Chapter 2: Is Matter around Us Pure ?

 Purity in Consumables

• Pure vs Mixtures:

  • Common Definition of Pure: "Pure" is generally understood by people as having no adulteration or contamination.
  • Scientific Definition of Pure: A pure substance contains only one type of particle, chemically the same in nature (e.g., a single element or compound).

• Examples of "Pure" Substances:

  • Milk: A mixture of water, fats, proteins, etc.
  • Ghee, Butter, Salt, Spices, Mineral Water, Juice: All of these are mixtures, as they consist of multiple components.

• 

• Mixtures in Everyday Life:

  • Sea Water, Minerals, Soil: These are examples of mixtures, which consist of different substances combined together.

• Market Labeling:

  • Many consumables like milk, ghee, butter, etc., may be labeled as "pure" on the packaging, but scientifically, these are mixtures rather than pure substances.

 Mixtures

• Definition of a Mixture:

  • A mixture is composed of two or more pure substances.
  • The substances in a mixture retain their individual properties and can be physically separated.

• Examples:

  • Sodium Chloride (NaCl): It is a pure substance that cannot be separated into its chemical components through physical means.
  • Sugar: A pure substance, composed of only one type of matter, maintaining consistent composition throughout.
  • Soft Drink and Soil: These are mixtures because they consist of various substances combined together, not just a single pure substance.

• Key Point:

  • Mixtures contain more than one pure substance and can be separated by physical processes (e.g., evaporation, filtration, etc.).

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Types of Mixtures

1. Homogeneous Mixtures:

• Definition: Mixtures that have a uniform composition throughout.
• Characteristics: The components are evenly distributed, and the mixture appears the same throughout.
• Examples:
  • Salt dissolved in water.
  • Sugar dissolved in water.
• Activity Results (Groups A & B):
  • Both groups made a homogeneous mixture (copper sulfate solution), though the intensity of color varied due to different amounts of copper sulfate.
• Key Point: A homogeneous mixture can have a variable composition but remains uniform in appearance.

2. Heterogeneous Mixtures:

• Definition: Mixtures that have physically distinct parts and non-uniform composition.
• Characteristics: The different components are not evenly distributed, and individual particles can be seen.
• Examples:
  • Mixture of sodium chloride and iron filings.
  • Salt and sulfur mixture.
  • Oil and water mixture.
• Activity Results (Groups C & D):
  • Groups C and D made heterogeneous mixtures.

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Activity Observations (2.2):

• Group A (Copper Sulfate Crystals in Water):

  • Type of Mixture: Solution (homogeneous).
  • Visibility of Particles: Particles are not visible.
  • Light Path: The light beam path is not visible.
  • Stability: Stable, no settling.
  • Filtration Result: No residue on filter paper.

• Group B (Copper Sulfate Powder in Water):

  • Type of Mixture: Solution (homogeneous).
  • Visibility of Particles: Particles are not visible.
  • Light Path: The light beam path is not visible.
  • Stability: Stable, no settling.
  • Filtration Result: No residue on filter paper.

• Group C (Chalk Powder/Wheat Flour in Water):

  • Type of Mixture: Suspension (heterogeneous).
  • Visibility of Particles: Particles are visible.
  • Light Path: The light beam path is visible (Tyndall effect).
  • Stability: Particles begin to settle after some time.
  • Filtration Result: Residue remains on filter paper.

• Group D (Milk or Ink in Water):

  • Type of Mixture: Colloidal Solution.
  • Visibility of Particles: Particles are not visible individually, but the mixture may appear cloudy.
  • Light Path: The light beam path is visible (Tyndall effect).
  • Stability: Stable, particles do not settle.
  • Filtration Result: No residue on filter paper.

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Summary of Types of Mixtures:

1. Solution (Homogeneous Mixture):
   • Small particles, cannot be seen, no settling, stable (e.g., salt in water, copper sulfate in water).
2. Suspension (Heterogeneous Mixture):
   • Larger particles visible, particles settle over time, visible light path (e.g., chalk powder in water).
3. Colloidal Solution:
• Medium-sized particles, visible light path (Tyndall effect), stable (e.g., milk in water, ink in water).

