Chapter 5: Changes Around Us: Physical and Chemical

Step 1 – Recall the defining features of a physical change
In a physical change:

• The identity and chemical composition of the substance remain the same; hence no new substance is produced.
• Only physical properties such as shape, size, colour or state (solid ↔ liquid ↔ gas) may change. These changes are usually reversible by simple physical means.

Step 2 – Examine each given statement

Step 3 – Pick the option that lists only the true statements
True statements: (i) and (iii)
Option (c) corresponds to these two statements.

Therefore, the correct choice is option (c).

Step 1 ‒ Recall the two kinds of changes
• Physical change: only the physical state, shape or size changes; no new substance is formed, and the change is usually reversible.
• Chemical change: a new substance with a new chemical composition appears; the change is generally irreversible by simple physical means.

Step 2 ‒ Change of water to ice
When liquid water is cooled to 0 °C, it freezes:

Step 3 ‒ Change of water to steam
On heating to 100 °C, water boils to give steam (water vapour):

Step 4 ‒ Conclusion
Both freezing and boiling of water involve only a change of physical state (liquid ↔ solid or liquid ↔ gas). Since the composition $$\mathrm{H_2O}$$ is unchanged and the changes are reversible, both are physical changes.

Step 1  – Recall the two kinds of changes.

• A physical change affects only the physical state, size or shape of a substance. The substance itself (its chemical composition) remains the same and the change is usually reversible.
• A chemical change produces one or more new substances with different properties and composition. Such a change is generally irreversible.

Step 2  – What happens when milk curdles?

1. Milk contains the sugar lactose and the proteins casein and whey proteins.
2. When a little curd (or a few drops of lemon) is mixed with warm milk, helpful bacteria enter the milk. These bacteria convert lactose into lactic acid.
3. The produced lactic acid lowers the pH of milk; the casein molecules gather together and form a semi-solid mass that we call curd / yoghurt.

Thus new substances – mainly lactic acid and coagulated casein proteins – are formed. This alters:

• Composition – milk (sweet) → curd (sour because of lactic acid).
• Taste and smell – both become tangy.
• Texture – liquid milk becomes semi-solid curd.

Step 3  – Test for reversibility.

Once milk has set into curd it cannot be converted back to fresh milk by any simple physical means such as heating, cooling or filtering. This irreversibility is a typical sign of a chemical change.

Conclusion.

Since curdling of milk produces new substances with new properties, involves a permanent change in composition and is not easily reversible, it is classified as a chemical change.

Step 1 — Recall the two kinds of changes

• Physical change: The substance keeps the same chemical composition; only its shape, size, or state changes. No new substance is produced.
• Chemical change: The original substance is converted into one or more new substances having different chemical compositions.

Step 2 — Describe what happens to rocks

When rocks are exposed to natural factors such as wind, flowing water (rain or rivers), temperature differences, and the actions of plant roots, they gradually break down into smaller and smaller pieces. This slow breaking process is called weathering.

Step 3 — Compare with the definitions

• The pieces obtained after weathering (gravel, sand, silt, clay) have the same chemical composition as the original rock. Only the size and shape of the particles change.
• No new chemical substance (like a different mineral or compound) is formed in this mechanical breakdown.

Step 4 — Classify the change

Because the composition remains unchanged and only the physical form alters, the formation of soil from rocks under the action of wind and rain is a physical change.

Step 5 — Conclude

Thus, natural weathering of rocks into soil is physical, not chemical, since no new substance is produced; only the size and texture of the rock fragments vary.

Suitable Title: Prithvi’s Eco-friendly Kitchen Garden

To label every activity we recall the basic ideas of the chapter.

• Physical change: no new substance is formed; only form, size, state or appearance alters; usually reversible.
• Chemical change: one or more new substances having different properties are produced; usually irreversible.

1. Chopping vegetables, peeling potatoes, cutting fruits
   Only shape/size changes, identity remains the same → physical change.
2. Collecting the seeds, fruit and vegetable peels in a clay pot
   Simply putting the same material in another container → physical change.
3. Scraps decomposing to compost by bacteria and fungi
   Organic matter is converted into new substances such as humus, $$\mathrm{CO_2}$$ and water → chemical change.
4. Seeds germinating and plants flowering
   Irreversible biochemical reactions create new tissues; the seed is no longer the same substance → chemical change.

Therefore the correct ticks are:

Step 1 ︱ Recall the two ideas

• Physical change – no new substance is formed; the change can usually be reversed by simple physical means, mass of the substance remains the same.
• Chemical change – one or more new substances are formed with different properties; the change is generally irreversible by simple physical methods.

Sometimes both kinds of changes take place together. One part of the event is only a change of state (physical) while another part involves a chemical reaction.

Step 2 ︱ Test every change given

Step 3 ︱ Fill the three areas

Hence the classification is complete.

