Elements, Compounds, and Mixtures

CHAPTER 8: NATURE OF MATTER - ELEMENTS, COMPOUNDS, AND MIXTURES

INTRODUCTION: UNDERSTANDING MATTER AROUND US

All the things we observe in our surroundings—the staircase, air, water, food, clothes, books, trees, and balls—are made of **matter**. Matter is the substance that makes up everything around us and occupies space.

Everything around us is made up of tiny particles. Most things are not made of just one substance; they are made of two or more substances mixed together. Understanding how different substances come together to form mixtures, compounds, and elements is fundamental to understanding the nature of matter.

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8.1 WHAT ARE MIXTURES?

Definition of Mixture

A **mixture** is formed when two or more substances are combined such that each substance retains its own properties. The individual substances that make up a mixture are called its **components**.

**Key Characteristics of Mixtures:**

The components do not react chemically with each other

Each component keeps its original properties

The components can be separated by physical methods

The ratio of components can be varied

**Indian Real-Life Examples:**

Poha (beaten rice with vegetables) - a traditional breakfast dish

Sprout salad (mixture of green gram, chickpeas, onion, and tomato)

Sugar dissolved in water

Soups and lemonade

Types of Mixtures Based on Visibility

#### Non-Uniform (Heterogeneous) Mixtures

**Non-uniform mixtures** are those where the different components are visible with the naked eye or with a magnifying device. The composition is not the same throughout.

**Examples:**

Sprout salad (you can see individual components: gram, chickpeas, onion, tomato)

A mixture of rice and beans

Sand and pebbles

Salt and sugar mixed together without dissolving

#### Uniform (Homogeneous) Mixtures

**Uniform mixtures** are those where the components are evenly distributed and cannot be distinguished, even with a microscope. The mixture appears the same throughout.

**Examples:**

Sugar dissolved in water

Salt dissolved in water

Seawater (water with dissolved salt and minerals)

Air (invisible mixture of gases)

Important Heritage Note: Indian Knowledge of Mixtures

**Mishraloha** was the name given to mixtures of two or more metals that had properties distinct from their constituent metals. Ancient Indian texts mention the use of alloys for medicinal purposes:

**Bronze (Kamsya)**: Made from Copper (Tamra, 4 parts) and Tin (Vanga, 1 part) - used to improve digestion and boost immunity

These concepts were documented in ancient texts like Charaka Samhita, Susruta Samhita, Rasaratna Samucchaya, and Rasa Jala Nidhi

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8.1.1 IS AIR A MIXTURE?

Composition of Air

Air is a **uniform mixture** composed mainly of:

**Nitrogen (N₂)**: approximately 78%

**Oxygen (O₂)**: approximately 21%

**Argon (Ar)**: approximately 0.9%

**Carbon dioxide (CO₂)**: small amount (~0.04%)

**Water vapour (H₂O)**: variable amount

Function of Components

**Oxygen**: Required by most living beings for respiration; helps in combustion

**Nitrogen**: Does not take part in combustion; essential for plant growth

**Water vapour**: Present in air; condenses into liquid water when warm air touches cool surfaces

**Carbon dioxide**: Essential for photosynthesis in plants; exhaled by animals during respiration

Activity 8.1: Testing for Carbon Dioxide in Air

**Objective**: To confirm the presence of carbon dioxide in the air using lime water

**Materials Required:**

Glass tumbler

Water

Calcium oxide (quick lime)

Petri dish

Filter paper and funnel

**Procedure:**

1. Fill a glass tumbler half with water

2. Add small amount of calcium oxide slowly (use appropriate safety precautions)

3. Observe: Calcium oxide reacts vigorously with water, releasing heat

4. Chemical reaction: CaO + H₂O → Ca(OH)₂ + Heat

5. Stir continuously to make a solution

6. Filter the solution - the filtrate is **lime water** (calcium hydroxide solution)

7. The lime water is **colourless**

8. Leave this solution in a petri dish for a few hours, stirring at regular intervals

9. Observe what happens to the solution

**Observations:**

The lime water turns **milky white** when exposed to air

This demonstrates the presence of carbon dioxide in the air

**Chemical Equation:**

Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water

Ca(OH)₂ + CO₂ → CaCO₃↓ + H₂O

The white precipitate of calcium carbonate causes the milky appearance.