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 Solutions

1. Definition of a Solution:

• A solution is a homogeneous mixture of two or more substances, where the components are uniformly distributed at the particle level.
• Solutions can be in various forms: solid, liquid, or gas.

2. Types of Solutions:

• Liquid Solutions: Usually a liquid with a solid, liquid, or gas dissolved in it (e.g., lemonade, soda water).
• Solid Solutions: These are mixtures of metals or a metal and a non-metal, known as alloys (e.g., brass, which is a mixture of copper and zinc).
• Gaseous Solutions: Mixtures of gases, such as air (a homogeneous mixture of gases, primarily nitrogen and oxygen).

3. Components of a Solution:

• Solvent: The component in the largest quantity that dissolves the other substance (e.g., water in a sugar solution).
• Solute: The component in the lesser quantity that dissolves in the solvent (e.g., sugar in a sugar solution).

Examples of Solutions:

• Sugar in Water: Sugar (solute) dissolved in water (solvent) – a solid in liquid solution.
• Iodine in Alcohol (Tincture of Iodine): Iodine (solute) dissolved in alcohol (solvent) – a solid in liquid solution.
• Aerated Drinks (Soda Water): Carbon dioxide (gas) dissolved in water (liquid) – a gas in liquid solution.
• Air: A mixture of gases, primarily nitrogen (78%) and oxygen (21%) – a gas in gas solution.

4. Properties of a Solution:

• Homogeneous: The composition is uniform throughout.
• Particle Size: The particles of a solution are smaller than 1 nm (10^-9 meters), so they cannot be seen with the naked eye.
• Light Scattering: Because of the very small particle size, solutions do not scatter light, and the path of light is not visible in the solution.
• Filtration: The solute particles cannot be separated by filtration.
• Stability: The solute particles do not settle down when the solution is undisturbed, meaning the solution is stable.

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 Concentration of a Solution

1. Types of Solutions Based on Concentration:

• Dilute Solution: Contains a small amount of solute relative to the solvent.
• Concentrated Solution: Contains a large amount of solute relative to the solvent.
• Saturated Solution: A solution that has dissolved the maximum amount of solute it can at a given temperature. Any additional solute will not dissolve.
  • Solubility: The amount of solute that can dissolve in a solvent at a specific temperature to form a saturated solution.
• Unsaturated Solution: A solution where the amount of solute is less than the saturation level, meaning more solute can still dissolve.

2. Activity Observations (2.3):

• When different solutes (salt, sugar, or barium chloride) are added to water, they dissolve at different rates and capacities.
• Heating the solution can increase the solubility by allowing more solute to dissolve.
• Saturated Solution: When no more solute dissolves at a given temperature, the solution is saturated.
• Different Solubilities: Different substances have different solubilities in the same solvent at the same temperature.

3. Concentration of a Solution:

• The concentration refers to the amount of solute present in a given amount of solution.

• It can be expressed in different ways:

  (i) Mass by Mass Percentage:

  • Formula: $$\text{Mass of Solute} = \frac{\text{Mass of Solute}}{\text{Mass of Solution}} \times 100$$

  (ii) Mass by Volume Percentage:

  • Formula: $$\text{Mass of Solute} = \frac{\text{Mass of Solute}}{\text{Volume of Solution}} \times 100$$

  (iii) Volume by Volume Percentage:

  • Formula: $$\text{Volume of Solute} = \frac{\text{Volume of Solute}}{\text{Volume of Solution}} \times 100$$

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Suspensions

1. Definition of a Suspension:

• A suspension is a heterogeneous mixture where solid particles are dispersed in a liquid but do not dissolve in it. The solid particles remain suspended throughout the liquid.
• Example: The mixture obtained by Group C in Activity 2.2 (e.g., chalk powder or wheat flour in water).

