Comprehensive Class VII Science Study Notes: Physical & Chemical Changes, Heat, Acids & Bases, and Physiology

Matter, Elements, Compounds, and Chemical Changes

• Element: An element is a pure substance that cannot be split into simpler substances by any known chemical methods.

• Atom: An atom is the smallest particle of an element that retains all the chemical properties of that element.

  • Each element is composed of only one specific kind of atom.

  • The atoms of one element are distinct from the atoms of any other element.

  • Examples: Oxygen, Hydrogen.

• Molecule: A molecule is the smallest particle of an element or a compound that can exist independently under normal conditions.

  • A molecule of an element contains identical atoms (e.g., a molecule of nitrogen, $N_2$).

  • A molecule of a compound contains two or more different atoms combined chemically (e.g., a molecule of water, $H_2O$).

• Chemical Formula Examples:

  • Water: $H_2O$

  • Carbon Dioxide: $CO_2$

  • Copper Sulphate: $CuSO_4$

  • Sodium Bicarbonate: $NaHCO_3$

  • Nitric Acid: $HNO_3$

    • One molecule of nitric acid ($HNO_3$) consists of $1$ atom of hydrogen, $1$ atom of nitrogen, and $3$ atoms of oxygen.

• Properties of Compounds:

  1. Fixed Composition: The constituent elements in a compound are always present in a fixed ratio by mass, regardless of how or where the compound is prepared.

  2. Separation: The constituent elements of a compound cannot be separated by physical methods (such as filtration, evaporation, or magnetic separation).

  3. Distinct Properties: The chemical and physical properties of a compound are entirely different from those of its individual constituent elements.

• Physical Changes:

  • A physical change alters only the physical properties of a substance, such as its size, shape, appearance, or state of matter.

  • No new chemical substance is formed during a physical change.

  • The substance remains chemically identical before and after the change.

  • Physical changes can be reversible or irreversible.

  • Examples: Melting of butter, tearing of paper, breaking of glass, freezing of water, crushing chalk.

• Chemical Changes:

  • A chemical change occurs when one or more new substances with entirely different chemical properties are formed.

  • When compound $A$ reacts with compound $B$ to form compound $C$, a chemical change has taken place.

  • Characteristics of chemical changes:

    • Change in colour.

    • Evolution of gas or effervescence.

    • Formation of a precipitate.

    • Release or absorption of heat or light energy.

  • Examples: Rusting of iron, cooking of food, curdling of milk, burning of paper.

• Prevention of Fruit and Vegetable Browning:

  • When cut surfaces of fruits (like apples) or vegetables are exposed to oxygen in the air, enzymatic browning occurs.

  • Prevention methods:

    1. Rubbing lemon juice over the cut surfaces (citric acid inhibits enzymatic action).

    2. Submerging cut pieces under water to cut off contact with atmospheric oxygen.

• Crystallization:

  • Crystallization is the physical process of obtaining pure solids in the form of crystals from their saturated solutions.

  • Experimental Procedure for Alum Crystals:

    1. Take $200\,ml$ of water in a beaker, add alum powder, and stir continuously until no more alum dissolves.

    2. Heat the solution over a Bunsen burner to dissolve any remaining undissolved alum, creating a saturated solution.

    3. Allow the solution to cool undisturbed to form a supersaturated solution, where initial small crystals begin to form.

    4. Pick out a well-formed crystal to act as a "seed crystal" and tie it with a piece of thread attached to a pencil or glass rod.

    5. Reheat the solution to completely dissolve all remaining solids, then suspend the seed crystal into the solution using the rod.

    6. Leave the beaker undisturbed for a few days; the seed crystal acts as a core for the growth of a much larger, pure alum crystal.

• Rusting of Iron and Prevention:

  • Rusting is a chemical change where iron reacts with oxygen and water vapour present in the air to form hydrated iron oxide (rust).

  • Essential Conditions: Both Oxygen (Air) and Water (Moisture) are mandatory for rusting to take place.

