Changes are happening around us constantly. We can observe these changes using our **senses** - sight, smell, touch, hearing, and taste.
**Real-life examples:**
These changes can be classified into **two main types:**
1. Physical changes
2. Chemical changes
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**Definition:** A **physical change** is a change in which only the **physical properties** (like shape, size, or state) of a substance change, but **no new substance is formed**. The original material remains the same.
**Key characteristics:**
**Examples of physical changes:**
1. **Folding paper** → Creates new shapes but paper remains paper. When unfolded, original paper comes back.
2. **Inflating and deflating a balloon** → The rubber balloon changes shape but remains rubber. Once deflated, it returns to original form.
3. **Crushing chalk into powder** → Chalk powder can be compressed back (though difficult). Still remains calcium compound.
4. **Melting ice** → Ice (solid water) becomes liquid water when heated, but it's still water. Can be refrozen.
5. **Boiling water** → Water becomes steam (water vapor) but is still water. Can condense back to liquid.
6. **Cutting vegetables** → Changes size and shape but remains the same vegetable.
7. **Tearing paper** → Changes shape but remains paper.
8. **Rolling dough into chapati** → Changes shape and size but dough remains dough.
9. **Drying wet clothes** → Water evaporates, clothes dry but remain same fabric.
**Indian example:** When you heat milk in a container, it becomes hot (temperature change - physical change). The milk is still milk, just at a different temperature.
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**Definition:** A **chemical change** is a change in which **one or more new substances are formed**. The original substance transforms into a completely different substance through a **chemical reaction**.
**Key characteristics:**
**Experiment setup:**
**What happened?**
Carbon dioxide from your breath reacted with calcium hydroxide (lime water) to form a new white substance:
**Chemical equation:**
**Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water**
(Lime water) (from breath) (white sediment) (new substance)
**Note:** Turning lime water milky is used as a **test to detect carbon dioxide gas**.
**Experiment:**
**Observations:**
**Chemical equation:**
**Vinegar + Baking soda → Carbon dioxide + Other substances**
**Important note:** When baking soda is mixed with water only (no vinegar), there is **no bubbling** - this shows it's a **physical change** (just dissolving), not a chemical change.
**Indian example:** When you add soda (baking soda) to buttermilk or yogurt while cooking, it fizzes and bubbles. This is the same chemical reaction.
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**Definition:** **Rusting** is the formation of a **reddish-brown coating** on the surface of iron or iron objects when exposed to air (oxygen) and moisture.
**What forms:** A new brown substance called **rust** (iron oxide - Fe₂O₃) is formed.
**Why it's a chemical change:**
**Equation:**
**Iron + Oxygen (in air) + Moisture → Iron oxide (Rust)**
**Indian examples:**
**How to prevent rusting:**
**Definition:** **Combustion** is a chemical reaction in which a **substance reacts with oxygen** and produces **heat and/or light**.
**Combustible substances:** Materials that can burn are called **combustible substances**. Examples:
**Example: Burning of magnesium ribbon**
**Observation:**
**Chemical equation:**
**Magnesium + Oxygen → Magnesium oxide + Heat + Light**
(Ribbon) (from air) (white powder)
**Why this is chemical change:**
**Activity 5.5: Candle burning experiment**
**Setup:**
**Observations:**
**Why?**
**Test for carbon dioxide formed:**
Add lime water to the petri dish under glass tumbler. It turns **milky**, confirming carbon dioxide was produced by combustion.
**Important:** Carbon dioxide was formed from:
**For combustion to occur, THREE things are necessary:**
1. **Combustible substance (Fuel)** - Material that can burn (wood, paper, kerosene, etc.)
2. **Oxygen** - Essential component of air that supports burning. Without oxygen, combustion cannot occur.