2. Properties of a Suspension:

• Heterogeneous Mixture: The components are not uniformly distributed.
• Visible Particles: The particles in a suspension are large enough to be seen with the naked eye.
• Light Scattering: The particles scatter light, making the path of the light visible (known as the Tyndall effect).
• Instability: The solute particles eventually settle down when the suspension is left undisturbed, meaning the suspension is unstable.
• Separation by Filtration: The solid particles can be separated from the liquid by filtration.
• When the particles settle, the suspension "breaks" and no longer scatters light.

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 Colloidal Solution

1. Definition of a Colloidal Solution:

• A colloid or colloidal solution is a heterogeneous mixture in which the solute particles are uniformly spread throughout the solution.
• Example: The mixture obtained by Group D in Activity 2.2 (e.g., milk in water).

2. Properties of a Colloid:

• Heterogeneous Mixture: Despite appearing homogeneous, colloids are actually heterogeneous.
• Invisible Particles: The particles are small enough that they cannot be seen with the naked eye.
• 
  
  
• Tyndall Effect: Colloidal particles scatter light, making the path of light visible. This phenomenon is called the Tyndall Effect.
• 
  • Example: The scattering of light observed in a mixture of water and milk.
  • The Tyndall Effect can also be observed when sunlight passes through a forest canopy, where mist (tiny water droplets) scatters the light.
• Stability: Colloids are stable and do not settle down when undisturbed.
• Separation: Colloidal particles cannot be separated by filtration. However, a special technique called centrifugation can be used for separation.

3. Components of a Colloidal Solution:

• Dispersed Phase: The solute-like particles in a colloid, which are dispersed throughout the medium.
• Dispersion Medium: The substance (liquid, solid, or gas) in which the dispersed phase is suspended.

4. Classification of Colloids:

• Colloids are classified based on the state of both the dispersing medium and the dispersed phase (solid, liquid, or gas).
  • Examples:
    • Solid in liquid (e.g., paint)
    • Liquid in solid (e.g., gel)
    • Gas in liquid (e.g., foam)

Dispersed Phase	Dispersing Medium	Type	Example
Liquid	Gas	Aerosol	Fog, clouds, mist
Solid	Gas	Aerosol	Smoke, automobile exhaust
Gas	Liquid	Foam	Shaving cream
Liquid	Liquid	Emulsion	Milk, face cream
Solid	Liquid	Sol	Milk of magnesia, mud
Gas	Solid	Foam	Foam, rubber, sponge, pumice
Liquid	Solid	Gel	Jelly, cheese, butter
Solid	Solid	Solid Sol	Coloured gemstone, milky glass

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 Separating Components of a Mixture

1. Separation Methods:

• Most natural substances are not chemically pure, and separation techniques are used to isolate individual components from a mixture.
• These methods allow for the study and utilization of the separate components in various applications.

2. Simple Separation Methods for Heterogeneous Mixtures:

• Handpicking: Manually removing unwanted components from a mixture.
• Sieving: Using a sieve to separate particles of different sizes.
• Filtration: Separating solid particles from liquids or gases using a filter paper or similar material.
• 

3. Special Separation Techniques:

• For more complex mixtures, specialized techniques are required, such as distillation, chromatography, and others.

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Activity 2.4: Obtaining the Coloured Component (Dye) from Blue/Black Ink

Objective: Separate the volatile solvent (water) from the non-volatile solute (dye) in ink using the process of evaporation.

Procedure:

1. Set Up: Fill half a beaker with water and place a watch glass on top of the beaker.
2. Add Ink: Put a few drops of ink on the watch glass.
3. Heating: Gently heat the beaker, ensuring that the ink itself is not heated directly.
4. Observation: As the water evaporates, observe the changes on the watch glass.
   • Continue heating until no further changes are observed.
5. Questions to Consider:
   • What evaporated from the watch glass?
   • Was there any residue left on the watch glass?
   • Based on your observations, do you think ink is a single substance or a mixture?

Results and Interpretation:

• Ink as a Mixture: The evaporation process separates the volatile solvent (water) from the non-volatile solute (dye), confirming that ink is a mixture of water and dye.

Conclusion:

• Evaporation helps separate a volatile component (solvent) from a non-volatile component (solute), and in this case, it shows that ink is a mixture, not a pure substance.