  • Experimental Verification:

    • Boil water in a conical flask for 1–2 minutes to drive off dissolved air.

    • Place clean iron nails inside the boiled water and seal the flask with an airtight stopper (Nails inside do not rust due to lack of air).

    • Place another set of clean iron nails in a moist muslin cloth exposed to the atmosphere (Nails in moist cloth rust rapidly due to exposure to both air and moisture).

  • Prevention Methods:

    1. Applying a coat of paint, grease, or oil to cut off air and moisture contact.

    2. Galvanisation: The process of depositing a protective layer of zinc metal on iron objects.

• Iron Reaction with Copper Sulphate:

  • When grey iron nails are immersed in a blue copper sulphate solution overnight, a chemical displacement reaction occurs:         $\text{Iron} + \text{Copper sulphate} \rightarrow \text{Copper} + \text{Iron sulphate}$         $Fe + CuSO_4 \rightarrow Cu + FeSO_4$

  • Observations: The blue colour of the solution changes to light green (due to the formation of iron sulphate, $FeSO_4$), and a reddish-brown deposit of copper metal ($Cu$) forms over the iron nails.

Photosynthesis and Plant Physiology

• Autotrophic Nutrition and Photosynthesis:

  • Green plants are termed autotrophs because they synthesize their own organic food from simple inorganic raw materials in the presence of sunlight.

  • Photosynthesis: The biological process by which green leaves convert carbon dioxide ($CO_2$) and water ($H_2O$) into food (carbohydrates/starch) in the presence of sunlight absorbed by chlorophyll.

  • Raw materials required: Carbon dioxide ($CO_2$) and Water ($H_2O$).

  • Role of key components:

    • Chlorophyll: The green pigment located in plant leaves that captures solar light energy.

    • Sunlight: Provides the radiant energy necessary to drive the chemical reaction.

    • Carbon Dioxide: Gas absorbed directly from atmospheric air through stomata.

    • Water: Absorbed from the soil by the roots and transported to the leaves.

  • Producers: Green plants are called producers because they create biomass that serves as the primary energy source for all heterotrophic organisms in ecosystems.

• Stomata:

  • Stomata are microscopic pores located on the surface of plant leaves, surrounded by guard cells.

  • Importance:

    1. Facilitate gaseous exchange ($CO_2$ intake and $O_2$ release) during photosynthesis and respiration.

    2. Regulate water loss through transpiration.

• Starch Test in Leaves:

  • Primary stored food product in leaves is starch.

  • Experimental Demonstration:

    • A potted plant is kept in sunlight, with a portion of a leaf covered with black paper.

    • After several hours, testing the leaf with iodine solution reveals that only the uncovered portion exposed to light turns blue-black (indicating starch formation), while the covered portion remains unchanged because photosynthesis requires light energy.

Animal Fibres and Fabrics

• Sources of Wool:

  • Wool fibres are obtained from the fleece (hair) of animals such as sheep, goats, yaks, camels, llamas, and alpacas.

• Shearing:

  • The process of shaving off or removing the fleece from the body of a sheep along with a thin layer of skin is called shearing.

• Suitability for Winter Clothing:

  • Animal hair fibres trap a substantial amount of air within their porous weave.

  • Air is a poor conductor (insulator) of heat, preventing body heat from escaping into the cold environment, thereby keeping the body warm.

Human Nutrition and Digestive System

• Digestive Movements and Processes:

  • Peristalsis: The continuous, wave-like muscular contraction and relaxation that pushes food downward through the alimentary canal.

  • Absorption: The process by which digested nutrient molecules pass through the intestinal wall into the bloodstream.

• Digestive Organs and Functions:

  • Mouth:

    • Mechanical breakdown of food through chewing (mastication).

    • Saliva, secreted by salivary glands, mixes with food. It contains enzymes that digest complex carbohydrates (starch) into simple sugars and lubricates food for smooth swallowing.