Activity 8.2: Observing Dust Particles in Air

**Objective**: To observe that dust particles are suspended in the air

**Materials Required:**

Black sheet of paper

Magnifying glass

Window or garden space

**Procedure:**

1. Take a black sheet of paper free from visible dust

2. Place it undisturbed near an open window or in the garden for a few hours

3. After a few hours, examine the surface of the paper

**Observations:**

Tiny particles are settled on the surface of the paper

When examined with a magnifying glass, individual dust particles are visible

Different types of particles may be observed (dust, soot, pollen, etc.)

**Conclusion:**

Dust particles are suspended in the air

They are not an integral part of air composition

They are considered **air pollutants**

The nature and number of dust particles vary from time to time and place to place

**Major Pollutants in Air:**

**Particulate matter**: Dust, soot, smoke particles

**Gaseous pollutants**:

Carbon monoxide (CO)

Ozone (O₃)

Nitrogen dioxide (NO₂)

Sulfur dioxide (SO₂)

**Air Quality Index (AQI)**: A tool used to describe and measure the quality of air in a given location.

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8.1.2 TYPES OF MIXTURES

Mixtures can be classified based on the **physical states** of their components. A comprehensive table shows different types:

Classification Table of Mixtures

| S.No. | Mixture Type | Examples | Nature |

|-------|-------------|----------|--------|

| 1 | Gas and Gas | Air, Nitrogen and oxygen mixture | Uniform |

| 2 | Gas and Liquid | Aerated water (soda water), Oxygen dissolved in water, Carbon dioxide in carbonated drinks | Uniform (usually) |

| 3 | Solid and Gas | Smoke (carbon particles in air), Dust in air | Non-uniform |

| 4 | Liquid and Liquid | Acetic acid in water (vinegar), Miscible liquids like alcohol and water | Uniform |

| 5 | Liquid and Liquid | Oil and water (immiscible) | Non-uniform |

| 6 | Solid and Liquid | Sand and water, Seawater (salt dissolved in water) | Non-uniform (sand+water), Uniform (salt+water) |

| 7 | Solid and Solid | Baking powder (baking soda + tartaric acid), Alloys (brass, bronze) | Non-uniform (usually) |

Purpose of Separating Mixtures

In everyday life: Separation is done to obtain the desired component

In science: The purpose is to **obtain pure substances** by separating the components of a mixture

Additional Information: Alloys

**Alloys** are uniform mixtures of two or more metals that have properties distinct from their constituent metals.

**Examples:**

**Stainless steel**: Contains iron, nickel, chromium, and small amount of carbon. Used for utensils, cutlery, and decorative items.

**Brass**: Mixture of copper and zinc. Used for decorative items, vessels, and musical instruments.

**Bronze**: Mixture of copper and tin. Used for sculptures and ceremonial items.

**Note**: Did you know that more than 45 different elements are used in manufacturing a mobile phone, including its screen, battery, and other components?

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8.2 WHAT ARE PURE SUBSTANCES?

Understanding "Pure" in Common Usage vs. Science

#### Common Usage of "Pure"

In everyday language, "pure" means **unadulterated products**—products without any added inferior substances.

**Adulteration** is an illegal process of adding substances that are:

Cheaper than the original

Of poor quality

Done to increase quantity or reduce manufacturing cost

Results in deterioration of product quality

Can be hazardous to health

**Examples**: Pure milk, pure ghee, pure spices (as claimed on packages)

#### Scientific Definition of Pure Substances

A **pure substance** is matter that has no other substance present in it. In science:

A pure substance **cannot be separated** into other kinds of matter by any **physical process**

It consists of **identical particles throughout**

It has fixed physical and chemical properties

**Key Point**: Even products marketed as "pure" (like commercial milk or fruit juice) may not be pure according to scientific definition, as they contain more than one substance.

Definition

A **pure substance** is a kind of matter with the same type of particles throughout, which cannot be separated into other kinds of matter by any physical method. This includes both **elements** and **compounds**.

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8.3 WHAT ARE THE TYPES OF PURE SUBSTANCES?