Key fact recalled
Lime-water is a dilute solution of calcium hydroxide, $$\mathrm{Ca(OH)_2}$$. Whenever carbon dioxide gas is bubbled through it, the following reaction occurs:

$$\mathrm{Ca(OH)_2 + CO_2 \;\longrightarrow\; CaCO_3 \downarrow + H_2O}$$

The insoluble calcium carbonate, $$\mathrm{CaCO_3}$$, makes the clear lime-water turn milky. Therefore lime water will turn milky only if the experiment produces the gas $$\mathrm{CO_2}$$.

Step-by-step check of every set-up

Conclusion
Lime water turns milky only in experiments (a) and (d) because these are the ones that release carbon dioxide gas. The gas reacts with lime water forming insoluble calcium carbonate, which gives the milky appearance.

Step 1 — Understanding what happens when the ink dries
Lemon juice is mostly water with some acids (citric acid) and other organic substances dissolved in it. When you write and then leave the sheet in air, the water in the juice evaporates:

$$\text{Lemon-juice solution}\;\xrightarrow[\text{air}]{\text{evaporation}}\;\text{dry lemon solids on paper}$$

Only a physical change (loss of water) has occurred. If you again moisten the same spot with a few drops of water, the dry solids dissolve back and the written area looks wet. Thus the drying of the juice can be reversed; it is a physical change.

Step 2 — What happens when the paper is warmed
On heating, the acids and sugars left in the lemon juice react with oxygen from the air and also begin to char (partial burning of the paper surface):

$$\text{Organic matter}\;\xrightarrow[\text{heat}]{\text{oxidation}}\;\text{brown/black compounds}$$

New chemical substances are produced, giving the letters a dark-brown colour. Because entirely new substances form, this stage is a chemical change.

Step 3 — Can we reverse the second change?
A chemical change is generally permanent. After charring, no simple cooling or addition of water will convert the brown substances back into the original colourless acids and sugars of lemon juice. Hence the dark-brown letters cannot be changed back to the invisible form.

Conclusion

• The drying of lemon juice before heating is a physical change and can be reversed by re-adding water.
• The browning of the letters on heating is a chemical change and cannot be reversed.

Understanding the problem first

• Rocks on a hill–side are constantly broken down by two kinds of changes you have learnt in this chapter:
  1. Physical change — Weathering:
  • Heating by Sun and cooling at night,
  • Water entering the cracks and freezing,
  • Roots of plants exerting a push.
• Chemical change — Chemical weathering:
• Rainwater (which is weak $$\mathrm{H_2CO_3}$$) reacts with minerals,
• Oxygen in moist air rusts iron-rich rocks.
Both these changes make the rocks loose and, during heavy rain or earthquake, large masses of this loosened material suddenly slide downhill. This is called a landslide.

Steps that can be taken to reduce landslides and rock erosion

1. Afforestation and protection of existing vegetation
   Roots form a natural network that ties soil and small rock pieces, so planting trees and grasses on slopes gives mechanical support and also reduces direct impact of rain drops.
2. Terracing and contour bunding
   Cutting long slopes into a series of almost horizontal steps (terraces) decreases the effective slope angle $$\theta$$ (smaller $$\theta$$ → smaller downward component of weight $$W \sin\theta$$), thus reducing the tendency of the mass to slide.
3. Proper drainage systems
   Laying perforated pipes, side drains and catch-water drains quickly removes rain water before it can soak the ground. Less water means less additional mass and smaller pore-water pressure, both of which otherwise act to push the material outward.
4. Retaining structures
   • Stone masonry or RCC retaining walls,
   • Gabion walls (wire-mesh boxes filled with stones),
   • Soil-nailed walls or rock-bolts that literally ‘pin’ loose rocks to the stable bed-rock.
   These act against the downslope sliding force.
5. Rock fall nets and shot-creting (sprayed concrete)
   Nets catch individual falling blocks; a thin concrete skin binds together the outer few centimetres of a weathered rock face and stops small cracks from widening further (a purely physical change).
6. Controlled construction and mining
   
   • No cutting at the foot of an already steep slope,
   • No dumping of excavated soil on the crest,
   • Regulating blasting vibration and quarry depth.
   Such regulations keep the natural slope in its angle of repose.
7. Providing vegetation cover quickly after a fire or deforestation event
   Fast-growing grasses or hydro-seeding prevent direct exposure of bare soil to wind and rain.
8. Channel diversion and check dams
   Small concrete/stone barriers slow down flowing water, force it to deposit its sediment load and therefore lessen the erosive power downstream.
9. Monitoring and early-warning systems
   Instruments such as tilt-meters, rain gauges and ground-movement sensors send alerts when preset thresholds are crossed, giving enough time to evacuate people.
10. Community awareness
    Signs of an imminent slide—cracks on walls, tilted trees, jamming of doors—must be reported early so that point-6 regulations can be enforced.

Summary

• Landslides happen because of a combination of physical and chemical changes that weaken rocks.
• Practical preventive steps centre around (a) binding the soil and rocks (vegetation, bolts, concrete), (b) lowering the slope angle and removing water (terracing and drainage), and (c) controlling man-made disturbances and setting up early-warning arrangements.