  • Oesophagus (Food Pipe):

    • Muscular tube that conveys food from the mouth to the stomach via peristaltic movement.

  • Stomach:

    • Churns food and mixes it with gastric juices, hydrochloric acid ($HCl$), and digestive enzymes to digest proteins and kill bacteria.

  • Small Intestine:

    • Receives bile from the liver and pancreatic juice from the pancreas.

    • Bile Action: Bile emulsifies fats, breaking large fat globules into small droplets. Without bile reaching the small intestine, fats become the most difficult nutrient to digest effectively.

    • Completes digestion of proteins, carbohydrates, and fats.

    • Villi: Finger-like projections on the inner wall of the small intestine that increase the surface area for maximum absorption of nutrients into the blood.

• Structure and Layers of a Human Tooth:

  • Enamel: The outer white, extremely hard layer protecting the crown of the tooth.

  • Dentine: The bone-like structure lying directly beneath the enamel, providing structural strength.

  • Pulp: The innermost soft tissue core containing blood vessels, connective tissues, and nerve endings.

Acids, Bases, Indicators, and Neutralization

• Classification of Substances:

  • Acidic: Substances that taste sour and turn blue litmus red (e.g., Vinegar, Citric acid/Lemon juice).

  • Basic: Substances that taste bitter, feel soapy or slippery to touch, and turn red litmus blue (e.g., Quick lime, Baking soda, Soap solution, Magnesium hydroxide, Sodium hydroxide).

  • Neutral: Substances that are neither acidic nor basic and do not alter the colour of litmus indicators (e.g., Sugar solution, Common salt solution, Sodium sulphate).

• Indicators:

  • Substances that exhibit distinct colour changes in acidic or basic solutions.

  • Natural Indicators: Litmus (derived from lichens), Turmeric, China rose, Purple cabbage juice.

  • Indicator Reactions:

    • Litmus: Turns red in acidic medium; turns blue in basic medium.

    • Purple Cabbage Juice: Turns red in acidic solutions, turns green in basic solutions, and remains light purple/unchanged in neutral solutions.

• Strong and Weak Acids:

  • Organic Acids: Weak acids naturally found in plants and animals (e.g., Citric acid in citrus fruits, Lactic acid in curd, Acetic acid in vinegar).

  • Mineral Acids: Strong acids derived from minerals, heavily used in laboratories (e.g., Hydrochloric acid, Nitric acid, Sulphuric acid). They are highly corrosive and cause severe chemical burns to skin, cloth, and wood.

• Properties and Uses of Common Bases:

  • Magnesium Hydroxide: Used to manufacture antacid liquids to neutralize stomach acidity.

  • Sodium Hydroxide: Used in the industrial manufacturing of soaps and detergents.

• Neutralization Reaction:

  • The chemical reaction between an acid and a base resulting in the formation of salt, water, and heat evolution:         $\text{Acid} + \text{Base} \rightarrow \text{Salt} + \text{Water} + \text{Heat}$

  • Nature of Formed Salt:

    • Equal strength Acid + Base = Neutral Salt.

    • Strong Acid + Weak Base = Acidic Salt.

    • Weak Acid + Strong Base = Basic Salt.

• Practical Applications of Neutralization:

  • Ant Bites and Wasp Stings:

    • Ant bites inject acidic fluid (formic acid) into skin, treated by applying mild basic substances like calamine solution (containing zinc carbonate) or moist baking soda.

    • Wasp stings are alkaline, treated by applying a mild acid such as vinegar.

  • Soil Treatment: Excessive chemical fertilizers make agricultural soil acidic. Farmers treat the field by sprinkling basic white lime powder (quick lime/calcium oxide) to neutralize soil acidity.

  • Factory Effluents: Industrial wastewater contains acidic chemicals that harm aquatic life; effluents are neutralized with basic substances before release into water bodies.