3. **Heat (Ignition temperature)** - Minimum temperature at which a substance catches fire
**Fire Triangle diagram (to draw):**
Draw triangle with three sides labeled:
Center of triangle: FIRE/COMBUSTION
**Definition:** **Ignition temperature** is the **minimum temperature** at which a substance **catches fire** in the presence of oxygen and a source of heat.
**Activity 5.6: Finding ignition temperature**
**Experiment 1: Using matchstick**
**Experiment 2: Using sunrays**
**Observations:**
**Indian example:** When you use a matchstick, it provides enough heat to reach the paper's ignition temperature. But when you keep paper in sunlight, it doesn't burn because sunlight temperature is below paper's ignition temperature.
**How to extinguish fire:**
Remove any one element from fire triangle:
**Science & Society - Fire Safety:**
If someone's clothes catch fire:
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**Question:** What changes occur when a candle burns?
**Analysis of candle burning:**
When a candle burns, **BOTH physical and chemical changes occur simultaneously**:
**Physical changes:**
1. **Melting** → Solid wax melts into liquid wax (change of state: solid → liquid)
2. **Flow** → Liquid wax flows up the wick (change in position)
3. **Evaporation** → Liquid wax evaporates into wax vapor due to heat (change of state: liquid → gas)
4. **Solidification** → Melted wax at the base solidifies again (change of state: liquid → solid)
**Chemical change:**
1. **Combustion** → Wax vapor burns in oxygen to produce flame
2. New substances formed: Carbon dioxide (COâ‚‚), water (Hâ‚‚O), and heat/light
3. Chemical equation: **Wax vapor + Oxygen → Carbon dioxide + Water + Heat + Light**
**Complete process (in order):**
1. Wick draws up liquid wax
2. Heat of flame evaporates wax into vapor
3. Wax vapor burns (combustion) - CHEMICAL CHANGE
4. Heat from combustion melts more wax - PHYSICAL CHANGE
5. Wax vapor escapes and burns in air
6. At the base, wax solidifies again - PHYSICAL CHANGE
**Conclusion:** A single process (burning candle) can involve both types of changes.
**Know a Scientist - Michael Faraday:**
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**Definition:** **Reversible changes** are changes in which the **original substance or object can be brought back** to its initial form.
**Characteristics:**
**Examples of reversible changes:**
1. **Melting ice:** Ice melts into water → Water can be refrozen into ice (repeated many times)
2. **Boiling water:** Liquid water becomes steam → Steam condenses back into liquid water
3. **Inflating/deflating balloon:** Balloon becomes inflated → Can be deflated to original shape
4. **Folding paper:** Paper folds into shape → Paper can be unfolded to original form
5. **Twisting string:** Straight string gets twisted → Can be untwisted to straight form
6. **Rolling mat:** Rolled up mat → Can be unrolled to flat form
7. **Dissolving sugar in water:** Sugar dissolves → Water can be evaporated to get sugar crystals back
**Indian example:** When you heat water to make tea, it becomes hot. When you let it cool, it returns to normal temperature. This process is reversible and can be done repeatedly.
**Definition:** **Irreversible changes** are changes in which the **original substance cannot be brought back** to its initial form.
**Characteristics:**
**Examples of irreversible changes:**
1. **Making popcorn:** Corn kernels heated → Popcorn cannot return to corn kernels
2. **Chopping vegetables:** Vegetables cut into pieces → Cannot return to original size/shape
3. **Making idlis:** Batter cooked to make idlis → Cannot be converted back to batter
4. **Ripening fruits:** Unripe fruit becomes ripe → Cannot return to unripe state
5. **Cooking food:** Raw food cooked → Cannot become raw again
6. **Burning wood:** Wood burns to ash → Cannot get original wood back
7. **Rusting iron:** Iron forms rust → Cannot easily reverse to pure iron
8. **Making curd from milk:** Milk becomes curd → Cannot become milk again
9. **Grinding wheat to flour:** Wheat grains ground to flour → Cannot get grains back
**Important:** Most chemical changes are irreversible because new substances are formed.
**Indian example:** When milk is boiled to make milk-based sweets like kheer, the milk changes permanently. You cannot get fresh milk back from kheer.