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Separation of Mixture of Two Immiscible Liquids

Activity:

• Objective: To separate kerosene oil from water using a separating funnel.
• Procedure:
  1. Pour the mixture of kerosene oil and water into a separating funnel.
  2. Let it stand undisturbed to allow the separation of layers (oil and water).
  3. Open the stopcock and pour out the lower layer (water) carefully.
  4. Close the stopcock when the oil layer reaches it.

Applications:

• Separating mixtures of oil and water.
• In industries like iron extraction, lighter slag is separated from molten iron using this method.

Principle: Immiscible liquids separate into layers based on their different densities.

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 Separating Cream from Milk

1. Milk Varieties:

• In the market, we get different varieties of milk such as full-cream, toned, and double-toned milk, which contain varying amounts of fat.

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Activity 2.5: Separating Cream from Milk

Objective: To separate the cream (fat) from milk using centrifugation.

Procedure:

1. Take Full-Cream Milk: Fill a test tube with some full-cream milk.
2. Centrifuge: Use a centrifuge machine for two minutes. If a centrifuge is not available, you can use a milk churner at home.
3. Visit a Dairy (Optional): You can visit a local dairy and ask about:
   • How they separate cream from milk.
   • How they make cheese (paneer) from milk.

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Observations and Explanation:

• What Happens on Churning?
  • When you churn the milk, the cream, which is lighter, rises to the top, while the denser liquid (buttermilk) stays at the bottom.
• How Does Centrifugation Separate Cream from Milk?
  • Centrifugation uses rapid spinning to separate components based on their density.
  • The denser particles (buttermilk) are forced to the bottom, and the lighter particles (cream) remain at the top.

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Principle of Centrifugation:

• Centrifugation works on the principle that when a mixture is spun rapidly, denser particles move to the bottom, while lighter particles stay at the top. This method is used when particles are too small to be separated by filtration.

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Applications of Centrifugation:

• Diagnostic Laboratories: Used to separate components of blood and urine for tests.
• Dairies and Homes: Used to separate butter from cream.
• Washing Machines: Used to squeeze out water from wet clothes.

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 Separating a Mixture of Salt and Camphor

1. Sublimation Process:

• Sublimation is a process in which a substance changes directly from a solid to a gas without passing through the liquid state.
• Camphor is a substance that sublimes when heated, changing directly from solid to gas.
• This property can be used to separate camphor from a mixture that contains a sublimable volatile component (like camphor) and a non-sublimable impurity (like salt).

2. Separation of Salt and Camphor Using Sublimation:

• Mixture: A mixture of salt and camphor can be separated by heating the mixture.
• Camphor's Sublimation: When heated, camphor will directly sublimate into the gaseous state, leaving the salt behind as a residue.

3. Examples of Sublimable Solids:

• Camphor, ammonium chloride, naphthalene, and anthracene are common examples of substances that sublime.

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Conclusion:

• Sublimation is a useful method for separating mixtures that contain both sublimable substances (like camphor) and non-sublimable substances (like salt). By heating the mixture, camphor will sublimate, and salt will remain behind.

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 Chromatography

1. What is Chromatography?

• Chromatography is a technique used for separating the components of a mixture, especially those that dissolve in the same solvent.
• The term "chromatography" comes from the Greek word kroma meaning "colour," as it was first used to separate different colours in dyes.

2. Activity to Demonstrate Chromatography:

• Materials Needed:

  • Thin strip of filter paper
  • Pencil
  • Ink (water-soluble, like from a sketch pen or fountain pen)
  • Water
  • Glass/jar/beaker/test tube

• Steps:

  1. Draw a line using a pencil about 3 cm above the lower edge of the filter paper.
  2. Place a small drop of ink at the center of the line and let it dry.
  3. Lower the filter paper into a jar or container with water, ensuring the ink drop stays above the water level.
  4. Allow the water to rise up the paper and observe what happens.

3. Observations:

• As the water rises up the paper, it carries the dye particles with it.
• Different Colours: If the ink is a mixture of colours, different coloured bands will appear on the filter paper. The dye that is more soluble in water will rise faster than the others.

