Class 7: Physical & Chemical Changes-Boojho & Paheli Questions

Physical & Chemical Changes

Boojho & Paheli's Questions Answered

Changes happen all around us. Some changes are temporary, while others are permanent. Let's explore the difference between physical and chemical changes!

Q

We learnt that plants produce food by photosynthesis. Can we call photosynthesis a chemical change?

Yes, photosynthesis is a chemical change.

Why it's a chemical change

Photosynthesis transforms carbon dioxide and water into glucose and oxygen. This creates new substances that can't be changed back through physical means.

CO₂ + H₂O + sunlight → C₆H₁₂O₆ + O₂

(Carbon dioxide + Water → Glucose + Oxygen)

Characteristics of chemical change

• New substances are formed
• Change is usually permanent
• Energy is absorbed or released
• Chemical composition changes

Q

Paheli said that even digestion is a chemical change.

Yes, digestion is a chemical change.

How digestion works

During digestion, complex food molecules are broken down into simpler substances through chemical reactions.

Food we eat

Complex carbohydrates, proteins, and fats

After digestion

Simple sugars, amino acids, and fatty acids

Evidence of chemical change

• New substances are formed
• Change cannot be reversed
• Energy is released (which our body uses)

B

"Oh, that is why my friend Rita is always complaining about iron articles rusting so fast. She lives near the coast."

Rusting happens faster near the coast because of salt and moisture in the air.

Why rusting is faster near coast

• Sea air contains salt particles that speed up rusting
• Humidity (moisture in air) is higher near the coast
• Salt water is a better conductor than fresh water

Near Coast

Rusting occurs quickly

Due to salt and moisture

+ →

Inland Areas

Rusting occurs slowly

Less salt in air

→ (slow)

Preventing Rust

To prevent rusting near coastal areas:

• Paint iron surfaces
• Apply grease or oil
• Use galvanization (zinc coating)
• Store in dry places

P

How can we tell if a change is chemical or physical?

We can tell by checking if new substances are formed.

Physical Change

• No new substance formed
• Reversible
• Only state or shape changes
• Example: Ice melting

Chemical Change

• New substance formed
• Usually irreversible
• Chemical composition changes
• Example: Burning wood

Signs of chemical change

• Color change
• Gas produced (bubbles)
• Temperature change
• New smell
• Precipitate formed

B

Is dissolving sugar in water a physical or chemical change?

Dissolving sugar in water is a physical change.

Why physical change?

• No new substance is formed
• You can get sugar back by evaporating the water
• Only the state of sugar changes (solid to solution)

Sugar crystals

Sugar solution

Recovered sugar

Dissolving and recovering sugar - a physical change

Understanding physical and chemical changes helps us understand the world around us!

Class 7 Science - Physical and Chemical Changes