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**Definition:** **Desirable changes** are those that are **beneficial and useful** to us.
**Examples:**
1. **Cooking of food** → Makes food digestible, tastes better, kills bacteria
2. **Ripening of fruits** → Fruits become sweet and ready to eat
3. **Making curd from milk** → Provides nutritious food, aids digestion
4. **Cutting of fruits/vegetables** → Makes them ready for cooking or eating
5. **Making compost from waste** → Converts waste into useful fertilizer
6. **Baking of bread** → Raw dough becomes edible bread
7. **Growing of plants** → Provides food and oxygen
**Indian examples:**
**Definition:** **Undesirable changes** are those that are **harmful or unwanted**.
**Examples:**
1. **Rusting of iron** → Destroys iron tools and structures, weakens them
2. **Decay/rotting of food** → Food becomes inedible and unhygienic
3. **Burning of forests** → Destroys habitat and increases pollution
4. **Melting of glaciers** → Causes environmental change and flooding
5. **Spoilage of milk** → Milk becomes unfit to drink
6. **Fading of clothes** → Color loss, reduces clothing quality
**Important concept:** A change can be desirable in one situation but undesirable in another.
**Example: Decomposition of food**
**Negative impacts caused by human activities:**
1. **Increased fuel consumption**
2. **Drying of paint**
3. **Industrial emissions**
**Indian example:** Heavy traffic in cities causes emission of carbon dioxide and other gases, which contributes to air pollution, especially during winters when smog increases.
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**Definition:** **Weathering** is the **slow breakdown of rocks** into smaller pieces through **physical and chemical processes** over long periods.
**What are sediments?**
**Sediments** are small pieces of rocks, soil, and sand that form as a result of weathering and erosion. They are found:
**Types of weathering:**
#### **1. Physical Weathering (Breaking process)**
Rocks break into smaller pieces without changing their chemical composition:
**Causes:**
**Result:** Large rocks break into smaller pieces (sediments), but remain the same material
#### **2. Chemical Weathering (Changing process)**
Rocks chemically change when exposed to water, air, or acids in rain:
**Process:**
**Example: Basalt rock turning red**
**Indian example:** Red soil in southern India is formed due to weathering of basaltic rocks. The red color comes from iron oxide created by chemical weathering.
**Result of weathering:**
Weathering gradually breaks down rocks and helps form **soil**, which is essential for plant growth.
**Process timeline:** Takes thousands to millions of years to form significant soil from rock weathering.
**Definition:** **Erosion** is the **movement and transportation of weathered rock pieces** (sediments) from one location to another by natural forces like water, wind, and gravity.
**Key point:** Erosion doesn't create new substances - it's a **physical change**. It moves materials that were created by weathering.
**Agents of erosion:**
1. **Flowing water** → Rivers, streams wear away banks and transport sediments
2. **Wind** → Desert winds transport sand and small particles
3. **Gravity** → Landslides move rocks downslope
4. **Waves** → Ocean waves erode coastal rocks and beaches
**Effects of erosion:**
1. **Smoothing of rocks**
2. **Formation of new rocks (Sedimentation)**
3. **Valley formation**
4. **Delta formation**
**Indian examples:**
**Rock cycle (brief):**
Rocks → Weathering → Sediments → Erosion → Deposition → Compaction → New rocks
**Important notes:**
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**Property** | **Physical Change** | **Chemical Change**
---|---|---
**New substance** | No new substance formed | New substance(s) formed
**Properties** | Physical properties change (shape, size, state, color) | Chemical properties change, new substance has different properties
**Reversible** | Usually reversible | Usually irreversible
**Examples** | Melting, boiling, folding, crushing | Burning, rusting, cooking, decomposition
**Chemical bonds** | Not broken/formed | Chemical bonds break and form
**Equation** | No chemical equation | Chemical equation can be written
**Particle level** | Atoms rearrange, no change in type | Atoms combine differently, forming new compounds
**Indian example** | Melting ice for cooling drinks | Cooking rice to make it edible
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**Physical change:** Change affecting physical properties only; no new substance formed; usually reversible.