Step 1 – Look carefully at routine kitchen work
Typical activities you can spot are

• Ice is taken out of the freezer and kept on the table → it melts to liquid water.
• Water is heated for tea → it starts boiling and gives off steam.
• Steam from the kettle strikes a cold plate held above it → tiny drops of liquid water collect (condensation).
• Butter / ghee is warmed slightly → it changes from solid to liquid.
• Salt or sugar is stirred into a glass of water → it dissolves and the solid seems to disappear.

Step 2 – Test whether each change can be reversed easily

Step 3 – Classify the reversible changes
In every example above, the substance itself remains $$\mathrm{H_2O}$$ or butter or salt; no new material having a different composition is produced. Therefore each of these is a physical change.

Step 4 – Write the final observation
The kitchen changes that can be reversed are melting / freezing of water or butter, boiling / condensing of water, and dissolving / crystallising of salt or sugar. All of them are reversible physical changes, not chemical changes.

Material used : A small bottle, fresh baker’s yeast, about 2 teaspoons of sugar, water, a balloon and another small bottle containing freshly prepared lime-water (clear solution of $$\mathrm{Ca(OH)_2}$$).

Step 1 : Preparing the sugar solution (set-up A)
Two teaspoons of sugar are stirred in a little water taken in the first bottle till the crystals disappear.

• Dissolving sugar does not create any new substance; it is only a change of physical state (solid $$\rightarrow$$ aqueous).

Step 2 : Adding yeast and fitting the balloon
A spoonful of fresh yeast is dropped into the sugary liquid and the mouth of the bottle is closed tightly with a limp balloon.

• At the start the balloon hangs flat because there is almost no gas above the liquid.

Observation after about one hour

• The balloon is found swollen or partially inflated.
• Bubbling is seen in the liquid and it smells slightly alcoholic (like fermented batter).

Explanation
Yeast contains the enzyme complex zymase. In the absence of air it brings about fermentation of the dissolved sugar (glucose/sucrose):

\[ \mathrm{C_6H_{12}O_6\,(aq) \xrightarrow[\;\text{zymase}\;]{\text{yeast}} 2\,C_2H_5OH\,(aq)+2\,CO_2\,(g)} \]

• The $$\mathrm{CO_2}$$ produced collects above the liquid and stretches the balloon. Inflating the balloon is only a physical effect; the real change is chemical (fermentation).

Step 3 : Passing the gas into lime-water (set-up B)
The balloon is pinched at its neck, removed carefully and fitted over a second bottle that contains clear lime-water. On gentle shaking the gas from the balloon mixes with the solution.

Second observation

• Lime-water turns milky (a white suspension forms).

Chemical test for carbon dioxide

\[ \mathrm{Ca(OH)_2\,(aq)+CO_2\,(g)\;\rightarrow\;CaCO_3\,(s)\downarrow+H_2O\,(l)} \]
The white solid $$\mathrm{CaCO_3}$$ makes the liquid look milky, proving that the gas inside the balloon is carbon dioxide.

What does the experiment tell us?

1. Yeast breaks down sugar into alcohol and carbon dioxide — a chemical change called fermentation.
2. The carbon dioxide produced is responsible for the balloon inflating in the model experiment and, in real bread-making, for the soft, spongy texture (gas bubbles get trapped in the dough and expand on baking).

Classification of every change that occurred

Final summary
Yeast makes bread fluffy because, during fermentation, it converts the sugar present in the dough into carbon dioxide gas. The gas forms tiny bubbles that get trapped in the dough, causing it to rise and giving the baked bread a soft, airy texture.

Step 1 – Recall the definitions

• Reversible change: A change that can be undone by reversing the conditions. The original substance or state can be obtained again. Examples learned in class are stretching a rubber band and melting/freezing water.
• Irreversible change: A change that cannot be undone easily. The substance formed or the condition achieved is permanent for everyday purposes. Examples are burning paper or curdling milk.

Step 2 – Observe what actually happens in a chameleon

• The skin of a chameleon contains special cells called chromatophores. Inside every chromatophore there are tiny grains of different coloured pigments (mainly black/brown, red/yellow and a reflective layer that looks blue/white).
• By expanding or contracting these grains a chameleon can let some colours show more and others less. This rearrangement changes the colour that we finally see.
• No new substance is produced in the skin. The pigments were already there; only their position and thickness change.

Step 3 – Classify the change using science concepts

• The change is similar to shaking a box of coloured beads and then letting them settle again—the beads are the same, only their places differ.
• Because the same pigments come back to their previous arrangement when the surroundings or mood change again, the skin returns to its former colour.
• Thus the chameleon’s colour shift satisfies the description of a reversible, physical change: no chemical reaction, no new molecule, and the process can be repeated any number of times.

Step 4 – Answer in textbook language

Yes, the colour change shown by a chameleon is a reversible change. It is brought about only by redistribution of pigments in its skin and the process can return to the original state whenever the animal is calm or moves to a different background.