4. Explanation:

• The ink used in this experiment has water as the solvent, and the dye is soluble in water.
• The rise of water on the filter paper causes the dye particles to travel up the paper, and depending on their solubility, the dyes separate into distinct colours.

5. Conclusion:

• The dye in black ink is typically a mixture of multiple colours.
• Chromatography is the process that allows for the separation of these different colours based on their solubility in the solvent (water in this case).

6. Applications of Chromatography:

• Separating colours in dyes.
• Isolating pigments from natural sources.
• Extracting drugs from blood for analysis.

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 Separating a Mixture of Two Miscible Liquids

1. Method: Distillation

• Distillation is used to separate two miscible liquids (liquids that mix in all proportions) that have different boiling points.
• In this process, the liquid with the lower boiling point vaporizes first, and the vapor is then condensed back into liquid form, leaving the higher boiling point liquid behind.

2. Activity to Demonstrate Simple Distillation:

• Materials Needed:

  • Mixture of acetone and water
  • Distillation flask
  • Thermometer
  • Distillation apparatus

• Steps:

  1. Place the mixture of acetone and water in the distillation flask.
  2. Fit the flask with a thermometer and connect the setup to a condenser.
  3. Heat the mixture slowly, keeping an eye on the thermometer.
  4. As the mixture heats, the acetone will vaporize first, condense in the condenser, and be collected from the condenser outlet.
  5. Water will be left behind in the distillation flask.

3. Observations:

• As you heat the mixture, acetone vaporizes first due to its lower boiling point.
• The thermometer reading becomes constant once the temperature reaches the boiling point of acetone (56°C).
• The boiling point of acetone is 56°C, which is lower than water's boiling point (100°C).

4. Explanation:

• Distillation works because acetone has a lower boiling point than water, so it vaporizes first when the mixture is heated.
• The vaporized acetone is condensed back into liquid form, separating it from the water.

5. Fractional Distillation:

• Fractional distillation is used when the boiling point difference between the two liquids is less than 25°K (e.g., for separating gases in air or fractions in petroleum).
• The setup is similar to simple distillation but includes a fractionating column packed with glass beads.
• The beads allow the vapors to condense and re-evaporate multiple times, separating the components more efficiently.

6. Conclusion:

• Distillation is effective for separating liquids with significant differences in boiling points.
• When the difference in boiling points is smaller, fractional distillation is used for better separation.

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 Obtaining Different Gases from Air

1. Separation of Gases from Air:

• Air is a homogeneous mixture containing several gases, primarily nitrogen (78%), oxygen (21%), and small amounts of other gases like argon, carbon dioxide, and trace gases.
• Air can be separated into its components through fractional distillation.

2. Process of Separation:

• Compression and Cooling:
  • First, the air is compressed by increasing the pressure.
  • Then, it is cooled to very low temperatures to convert the gases into liquid form, producing liquid air.
• Fractional Distillation:
  • The liquid air is slowly warmed up in a fractional distillation column.
  • As the air warms up, the gases separate at different heights in the column based on their boiling points. Gases with lower boiling points will rise higher in the column, while those with higher boiling points will condense and collect lower down.

3. Order of Boiling Points of Gases in Air:

• Boiling Points in Increasing Order:
  1. Nitrogen (-195.8°C)
  2. Oxygen (-183°C)
  3. Argon (-185.7°C)
  4. Carbon dioxide (-78.5°C)
• Oxygen forms the liquid first as the air is cooled because it has a higher boiling point than nitrogen and other gases.

4. Key Points:

• Fractional distillation is the process used to separate air into its components.
• Nitrogen has the lowest boiling point and will remain gaseous until the temperature is very low.
• Oxygen forms liquid first during the cooling process because it has a higher boiling point than nitrogen.

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Obtaining Pure Copper Sulphate through Crystallization

1. Crystallization Process:

• Objective: To obtain pure copper sulfate from an impure sample.