**Chemical change:** Change producing one or more new substances; involves chemical reactions; usually irreversible.
**Combustion:** Chemical reaction of substance with oxygen producing heat and/or light.
**Combustible substance:** Material that can burn (fuel).
**Ignition temperature:** Minimum temperature at which substance catches fire.
**Weathering:** Breakdown of rocks into sediments through physical and chemical processes.
**Erosion:** Movement and transportation of weathered rocks by natural forces.
**Sediments:** Small pieces of rocks, soil, and sand formed by weathering.
**Reversible change:** Change in which original substance can be recovered.
**Irreversible change:** Change in which original substance cannot be recovered.
**Desirable change:** Beneficial, useful change.
**Undesirable change:** Harmful, unwanted change.
**Rusting:** Formation of rust (iron oxide) on iron surface.
**Rust:** Reddish-brown compound formed when iron reacts with oxygen and moisture.
**Bioluminescence:** Light production in living organisms through chemical changes (without producing heat).
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**Activity 5.2:** Folding paper, inflating balloon, crushing chalk → Physical changes (reversible)
**Activity 5.3:** Blowing air into lime water → Carbon dioxide test (lime water turns milky)
**Activity 5.4:** Vinegar + baking soda → Produces carbon dioxide gas (fizzing, bubbles)
**Activity 5.5:** Candle covered with glass → Shows oxygen needed for combustion
**Activity 5.6:** Magnifying glass focusing sunrays on paper → Shows ignition temperature concept
**Activity 5.7:** Burning candle → Shows physical and chemical changes occur together
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✓ **Physical change** = Property change, no new substance, usually reversible (melting, folding, boiling)
✓ **Chemical change** = New substance formed, cannot easily reverse (burning, rusting, cooking)
✓ **Combustion** requires three things: Fuel, Oxygen, Heat (Fire triangle)
✓ **Ignition temperature** = Minimum heat for substance to catch fire
✓ **Weathering** = Slow breakdown of rocks (thousands of years)
✓ **Erosion** = Transport of weathered materials by water, wind, gravity
✓ **Reversible changes** = Can be undone (melting, freezing, dissolving)
✓ **Irreversible changes** = Cannot be undone (cooking, burning, ripening)
✓ **Desirable changes** = Beneficial (cooking, ripening, composting)
✓ **Undesirable changes** = Harmful (rusting, decaying, pollution)
✓ **One process can have both physical and chemical changes** (burning candle)
✓ **Carbon dioxide test** = Lime water turns milky when CO₂ passes through
---
1. **Distinguish between physical and chemical changes** → Look for formation of new substance (chemical) vs. property change only (physical)
2. **Reversibility is key** → Physical changes are usually reversible; chemical are not
3. **Fire triangle** → Remember all three elements needed for combustion
4. **Lime water test** → Remember this specific test for detecting CO₂
5. **Weathering vs. Erosion** → Weathering breaks rocks; erosion moves them
6. **Real-life examples** → Use Indian examples (rice cooking, milk curdling, rusted gates) in answers
7. **Environmental impact** → Know examples of how human activities cause undesirable changes
8. **Process understanding** → Be able to explain step-by-step what happens in candle burning, weathering, erosion
9. **Chemical equations** → Know major reactions: combustion, rusting, lime water + CO₂
10. **Time scale** → Remember weathering and erosion take very long time (thousands of years)
Q1. Which of the following is a physical change?
Answer: A — Melting ice is a physical change because ice remains water (same substance) just in a different state; no new substance forms.