Pure substances are classified into two main categories: **Elements** and **Compounds**.

Understanding Water Through Experimentation

Recall that water exists in three states:

**Solid state**: Ice (when cooled to 0°C or 273K)

**Liquid state**: Water (at room temperature)

**Gaseous state**: Water vapour (when heated to 100°C or 373K)

During these **state changes** (melting, freezing, boiling, condensation), the particles of water remain the same—they only change their arrangement and speed of movement. Water is still water in all three states.

**But what happens when we pass electricity through water?** Let us find out using Activity 8.3.

Activity 8.3: Electrolysis of Water (Demonstration Activity)

**Objective**: To observe the decomposition of water into hydrogen and oxygen gases using electric current

**Materials Required:**

Two small test tubes

Beaker or glass tumbler

9 V battery (not lithium-ion)

Dilute sulfuric acid (a few drops)

Water

Test tube stand

Burning candle

**Safety Precautions:**

This activity must be performed under teacher supervision

Handle sulfuric acid with care

Do not use lithium-ion batteries

Maintain safe distance during gas testing

Perform in a well-ventilated area

**Procedure:**

**Step 1: Setup**

1. Collect two small test tubes, a beaker, and a 9V battery

2. Fill the beaker 2/3 with water

3. Add a few drops of dilute sulfuric acid to the water (this makes water a good conductor of electricity)

4. Fill both small test tubes completely with the prepared water

5. Place the 9V battery inside the beaker

**Step 2: Electrolysis**

1. Carefully place the water-filled test tubes over the positive and negative terminals of the battery (without spilling water)

2. Observe for a few minutes

**Observations During Activity:**

Gas bubbles are formed at both terminals inside the test tubes

Continue the activity for 10-15 minutes

The volume of gas collected in the two test tubes is **different**

One test tube collects **more gas** than the other

The test tubes should be removed carefully

**Step 3: Testing the Gases**

**Gas from first test tube** (where more gas is collected):

Remove the test tube carefully

Bring a burning candle close to the mouth of the test tube

**Observation**: The flame of the candle glows **brighter**

**This is oxygen gas** (O₂) - it supports combustion

More oxygen is produced because water has more oxygen atoms

**Gas from second test tube** (where less gas is collected):

Remove the test tube carefully

Bring a burning candle close to the mouth of the test tube

**Observation**: A **"pop" sound** is heard

**This is hydrogen gas** (H₂) - it is combustible and burns with a pop sound

**Key Observation:** These collected gases are **NOT water vapour** because water vapour would have condensed back into liquid water. These are entirely different substances.

Chemical Equation for Electrolysis of Water

**Water → Hydrogen + Oxygen**

2H₂O → 2H₂ + O₂ (with electric current)

Or simply: H₂O → H₂ + O₂ (simplified representation)

Conclusion from Activity 8.3

Water is composed of two different constituents: **hydrogen** and **oxygen**

These two substances are fundamentally different from water itself

By passing electricity (a chemical process), water can be broken down into simpler substances

This shows that water is **not an element** but a **compound**

Fascinating Observation

Oh! This is really fascinating:

**Hydrogen** is a fuel (burns readily)

**Oxygen** supports combustion (makes things burn)

**Water** extinguishes fire

This contrast shows how properties of compounds differ completely from their constituent elements!

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8.3.1 ELEMENTS

Definition of Elements

**Elements** are pure substances that **cannot be further broken down** into simpler substances by any chemical means. They are the **building blocks of all matter**.

From Activity 8.3, we identified that hydrogen and oxygen formed from water are elements.

**Other Examples of Elements:**

Gold (Au)

Silver (Ag)

Sulfur (S)

Carbon (C)

Iron (Fe)

Copper (Cu)

Oxygen (O₂)

Nitrogen (N₂)