Heat, Temperature, Thermometers, and Thermal Expansion

• Effects of Heat Energy:

  1. Rise in temperature.

  2. Thermal expansion in solids, liquids, and gases.

  3. Change of state (e.g., water evaporating to steam).

  4. Chemical or biological alterations (e.g., vegetables softening during cooking).

• Thermal Expansion Phenomena and Applications:

  • Electric Wires: Wires sag between poles during hot summer days because solids expand on heating. They are installed with calculated slack to accommodate winter contraction.

  • Fitting Metal Rims on Wooden Wheels: A metal rim with a diameter slightly smaller than a wooden wheel is heated so that it expands and slips over the wheel. Upon cooling, it contracts tightly around the wooden frame.

  • Expansion Gaps in Rails and Concrete Structures: Gaps are intentionally left between concrete road/bridge slabs and railway tracks to allow metal and concrete to expand freely during summer heat and friction without causing warping or structural cracking.

  • Air Expansion: Air expands upon heating and becomes less dense, creating upward convective air currents.

  • Experimental Proof of Solid Expansion (Ball and Ring Apparatus):

    *   *Procedure*: At room temperature, a metal ball easily passes through a metallic ring.
    *   *Observation*: After heating the metal ball over a Bunsen burner flame for a few minutes, it expands in volume and fails to pass through the ring. Upon cooling back to room temperature, it contracts and passes through the ring again.

• Thermometry and Temperature Scales:

  • Working Principle: Thermometers operate on the uniform thermal expansion and contraction of liquids with changing temperature.

  • Thermodynamic Scales: The three standard units/scales of temperature measurement are Celsius ($^\circ\text{C}$), Fahrenheit ($^\circ\text{F}$), and Kelvin ($\text{K}$).

  • Fixed Points: Fixed reference temperatures (e.g., ice point and steam point) are marked on thermometers to establish standardized calibrations. The Celsius scale contains $100$ equal divisions between its lower fixed point ($0^\circ\text{C}$) and upper fixed point ($100^\circ\text{C}$).

  • Fahrenheit to Celsius Conversion Formula:         $\text{C} = \frac{5}{9} \times (\text{F} - 32)$

    • Calculation for $68^\circ\text{F}$:             $\text{C} = \frac{5}{9} \times (68 - 32) = \frac{5}{9} \times 36 = 20^\circ\text{C}$

  • Alcohol vs Mercury Thermometer: Alcohol thermometers are used in extremely cold regions because alcohol has a vastly lower freezing point than mercury.

  • Types of Thermometers:

    • Clinical Thermometer:

        *   Graduated from $34^\circ\text{C}$ to $42^\circ\text{C}$ (or $94^\circ\text{F}$ to $108^\circ\text{F}$) to cover human body temperatures.
        *   Contains a constriction or **kink** in the capillary stem just above the bulb, which prevents the expanded mercury column from dropping back into the bulb automatically before reading.
        *   *Caution*: The bulb must not be touched during temperature readings because heat from human hands alters the measurement.
    *   **Laboratory Thermometer**:
        *   Graduated typically from $-10^\circ\text{C}$ to $110^\circ\text{C}$ for laboratory chemical testing.
        *   Does not contain a kink; temperature must be read while the bulb remains immersed in the medium.

Modes of Heat Transfer and Applications

• Modes of Heat Transfer:

  1. Conduction:

     • The mode of heat transfer in solids from a higher temperature region to a lower temperature region through direct contact, without any actual movement of constituent particles.

     • Mechanism: Particles at the heated end absorb heat energy and vibrate faster, passing vibrational kinetic energy to neighbouring particles along the length of the solid rod.

     • Conditions Needed: Direct physical contact between bodies, and a temperature gradient between them.

     • Conductors and Insulators:

       • Good Conductors: Materials allowing rapid heat transfer (e.g., Copper, Iron, Mercury). Vessels often feature copper bottoms for quick heating.

       • Insulators (Poor Conductors): Materials retarding heat transfer (e.g., Wood, Cork, Glass, Wool, Trapped air). Handles of cookware are made of wood/plastic to prevent burns.