Steps:

• Step 1: Take approximately 5 g of impure copper sulfate in a china dish.
• Step 2: Dissolve the copper sulfate in a minimum amount of water.
• Step 3: Filter the solution to remove any insoluble impurities.
• Step 4: Evaporate the water from the solution to obtain a saturated solution of copper sulfate.
• Step 5: Cover the dish with a filter paper and leave it undisturbed at room temperature for a day. This allows the copper sulfate to cool slowly.
• Step 6: As the solution cools, crystals of copper sulfate will form in the china dish.

2. Observations and Answering Questions:

• The crystals that form will appear similar in size and shape, as they are pure copper sulfate.
• Crystals can be separated from the remaining liquid by decanting or filtering the liquid.

3. Crystallization:

• Crystallization is a method used to purify solids by forming crystals.
• It is preferred over simple evaporation because:
  • Some solids may decompose or char (like sugar) if evaporated directly.
  • Some impurities may still remain dissolved in the solution even after filtration, and can contaminate the solid if evaporated.

4. Applications of Crystallization:

• Purification of salt obtained from sea water.
• Separation of crystals of alum (phitkari) from impure samples.

5. Advantages of Crystallization:

• It helps separate pure substances by forming crystals, while impurities stay dissolved in the liquid.

6. Connection to Water Purification:

• The crystallization method can be compared to the water purification process used in cities, where raw water is treated, and purified water is made available to homes. This involves a series of processes like filtration, sedimentation, and sometimes, crystallization techniques for specific impurities.

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 Physical and Chemical Changes

1. Physical Changes:

• Physical changes are those that affect the physical properties of a substance, such as color, hardness, density, melting point, boiling point, etc.
• In a physical change, the chemical composition of the substance does not change.
• Examples of physical changes include the interconversion of states (e.g., from solid to liquid or liquid to gas). For example:
  • Ice melting into water and water evaporating into water vapor.
  • Although these substances appear different (solid, liquid, or gas), they are chemically the same (all are water, H₂O).

2. Chemical Changes:

• Chemical changes occur when a substance reacts with another to form a new substance, changing its chemical composition.
• During chemical changes, the chemical properties of the substance are altered, and new substances are produced. This is also called a chemical reaction.
• A common example of a chemical change is burning, where one substance reacts with oxygen (air) and undergoes a transformation, producing heat and light. For example, burning oil or wood produces new substances like carbon dioxide and water.

3. Physical vs Chemical Changes (Example of a Candle Burning):

• Physical Changes in a Candle:
  • The candle melts as it heats up, changing from solid to liquid.
  • The wax evaporates when it gets hot enough to produce the flame, which is a physical change since it's a change in state, not composition.
• Chemical Changes in a Candle:
  • The wax burns and undergoes a chemical reaction with oxygen (combustion) to produce carbon dioxide (CO₂) and water vapor (H₂O).
  • This reaction alters the chemical composition of the wax and produces new substances, indicating a chemical change.

4. Key Differences:

• Physical changes are reversible (e.g., melting and freezing), while chemical changes are often irreversible (e.g., burning).
• Chemical changes result in the formation of new substances, while physical changes do not.

5. Conclusion:

• While physical changes involve no change in chemical composition, chemical changes result in the formation of new substances and a change in chemical properties.

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Types of Pure Substances

Pure substances are classified into two main categories: elements and compounds. These classifications are based on the chemical composition of the substance.

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1. Elements

• Definition: An element is a basic form of matter that cannot be broken down into simpler substances by chemical reactions.
• First use of the term: The term "element" was first used by Robert Boyle in 1661.
• Modern Definition: Antoine Laurent Lavoisier (1743-94), a French chemist, established the modern definition of an element.
• Categories of Elements: Elements are further classified into metals, non-metals, and metalloids.

Metals:

• Properties of Metals:
  • Lustrous (shine): Metals have a shiny appearance.
  • Color: Typically silvery-grey or golden-yellow.
  • Conductivity: Metals conduct heat and electricity.
  • Ductility: They can be drawn into wires.
  • Malleability: Metals can be hammered into thin sheets.
  • Sonorous: Metals produce a ringing sound when hit.
• Examples of Metals:
  • Gold, silver, copper, iron, sodium, potassium.
  • Mercury is the only metal that is liquid at room temperature.