Composition of Elements: Atoms and Molecules

#### Atoms

**Atoms** are the smallest particles of an element. Key points:

Each element is made up of identical particles called atoms

The atoms of one element are different from the atoms of any other element

Atoms are the fundamental units of elements

#### Molecules

**Molecules** are stable particles formed when two or more atoms of the same element combine together.

**Key Points:**

Atoms of most elements cannot exist independently in nature

Two or more atoms combine to form molecules

Molecules have fixed composition and properties

**Examples of Molecular Elements:**

1. **Hydrogen Gas (H₂)**

Two atoms of hydrogen combine to form one molecule of hydrogen

Depicted as: H—H

Exists as diatomic molecules at room temperature

2. **Oxygen Gas (O₂)**

Two atoms of oxygen combine to form one molecule of oxygen

Depicted as: O=O

Exists as diatomic molecules at room temperature

3. **Nitrogen Gas (N₂)**

Two atoms of nitrogen combine

Exists as diatomic molecules

Classification of Elements: Metals and Non-Metals

Elements are classified into broad categories:

#### Metals

Gold (Au)

Silver (Ag)

Magnesium (Mg)

Iron (Fe)

Aluminium (Al)

Copper (Cu)

#### Non-Metals

Carbon (C)

Sulfur (S)

Hydrogen (H)

Oxygen (O)

Nitrogen (N)

Helium (He)

#### Metalloids

Silicon (Si)

Boron (B)

Arsenic (As)

These have intermediate properties between metals and non-metals

You will study these in more detail in higher grades

Physical States of Elements at Room Temperature

**118 elements are known to exist:**

#### Solid Elements (majority)

Most of the 118 elements exist in solid state at room temperature

Examples: Gold, silver, iron, copper, carbon, sulfur, aluminum

#### Gaseous Elements (11 elements)

Eleven elements exist as gases at room temperature

All of these are non-metals

Examples:

Oxygen (O₂)

Nitrogen (N₂)

Helium (He)

Neon (Ne)

Argon (Ar)

Hydrogen (H₂)

Chlorine (Cl₂)

Fluorine (F₂)

#### Liquid Elements (2 elements)

Only two elements are liquid at room temperature:

1. **Mercury (Hg)**: A metal - liquid, silvery, toxic

2. **Bromine (Br)**: A non-metal - liquid, brown color, toxic vapors

#### Elements Liquid at Moderate Temperatures

**Gallium (Ga)**: Solid at room temperature (29°C or 302K) but becomes liquid at around 30°C (303K)

**Caesium (Cs)**: Solid at room temperature but becomes liquid at around 28°C (301K)

Important Note: Is Electrolysis a Chemical Change?

When electric current is passed through water, it breaks down into hydrogen and oxygen. This is a **chemical change** (not physical) because:

New substances are formed

The original substance (water) is destroyed

The process requires energy (electric current)

The change is irreversible by simple physical means

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8.3.2 COMPOUNDS

Definition of Compounds

**Compounds** are formed when **different elements combine in fixed ratios** through chemical bonds to form something entirely new.

**Key Characteristics:**

Made of two or more different elements

Elements combine in **fixed, definite proportions**

The properties of compounds are **completely different** from the properties of their constituent elements

Constituent elements **cannot be separated by physical methods** alone

Chemical methods are required to separate the elements

The particles of constituent elements are tightly bonded together

Why Water is a Compound

From Activity 8.3:

Water contains two elements: hydrogen and oxygen

In water, hydrogen and oxygen particles are **so tightly bonded** that they cannot be separated by physical means (like filtering, dissolving, or evaporation)

Chemical process (electrolysis) is required to separate them

Therefore, water is a **compound**

Composition of Water

**Molecular Formula**: H₂O

**Ratio of atoms**: The ratio of hydrogen atoms to oxygen atoms = **2:1**

This means:

Each water molecule contains 2 hydrogen atoms and 1 oxygen atom

This ratio is always fixed and constant

You cannot find water with a different ratio of these elements

**Diagram representation:**

Water molecule (H₂O):

Two hydrogen atoms bonded to one oxygen atom

Oxygen atom is in the center

Hydrogen atoms are on either side

Example: Sodium Chloride (Common Salt)

#### Elements and Properties

**Constituent Elements:**

1. **Sodium (Na)**: A soft, shiny metal

Highly reactive

Stored in mineral oil to prevent reaction with air and moisture

Can react explosively with water

2. **Chlorine (Cl₂)**: A hazardous, poisonous gas

Greenish-yellow color

Strong, pungent smell

Toxic to living organisms

Used as a disinfectant

#### Formation of Sodium Chloride

When sodium and chlorine combine, they form **sodium chloride (NaCl)**:

A **harmless, white crystalline solid**

Essential for human health

Enhances taste of food

Necessary for nerve and muscle function

Used for food preservation

#### Ratio in Sodium Chloride

**Ratio of sodium to chlorine = 1:1**

Each molecule has 1 sodium atom and 1 chlorine atom

#### Separation of Sodium Chloride

**From water (as a mixture):**

Sodium chloride dissolved in water forms a **mixture** (seawater)

The salt can be separated from water by **physical process of evaporation**

When water evaporates, salt crystals remain behind

**From its elements (as a compound):**

Sodium chloride **cannot be separated** into sodium and chlorine by any physical method

The chemical bonds between sodium and chlorine are very strong

Chemical processes (like electrolysis) would be required to separate them

Can We Separate Sodium Chloride into Its Elements?

**Answer: NOT by physical processes**

The ionic bond between sodium and chlorine is extremely strong. Only by passing electric current through molten (liquid) sodium chloride can we break this bond:

2NaCl → 2Na + Cl₂ (electrolysis)

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8.3.2 INVESTIGATING SUGAR: IS IT AN ELEMENT OR COMPOUND?

Activity 8.4: Heating Sugar to Observe Decomposition

**Objective**: To observe that sugar is a compound made of carbon, hydrogen, and oxygen

**Materials Required:**

Sugar (sucrose)

Boiling tube

Test tube holder

Spirit lamp

Watch glass

**Safety Precautions:**

This activity must be performed in the presence of a teacher

Handle hot glassware with appropriate tools

Ensure proper ventilation

Allow equipment to cool before handling

**Procedure:**

**Step 1: Preparation**

1. Put one teaspoon of sugar in a boiling tube

2. Heat the sugar gently using a spirit lamp

3. Support the boiling tube with a test tube holder

**Step 2: Observations During Heating**

**Initial observation (first few minutes):**

The sugar begins to turn **brown** in color

This is due to melting and caramelization

The sugar is still mostly sugar at this stage

**Continued heating:**

The brown color becomes darker

The sugar begins to **char** (turn blackish)

A black residue forms in the boiling tube

**Important Observation - Water Formation:**

Small droplets of **liquid water** are observed inside the boiling tube, especially near the open end

This water is **NOT from atmospheric moisture** because the tube is being heated (any atmospheric water would evaporate rather than condense)

This water **comes from the sugar itself** - it's produced by decomposition of sugar

**Final stage:**

A black solid **charcoal (carbon)** remains in the boiling tube

The black mass can be carefully scooped out using a watch glass

**Step 3: Testing the Residue**

The charcoal/carbon can be tested to see if it burns like coal

It will show properties of carbon (burns with a yellow flame, produces carbon dioxide)

Chemical Change During Heating of Sugar

When sugar is heated strongly, it undergoes **decomposition**:

**Sugar → Carbon + Water**

C₁₂H₂₂O₁₁ → 12C + 11H₂O

Breaking this down:

Sugar contains three elements: **Carbon (C), Hydrogen (H), and Oxygen (O)**

Upon heating, these elements separate

Water (containing hydrogen and oxygen) escapes as vapor and condenses as droplets

Carbon remains as black charcoal

Conclusion: Sugar is a Compound

**Sugar (Sucrose) is a COMPOUND because:**

It contains three different elements: Carbon, Hydrogen, and Oxygen

These elements are chemically bonded in fixed ratio: C₁₂H₂₂O₁₁

It cannot be separated into its elements by physical methods alone

It requires chemical process (heating/decomposition) to break it down

Its properties are completely different from carbon, hydrogen, and oxygen

**Molecular Formula of Sucrose**: C₁₂H₂₂O₁₁

Comparison: Three States of Water vs. Decomposition of Sugar

| Aspect | Water State Changes | Sugar Heating |

|--------|-------------------|--------------|

| Process | Melting, boiling, condensation | Heating to decompose |

| Nature of change | Physical change | Chemical change |

| Original substance | Water remains water | Sugar no longer exists |

| Particles | Same particles, different arrangement | New substances formed |

| Products | Ice, water, or water vapor (all water) | Carbon and water (completely different) |