Q2. What colour does lime water turn when carbon dioxide is passed through it?
Answer: C — Carbon dioxide reacts with lime water to form calcium carbonate, which is insoluble and makes the liquid appear milky.
Q3. Which of the following is a combustible substance?
Answer: C — Wood catches fire and burns in the presence of oxygen, making it a combustible substance; iron, water, and sand do not burn easily.
Q4. What happens when you fold a piece of paper and then unfold it?
Answer: C — Folding paper is a physical change where only the shape changes; unfolding returns it to the original form with no new substance created.
Q5. In your kitchen, you mix vinegar and baking soda. Bubbles form and the mixture fizzes. What gas is being produced?
Answer: B — Vinegar and baking soda react chemically to produce carbon dioxide gas, which can be confirmed by the lime water test turning milky.
Q6. Why does a candle stop burning when covered with a glass tumbler?
Answer: B — Combustion requires oxygen; covering the candle removes the continuous supply of air, causing the flame to extinguish.
Q7. A banana you bought yesterday has developed more brown spots today and smells stronger. Is this a physical or chemical change?
Answer: B — Banana ripening involves chemical changes where new flavour compounds and pigments form; it is not just a size or appearance change.
Q8. When magnesium ribbon burns in air, which of the following is true?
Answer: B — Burning magnesium is a chemical change where magnesium reacts with oxygen to form a new substance, magnesium oxide, with heat and light.
Q9. For a fire to continue burning, which component of air is essential?
Answer: C — Oxygen is the component of air that supports combustion; without it, fire cannot continue and will get extinguished.
Q10. A piece of chalk is crushed into powder. Can you recover the original chalk piece from this powder?
Answer: D — Crushing chalk is a physical change; the chalk remains chalk whether it is one piece or powder, so it cannot be recovered as a piece but the substance is unchanged.
What is a physical change?
A change in which only the appearance (shape, size, or state) changes but the substance remains the same and no new substance forms.
What is a chemical change?
A change in which one or more new substances are formed through a chemical reaction, and the original substance no longer exists in the same form.
Name the three things needed for combustion.
Combustible substance (fuel), oxygen, and heat (at or above the ignition temperature).
What is ignition temperature?
The minimum temperature at which a combustible substance catches fire and starts burning.
What is the test for carbon dioxide gas?
Pass the gas through freshly prepared lime water; if it turns milky or cloudy, carbon dioxide is present.
Give one example of a physical change.
Melting of ice, boiling of water, folding paper, or crushing chalk into powder.
Give one example of a chemical change.
Burning of wood, rusting of iron nails, ripening of banana, or burning of magnesium ribbon.
What happens when a burning candle is covered with a glass tumbler?
The flame stops burning after some time because the supply of oxygen (air) is cut off.
What new substance forms when magnesium ribbon burns?
Magnesium oxide, which is a white powder.
What is combustion?
A chemical reaction in which a substance reacts with oxygen and produces heat and/or light.
Define physical change with one example. [1 mark]
Physical change = only appearance changes, no new substance. Example: melting ice, folding paper, or crushing chalk.
When vinegar and baking soda are mixed, a gas is produced. How would you test this gas to confirm it is carbon dioxide? [2 marks]
Pass the gas through lime water. If lime water turns milky or cloudy, carbon dioxide is present. This is the lime water test.
Explain with an example why oxygen is necessary for combustion. What happens when a burning candle is covered with a glass tumbler? [3 marks]
Activity 5.5: candle without cover burns, with cover stops burning. Reason: oxygen is cut off. Example: fire extinguishers work by removing oxygen supply.
What is ignition temperature? List the three requirements for combustion to occur and draw a labelled diagram of the fire triangle showing all three components. [5 marks]
Ignition temperature = minimum temperature to start burning. Three requirements: combustible substance (fuel), oxygen, heat. Fire triangle diagram must show three sides labelled: Fuel, Oxygen, Heat.
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