in the water's appearance.
    • Reaction (Lime Water): Blowing air into lime water (Glass Tumbler B) turns the liquid milky or cloudy.
    • Observation: If left undisturbed, a white substance settles at the bottom.
    • The Reaction: Carbon dioxide (CO2CO_2) from exhaled air reacts with lime water (calcium hydroxide\text{calcium hydroxide}) to form calcium carbonate and water.
    • Chemical Equation:Calcium hydroxide (Lime water)+Carbon dioxide→Calcium carbonate (Insoluble substance)+Water\text{Calcium hydroxide (Lime water)} + \text{Carbon dioxide} \rightarrow \text{Calcium carbonate (Insoluble substance)} + \text{Water}

  • Activity 5.4: Mixing Vinegar and Baking Soda:

    • Procedure: Add a pinch of baking soda (sodium hydrogen carbonate\text{sodium hydrogen carbonate}) to a teaspoon of vinegar or lemon juice.
    • Observations: A fizzing, bubbling sound occurs, indicating the formation of gas bubbles.
    • Test for Carbon Dioxide: Passing this gas through freshly prepared lime water turns it milky, confirming the gas is carbon dioxide.
    • Chemical Equation:Vinegar+Baking soda→Carbon dioxide+Other substances\text{Vinegar} + \text{Baking soda} \rightarrow \text{Carbon dioxide} + \text{Other substances}

Processes Involving Chemical Changes

  • Rusting of Iron:

    • Rusting involves the formation of a new brown-colored substance called rust (iron oxide\text{iron oxide}).
    • This is a chemical change because the original iron is transformed into a different chemical compound.

  • Combustion:

    • Definition: A chemical reaction in which a substance reacts with oxygen and produces heat and/or light is called combustion.
    • Combustible Substances: Materials that undergo combustion reactions, such as wood, paper, cotton, and kerosene.
    • Burning Magnesium Ribbon:
      • When burnt, it reacts with oxygen in the air to form a white powder called magnesium oxide.
      • Equation: Magnesium (Ribbon)+Oxygen (Air)→Magnesium oxide (White powder)+Heat+Light\text{Magnesium (Ribbon)} + \text{Oxygen (Air)} \rightarrow \text{Magnesium oxide (White powder)} + \text{Heat} + \text{Light}

  • The Necessity of Oxygen for Combustion (Activity 5.5):

    • A candle burning in open air continue to burn because of a continuous supply of oxygen.
    • A candle covered with a glass tumbler extinguishes because the oxygen supply is cut off. Carbon dioxide is produced during the process, which can be tested by adding lime water to the dish (it will turn milky).

  • Bioluminescence: Some living organisms, like fireflies, produce light (without heat) through a internal chemical change.

The Fire Triangle and Ignition Temperature

  • Ignition Temperature: The minimum temperature at which a substance catches fire is called its ignition temperature.

    • Example: Paper does not burn in air at room temperature. However, using a magnifying glass to focus sunrays onto a small spot heats the paper. Once the paper reach its ignition temperature, it starts to smoke and eventually catches fire.

  • The Fire Triangle Requirements: For combustion to occur, three elements must be present:

    1. Fuel: A combustible substance.
    2. Oxygen: Provided by the air.
    3. Heat: To raise the fuel to its ignition temperature.

  • Fire Safety (Science and Society): To extinguish a fire on a person's clothes, wrap them in a blanket to cut off the oxygen supply. Caution: Never use synthetic blankets or cloth, as they can melt and stick to the skin.

Co-occurrence of Physical and Chemical Changes

  • The Burning Candle (Activity 5.7):
    • Physical Changes: The heat of the flame melts the solid wax into liquid wax. The liquid wax is then carried up the wick and evaporated into wax vapor. Liquid wax can also flow and solidify elsewhere.
    • Chemical Changes: The wax vapor reacts with oxygen in the air (combustion) to produce a flame, heat, light, and new substances like carbon dioxide and water vapor.
    • Scientific Context: Michael Faraday, in the 19th century, delivered a series of lectures titled Chemical History of a Candle, using the candle to explain the differences between physical and chemical processes.

Reversibility and Desirability of Changes

  • Reversible Changes: Changes where the original substance can be recovered.

    • Example: Melting ice (can be refrozen); evaporating water (can be condensed).

  • Irreversible Changes: Changes where the original substance cannot be recovered.

    • Example: Chopping vegetables; making popcorn; grinding wheat into flour; ripening of fruits.

  • Desirable Changes: Useful changes such as milk turning into curd, ripening of fruits, and cooking food.

  • Undesirable Changes: Harmful or unwanted changes such as the rusting of iron or the decay of food during storage.

  • Contextual Desirability: The decomposition of food is undesirable during storage but highly desirable when converting waste into compost.

Long-term Environmental Impacts and Natural Changes

  • Human Activity: Increased fuel consumption in transportation increases atmospheric carbon dioxide. Drying paint releases evaporated substances that contribute to pollution.

  • Weathering of Rocks: Physical and chemical changes that break down large rocks into smaller sediments, eventually forming soil.

    • Physical Weathering: Caused by temperature fluctuations, tree roots growing in cracks, or water freezing inside rocks.
    • Chemical Weathering: Water or chemicals reacting with minerals in rocks.
    • Example: Black basalt containing iron reacts with water/air to form red-colored iron oxide.

  • Erosion: A physical change where natural forces like wind and flowing water move sediments from one location to another.

    • River rocks become smoother due to constant erosion.
    • Landslides are a form of erosion causing physical change.
    • Sediments settling in oceans or lakes may harden over thousands of years to form new rocks (an irreversible process).

Questions & Discussion

  • Discussion on Burning Candles (Fig. 5.9):

    • Student 1: "The wax melts and evaporates…"
    • Student 2: "I think the wax burns, and new substances are formed…"
    • Student 3: "Maybe the wick draws up the liquid wax…"
    • Student 4: "Wax melts, flows, and solidifies in different shapes…"
    • Analysis: All students are correct in identifying various parts of the simultaneous physical (melting, vaporizing, solidifying) and chemical (combustion) changes.

  • Let Us Enhance Our Learning (Exercises):

    1. Characteristics of Physical Change: (i) State may/may not change and (iii) No new substance formed. [Option (c) in text].
    2. Reversibility Predictions:
      • Reversible: Twisting a string, dissolving sugar in water, rolling up a mat, drawing water from a well.
      • Irreversible: Stitching cloth to a shirt, making idlis from batter, ripening fruits, grinding wheat, forming soil from rocks.
    3. True/False:
      • (i) Melting of wax is necessary for burning a candle: True.
      • (ii) Condensing water vapor is a chemical change: False (It is a physical change).
      • (iii) Converting leaves to compost is a chemical change: True.
      • (iv) Mixing baking soda with lemon juice is a chemical change: True.
    4. Fill in the Blanks:
      • (i) Brown deposits are due to rusting, and this is a chemical change.
      • (ii) Folding a handkerchief is a physical change and can be reversed.
      • (iii) Reacting with oxygen/heat is combustion, and this is a chemical change.
      • (iv) Burning magnesium produces magnesium oxide. It is basic in nature. Burning magnesium is a chemical change.

  • Exploratory Project - Yeast in Bread:

    • Process: Mixing yeast, sugar, and water in a bottle and covering with a balloon. After an hour, the balloon inflates.
    • Test: Balloon contents mixed with lime water turn it milky.
    • Conclusion: Yeast undergoes a chemical change (fermentation) producing carbon dioxide (CO2CO_2).