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8.3.2 INVESTIGATING THE REACTION BETWEEN IRON AND SULFUR

Activity 8.5: Comparing a Mixture and a Compound (Demonstration Activity)

**Objective**: To understand the difference between a physical mixture and a chemical compound by observing the reaction between iron and sulfur

**Materials Required:**

Iron filings (5.6 grams)

Sulfur powder (3.2 grams)

Watch glasses

Mortar and pestle

China dish

Spirit lamp and test tube holder

Magnet

Burning candle

Filter paper

Dilute hydrochloric acid (HCl)

**Safety Precautions:**

This activity should be demonstrated under teacher supervision

Perform in a fume hood or well-ventilated area

Do not inhale the gases produced

Handle sulfur carefully as it can burn

Allow materials to cool before handling

Part 1: Creating the Mixture (Sample A)

**Procedure:**

1. Measure 5.6 grams of iron filings (reddish-brown powder)

2. Measure 3.2 grams of sulfur powder (yellow powder)

3. Observe each substance carefully:

**Iron filings**: Reddish-brown, magnetic, metallic

**Sulfur powder**: Yellow, non-magnetic, non-metallic

4. Mix them thoroughly in a watch glass

5. Label this as **Sample A**

**Observations of Sample A:**

The mixture appears **non-uniform**

Both **iron and sulfur are still visible** as separate substances

The yellow sulfur and reddish-brown iron particles are clearly distinguishable

You can still observe the original properties of both materials

**This is a PHYSICAL MIXTURE:**

Two substances mixed together but not chemically combined

Each substance keeps its own properties

They can be separated by physical means

Part 2: Creating the Compound (Sample B)

**Procedure:**

1. Take half of Sample A (the mixture of iron and sulfur)

2. Place it in a china dish

3. Heat it **gently** with **continuous stirring** using a spirit lamp

4. Continue heating until a **black mass** is formed

5. Allow the content to cool completely

6. Transfer the cooled black mass to a mortar

7. Grind it with a pestle until it becomes fine powder

8. Place the powder on a watch glass

9. Label it as **Sample B**

**Chemical Reaction During Heating:**

Iron + Sulfur → Iron Sulfide (compound)

Fe + S → FeS (with heat)

**Observations During Heating:**

The mixture starts to react

Colors change as reaction progresses

A black mass forms (iron sulfide - FeS)

Heat is released during the reaction (exothermic reaction)

A burnt smell may be noticed

**Observations of Sample B (After Cooling and Grinding):**

Appears as a **uniform black powder**

**No longer shows separate yellow and reddish-brown particles**

The entire sample looks the same throughout

Different in appearance from original Sample A

**This is a CHEMICAL COMPOUND (Iron Sulfide - FeS):**

Iron and sulfur have chemically combined

A new substance with new properties has formed

The original substances are no longer visible

Part 3: Comparative Analysis - Testing Sample A and Sample B

The following tests are performed to highlight differences between a mixture and a compound:

#### Test 1: Appearance and Physical Observation

**Sample A (Mixture):**

Non-uniform appearance

Color: Mixture of yellow and reddish-brown

Texture: Grainy, can see individual particles

Can distinguish iron filings and sulfur powder

**Sample B (Compound):**

Uniform appearance throughout

Color: Completely black

Texture: Fine, uniform powder

No visible separation of components

#### Test 2: Magnet Test (Magnetic Properties)

**Sample A (Mixture):**

**Strongly magnetic** - the magnet attracts the iron filings

Sulfur powder does not stick to the magnet

The yellow sulfur particles remain in the watch glass

Iron filings move toward the magnet and stick to it

**Conclusion for Sample A:**

Iron retains its magnetic property in the mixture

This shows that iron has NOT chemically combined

The mixture is just a physical combination

**Sample B (Compound):**

**Not magnetic** (or very weakly magnetic)

The magnet does not attract the black powder significantly

The iron sulfide does not respond to the magnet like pure iron does

Iron has lost its magnetic property

**Conclusion for Sample B:**

Iron in the compound (iron sulfide) is NO LONGER magnetic

This proves that iron has chemically combined with sulfur

A new substance with completely different properties has formed

#### Test 3: Chemical Reaction with Dilute Hydrochloric Acid

**Sample A (Mixture):**

When dilute HCl is added to Sample A:

Iron reacts: Fe + 2HCl → FeCl₂ + H₂↑

Sulfur does NOT react with dilute HCl

Only iron particles react and dissolve

Yellow sulfur remains undissolved

**Conclusion for Sample A:**

Components still retain their individual chemical properties

Each component reacts independently

This confirms it is a mixture

**Sample B (Compound):**

When dilute HCl is added to Sample B:

Iron sulfide reacts as a single substance: FeS + 2HCl → FeCl₂ + H₂S↑

The entire sample reacts together

A rotten egg smell (characteristic of H₂S gas) is produced

No part of the sample remains unreacted

**Conclusion for Sample B:**

The compound reacts as ONE substance

The reaction produces new products not seen in individual elements

This confirms it is a compound, not a mixture

Summary Table: Comparing Sample A and Sample B

| Property | Sample A (Mixture) | Sample B (Compound) |

|----------|-------------------|-------------------|

| **Appearance** | Non-uniform, yellow and reddish-brown visible | Uniform black powder |

| **Distinguishability** | Can see iron and sulfur separately | Cannot see separate substances |

| **Magnetic Property** | Iron filings attract to magnet | Not attracted to magnet (or very weakly) |

| **Reaction with dilute HCl** | Only iron reacts, sulfur remains | Entire sample reacts as one substance |

| **Nature** | Physical mixture - particles not chemically bonded | Chemical compound - chemically bonded |

| **Separation** | Can be separated by magnet (physical method) | Cannot be separated by physical methods |

| **Properties** | Components retain original properties | New properties different from components |

Key Learning Points from Activity 8.5

1. **Physical Mixture**: Iron and sulfur mixed but not chemically combined

Components retain individual properties

Can be separated by physical means (magnet)

Non-uniform in nature

Components visible

2. **Chemical Compound**: Iron sulfide (FeS) formed by chemical reaction

New substance with new properties

Cannot be separated by physical means

Uniform throughout

Components not distinguishable

Requires chemical process to break down

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KEY DIFFERENCES: MIXTURES vs. PURE SUBSTANCES

Comparison Table

| Feature | Mixture | Pure Substance |

|---------|---------|-----------------|

| **Definition** | Two or more substances combined without chemical reaction | Single type of substance with fixed composition |

| **Composition** | Variable ratio of components | Fixed ratio of elements |

| **Properties** | Components retain original properties | Has fixed, definite properties |

| **Appearance** | Can be uniform or non-uniform | Always has consistent appearance |

| **Separation** | Can be separated by physical methods | Elements cannot be separated by physical methods |

| **Examples** | Seawater, air, mixture of salt and sugar | Water, salt, sugar, oxygen, nitrogen |

Comparison Table: Elements vs. Compounds

| Feature | Element | Compound |

|---------|---------|----------|

| **Definition** | Cannot be broken down into simpler substances | Made of two or more elements chemically combined |

| **Composition** | Made of only one type of particle | Made of two or more types of atoms |

| **Number known** | 118 elements | Millions of compounds |

| **Properties** | Characteristic properties of that element | Properties different from constituent elements |

| **Separation** | Cannot be separated | Can be decomposed by chemical methods |

| **Examples** | Oxygen, hydrogen, gold, carbon | Water, sugar, salt, iron sulfide |

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SUMMARY OF CHAPTER CONCEPTS

The Hierarchy of Matter

**MATTER (Everything around us)**

↓

Can be divided into:

↓

├─ **MIXTURES** (Non-uniform and Uniform)

│ ├─ Non-uniform: Components visible (sprout salad, sand and water)

│ └─ Uniform: Components not visible (sugar in water, air, seawater)

│

└─ **PURE SUBSTANCES**

├─ **ELEMENTS** (Cannot be broken down)

│ ├─ Metals (Gold, silver, iron, copper)

│ ├─ Non-metals (Oxygen, carbon, sulfur, hydrogen)

│ └─ Metalloids (Silicon, boron)

│

└─ **COMPOUNDS** (Can be broken down into elements by chemical means)