Physical And Chemical Changes Class 7 Notes

Understanding Physical and Chemical Transformations

A physical change is a temporary change where only the physical properties of a substance (such as shape, size, state, or color) undergo alteration, and no new substance is formed. Most physical changes are reversible in nature; for example, the melting of ice can be easily reversed by freezing the water back into ice.

Conversely, a chemical change (also called a chemical reaction) is a permanent transformation where one or more new substances with entirely different chemical properties are formed. These changes are typically irreversible and are accompanied by energy changes (such as heat or light production), gas evolution, or distinct color changes. A classic example is the burning of a magnesium ribbon, which leaves behind a new powdery white ash called magnesium oxide ($MgO$).

Physical Change vs. Chemical Change

Key Chemical Processes: Rusting & Crystallisation

1. — Rusting is a slow chemical change that destroys iron objects. It requires the simultaneous presence of both Oxygen ($O_2$) and Water/Water vapour ($H_2O$). The reaction forms hydrated iron oxide (Rust): $4Fe + 3O_2 + 2xH_2O \rightarrow 2Fe_2O_3 \cdot xH_2O$.
2. — To prevent rusting, iron is protected from oxygen and moisture. This is done by applying a coat of paint, grease, or by depositing a thin layer of a metal like chromium or zinc on iron. The process of depositing a layer of zinc on iron is called Galvanisation.
3. — Crystallisation is used to obtain large crystals of pure substances from their solutions. For example, to get pure Copper Sulphate ($CuSO_4$) crystals: Add dilute sulphuric acid to water, boil it, dissolve copper sulphate powder until saturation, filter, and cool undisturbed to observe shiny blue crystals separating.

Key Chemical Reactions with Equations

• {"title":"Burning of Magnesium Ribbon","description":"Magnesium ($Mg$) burns with a brilliant white light in oxygen ($O_2$) to form a white powdery ash of Magnesium Oxide ($MgO$). \nEquation: $2Mg + O_2 \\rightarrow 2MgO$. \nWhen this ash is dissolved in water, it forms a basic compound, Magnesium Hydroxide: $MgO + H_2O \\rightarrow Mg(OH)_2$."}
• {"title":"Reaction of Copper Sulphate with Iron","description":"When an iron nail is placed in blue Copper Sulphate ($CuSO_4$) solution, a displacement reaction occurs. The blue solution turns green due to the formation of Iron Sulphate ($FeSO_4$), and a brown deposit of copper is formed on the iron nail. \nEquation: $CuSO_4 (Blue) + Fe \\rightarrow FeSO_4 (Green) + Cu (Brown\\,Deposit)$."}
• {"title":"Vinegar and Baking Soda Reaction","description":"Mixing Vinegar (acetic acid) with Baking Soda (sodium hydrogen carbonate) produces Carbon Dioxide ($CO_2$) gas, which bubbles out. \nEquation: $Vinegar + Baking\\,Soda \\rightarrow Carbon\\,Dioxide + Other\\,Substances$. \nWhen this gas is bubbled through lime water, it turns milky: $CO_2 + Ca(OH)_2 (Lime\\,Water) \\rightarrow CaCO_3 (Calcium\\,Carbonate) + H_2O$."}

Important Terms and Glossary

Physical Change

A change in which a substance undergoes a change in its physical properties like shape, size, state, or color without forming any new substance.

Chemical Change

A change in which one or more new substances are formed with entirely different chemical properties, accompanied by light/heat absorption or release.

Rusting

The slow oxidation process in which iron reacts with atmospheric oxygen and moisture to form a flaky, brown substance called rust (hydrated iron oxide).

Galvanisation

The industrial process of depositing a thin protective layer of zinc metal over iron objects to prevent them from rusting.

Crystallisation

A purification technique used to obtain large, highly pure crystals of a soluble solid substance from its hot, saturated solution.

Chemical Reaction

The actual process of chemical change where original reactant substances interact and transform into new product substances.

Must-Remember Key Revision Points

• A physical change is mostly temporary and reversible; no new chemical bonds are created or broken.
• Chemical changes are permanent, irreversible, and lead to the creation of completely new substances.
• Indicators of a chemical change include: evolution of gas, change of color, production of sound, change of smell, or heat/light emission.
• Rusting of iron requires both oxygen and moisture simultaneously. Salt water speeds up the process of rusting.
• The chemical formula of rust is Hydrated Iron Oxide ($Fe_2O_3 \cdot xH_2O$).
• Zinc-coated iron pipes are used in water supply lines to prevent rusting because of the galvanisation layer.
• Passing carbon dioxide through fresh lime water turns it milky; this is the standard laboratory test for detecting $CO_2$ gas.
• Crystallisation is a physical change because it only changes the state of the substance from dissolved solute to structured solid crystals.

Board Exam Tips & Common Traps

Watch out for the 'Burning of a Candle' question! In school exams, students often get confused about this. Remember: Both physical and chemical changes occur simultaneously when a candle burns. The melting of wax is a physical change (as solid wax turns to liquid and can solidify back without making a new substance). The burning of the wax wick is a chemical change (producing carbon dioxide, soot, heat, and light, which cannot be reversed). Be sure to specify both aspects to score full marks!

Practice Questions with Solutions

• Q: Why is rusting faster in coastal areas than in dry desert areas? A: Rusting requires both air (oxygen) and moisture (water vapor). Coastal areas have highly humid air with rich moisture levels, whereas deserts have extremely dry air, which slows down the oxidation of iron.
• Q: What happens when an iron nail is left in a copper sulphate solution? State the color changes. A: A chemical displacement reaction occurs. The blue copper sulphate solution turns light green due to the formation of iron sulphate, and a reddish-brown coating of copper is deposited on the surface of the iron nail.
• Q: Classify the following as physical or chemical changes: (a) Photosynthesis, (b) Dissolving salt in water, (c) Digestion of food, (d) Beating aluminum to make foil. A: (a) Chemical change, (b) Physical change, (c) Chemical change, (d) Physical change.
• Q: Why is crystallisation preferred over simple evaporation to obtain pure substances? A: Crystallisation is preferred because heating a solution to dryness during evaporation can char or decompose some delicate solids (like sugar), whereas crystallisation gently separates pure crystals leaving soluble impurities behind in the solution.

Frequently Asked Questions