Non-Metals:

• Properties of Non-Metals:
  • Color variety: Non-metals show a variety of colors.
  • Poor conductors: They are poor conductors of heat and electricity.
  • Non-lustrous: Non-metals are not shiny.
  • Not sonorous or malleable: They do not produce a ringing sound and cannot be hammered into sheets.
• Examples of Non-Metals:
  • Hydrogen, oxygen, iodine, carbon (coal, coke), bromine, chlorine.

Metalloids:

• Properties of Metalloids:

  • Metalloids have intermediate properties between metals and non-metals.
  • They may have some properties of both metals and non-metals.

• Examples of Metalloids:

  • Boron, silicon, germanium.

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Number of Elements Known

• Total Elements: There are more than 100 elements known at present.

• Naturally Occurring Elements: 92 elements occur naturally.

• Manmade Elements: The rest of the elements (beyond 92) are manmade.

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States of Elements at Room Temperature

• Solid Elements: The majority of elements are in the solid state at room temperature.

• Gaseous Elements: Eleven elements are in the gaseous state at room temperature.

• Liquid Elements:

  • Two elements are liquid at room temperature: Mercury (Hg) and Bromine (Br).
  • Gallium (Ga) and Cesium (Cs) become liquid at a temperature slightly above room temperature (303 K).

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Compounds And Mixture 

A compound is a substance composed of two or more elements that are chemically combined in a fixed proportion.

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Activity: Combining Iron and Sulphur

Objective: Observe the changes when iron and sulphur are combined in different conditions.

Group I (Physical Change - Mixture)

1. Materials: Mix 5g of iron filings and 3g of sulphur powder. Crush them together.
2. Observation: The material obtained is a mixture of iron and sulphur. It retains the properties of both substances.
3. Properties:
   • Magnetism: The material is magnetic (iron is attracted by a magnet).
   • Separation: Components can be separated (magnetic properties of iron can be used).
   • Gas on Adding Acid: When dilute sulphuric acid is added, hydrogen gas is produced. Hydrogen is colorless, odorless, and combustible.
   • Texture and Color: The texture and color of the mixture remain unchanged.

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Group II (Chemical Change - Compound)

1. Materials: Mix and crush iron filings and sulphur powder. Heat this mixture strongly until red hot. Let it cool.
2. Observation: The heated mixture forms a compound with new properties.
3. Properties:
   • Magnetism: The compound does not have magnetic properties (iron has reacted with sulphur).
   • Separation: The components cannot be separated easily (no magnetism to separate iron).
   • Gas on Adding Acid: When dilute sulphuric or hydrochloric acid is added, hydrogen sulphide is produced. It is a colorless gas with the smell of rotten eggs.
   • Texture and Color: The texture and color of the compound are uniform throughout.

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Summary:

• Group I demonstrates a physical change (mixture of elements) where properties of the individual substances remain.
• Group II demonstrates a chemical change (compound formation) where the properties of the individual elements are lost, and new properties emerge.

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Property	Group I (Mixture)	Group II (Compound)
Nature	Physical change (mixture)	Chemical change (compound)
Magnetism	Yes (magnetic iron remains)	No (compound is not magnetic)
Gas on Adding Acid	Hydrogen (colorless, odorless)	Hydrogen sulphide (smell of rotten eggs)
Separation	Can be easily separated	Cannot be separated easily
Composition	Varies; same as starting materials	Fixed, uniform throughout
Texture and Color	Same as iron and sulphur	Uniform and new compared to the elements

This experiment shows that when elements chemically combine, they form a compound with new properties, unlike mixtures where the properties of the individual elements remain.

Mixtures	Compounds
1. Elements or compounds just mix together to form a mixture and no new compound is formed.	1. Elements react to form new compounds.
2. A mixture has a variable composition.	2. The composition of each new substance is always fixed.
3. A mixture shows the properties of the constituent substances.	3. The new substance has totally different properties.
4. The constituents can be separated fairly easily by physical methods.	4. The constituents can be separated only by chemical or electrochemical reactions.

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