     • Insulation Comparison: A wool blanket provides far greater thermal protection than a conductive blanket of equal thickness because wool traps dead air pockets; trapped air acts as an effective insulator.

  2. Convection:

     • The mode of heat transfer in fluids (liquids and gases) via actual mass movement of heated molecules.

     • Convection Currents: Heated fluid expands, drops in density, and rises upward. Cooler, denser surrounding fluid sinks down to replace it, creating continuous circulating currents.

     • Absence in Solids: Convection cannot occur in solids because solid molecules are locked tightly in fixed crystalline lattice positions.

     • Ventilators: Placed high up near ceilings because warm air produced in rooms rises via convection and exits through top ventilators.

     • Coastal Breezes:

       • Sea Breeze (Daytime): Land heats up faster than the sea. Hot air over land rises, and cool air over the sea flows inland to replace it.

       • Land Breeze (Nighttime): Land cools down faster than the sea. Warmer air over the sea rises, and cool air over the land flows out toward the sea.

  3. Radiation:

     • The transfer of heat energy through electromagnetic waves without requiring any material medium; can occur across empty space/vacuum.

     • Influencing Factors: Temperature, surface colour, and surface texture of the body.

     • Reflectors and Clothing:

       • Electric room heaters feature polished curved metal reflectors behind heating coils to focus radiant heat forward.

       • Light-coloured/white clothing is preferred in summer because light surfaces reflect radiant sunlight without absorbing heat.

• Thermos Flask (Vacuum Flask) Design:

*   Designed to preserve liquid temperature by preventing heat transfer across all three modes:
    *   *Prevention of Conduction*: Cork stopper, glass body, and protective plastic outer casing are poor heat conductors.
    *   *Prevention of Convection*: An evacuated space (vacuum) between double glass walls contains no gas particles, preventing convective currents.
    *   *Prevention of Radiation*: Inner surfaces of the glass walls are silvered like mirrors to reflect radiant heat energy back into the flask.

Summary Assessment Review

• Multiple-Choice Reference Answers:

  • Raw material required by green plants during photosynthesis: Carbon dioxide.

  • Nutrient most affected if bile fails to reach small intestine: Fats.

  • Animal fibre obtained from fleece of goats: Wool.

  • Solution turning red litmus blue: Basic.

  • Scale with 100 divisions between fixed points: Celsius.

  • Chemical change example: Formation of curd.

  • Direction of sea breeze: Sea to land during the day.

  • Substance coating iron during galvanisation: Zinc.

  • Tooth layer immediately beneath enamel: Dentine.

  • Heat transfer mode across empty space: Radiation.

• Fill-in-the-Blank Reference Answers:

  • Light-absorbing green leaf pigment: Chlorophyll.

  • Rhythmic movement pushing food through canal: Peristalsis.

  • Process of removing fleece from sheep: Shearing.

  • Substance changing colour in acid/base solutions: Indicator.

  • Primary heat transfer mode in liquids and gases: Convection.

  • Change forming a new chemical substance: Chemical change.

• Assertion & Reason Concepts:

  • Green plants are autotrophs because they synthesize food using simple substances in sunlight (Assertion and Reason true, Reason explains Assertion).

  • Metal spoon gets hot in tea because metals are good conductors of heat (Assertion and Reason true, Reason explains Assertion).

  • Burning paper is a physical change (Assertion false; Reason that new substances form during burning is true).

  • Thermometer bulb should not be touched while measuring because hand heat alters readings (Assertion and Reason true, Reason explains Assertion).

• One-Word Terminology:

  • Process of digested food passing into blood: Absorption.

  • Substances retarding heat passage: Insulators.

  • Process of zinc layer deposition on iron: Galvanisation.

  • Biological relationship where one benefits and the other is harmed: Parasitism.

  • Muscular tube transferring food to stomach: Oesophagus.