Science (EVS K-5) · Class 7 · Chemical Changes and Matter · Term 1

Chemical Changes: New Substances Formed

Students will identify chemical changes by observing the formation of new substances with different properties.

• CBSE Learning Outcomes
• CBSE: Physical and Chemical Changes - Class 7

About This Topic

Chemical changes happen when two or more substances react to produce entirely new substances with different properties, such as colour, smell, or state. Students in Class 7 identify key signs: gas bubbles, heat or light release, colour change, or solid formation. For instance, burning paper turns it into ash and smoke, creating new materials, while tearing paper is a physical change that preserves the original substance.

This topic fits within the CBSE unit on physical and chemical changes, linking to everyday examples like rusting iron, curd formation from milk, or baking powder in cakes. It sharpens skills in observing, comparing reactants and products, and justifying classifications, which supports broader matter studies.

Active learning suits this topic well because safe, observable reactions make abstract ideas concrete. When students conduct vinegar and baking soda mixes or watch copper sulphate react with an iron nail, they directly see new substances form, discuss evidence, and correct their ideas through peer sharing, boosting understanding and enthusiasm.

Key Questions

1. Analyze the signs that indicate a chemical change has occurred.
2. Compare the properties of reactants and products in a chemical change.
3. Justify why burning paper is a chemical change, while tearing it is physical.

Learning Objectives

• Identify at least three observable signs that indicate a chemical change has occurred.
• Compare the properties of reactants before and products after a chemical change, such as burning paper to ash.
• Classify given examples as either physical or chemical changes, providing justification.
• Explain the difference between a physical change and a chemical change using examples like tearing versus burning paper.

Before You Start

Properties of Matter

Why: Students need to understand basic properties like colour, state, and smell to compare reactants and products.

States of Matter

Why: Understanding solid, liquid, and gas states is necessary to identify changes like gas formation.

Key Vocabulary

Chemical Change	A process where one or more substances are transformed into new substances with different chemical properties. This often involves the breaking and forming of chemical bonds.
Physical Change	A change in the form of a substance that does not alter its chemical composition. Examples include changes in shape, size, or state.
Reactant	The starting substance(s) that undergo a chemical change. These are the materials that react together.
Product	The new substance(s) formed as a result of a chemical change. These have different properties from the reactants.
Formation of Gas	An observable sign of a chemical change where bubbles of gas are produced, indicating a new substance has formed.
Formation of Precipitate	An observable sign of a chemical change where a solid forms and separates from a liquid solution.

Watch Out for These Misconceptions

Common Misconception

Dissolving sugar in water is a chemical change.

What to Teach Instead

No new substance forms; evaporating the water recovers the same sugar crystals. Hands-on evaporation trials in pairs let students test and see the original properties return, clarifying physical dissolution.

Common Misconception

Any irreversible change is chemical.

What to Teach Instead

Irreversibility alone does not define it; new substances must form, like in cooking an egg versus crushing ice. Group comparisons of examples during discussions help students focus on property changes as key evidence.

Common Misconception

Tearing paper produces a new substance.

What to Teach Instead

The pieces remain paper with same burn and dissolve properties. Simple pair tests, like burning small pieces, show same ash as whole paper, reinforcing physical change through direct observation.

Active Learning Ideas

Pairs Experiment: Vinegar and Baking Soda

Give pairs baking soda, vinegar, a test tube, and a balloon. Add baking soda to the tube, stretch balloon over top, pour in vinegar, and observe gas production inflating the balloon. Discuss how carbon dioxide gas is a new substance with different properties.

25 min·Pairs

Small Groups: Iron Nail in Copper Sulphate

Provide small groups with copper sulphate solution, iron nails, and beakers. Place nail in solution for 10 minutes, note blue colour fading and reddish deposit on nail. Compare initial and final properties to confirm new substances formed.

35 min·Small Groups

Whole Class Demo: Burning Magnesium Ribbon

Demonstrate safely with tongs: light magnesium ribbon, observe bright light and white ash. Students record signs like light and new powdery product. Follow with class discussion on why this differs from melting wax.

20 min·Whole Class

Stations Rotation: Curdling Milk

Set stations with milk and lemon juice. Groups add juice to milk, stir gently, watch lumps form. Filter and taste to note new solid (paneer-like) versus liquid whey, identifying precipitate as a chemical sign.

30 min·Small Groups

Real-World Connections

• Bakers use chemical changes when they add baking powder to dough. The reaction between baking powder and other ingredients produces carbon dioxide gas, causing the cake or bread to rise and become fluffy.
• Farmers observe chemical changes when they notice milk turning sour or curdling. Bacteria in the milk cause chemical reactions that change its taste, texture, and smell, forming curd.
• Metallurgists study chemical changes like rusting. Iron reacts with oxygen and moisture to form iron oxide (rust), a new substance with properties different from iron, which can weaken structures.

Assessment Ideas

Exit Ticket

Provide students with a list of scenarios (e.g., boiling water, rusting iron, dissolving sugar, burning wood). Ask them to circle the chemical changes and write one sentence for each circled item explaining why it is a chemical change, referencing the formation of a new substance.

Discussion Prompt

Pose the question: 'Imagine you are a food scientist developing a new type of biscuit. What chemical changes would you want to happen during baking to make the biscuit tasty and have the right texture? What signs would you look for?' Facilitate a class discussion on their ideas.

Quick Check

Show students two substances side-by-side, one before a reaction and one after (e.g., clear liquid and cloudy liquid, or a metal strip and a dark powder). Ask: 'What signs suggest a chemical change has occurred here? What are the reactants and what are the products?'

Frequently Asked Questions

What are the main signs of a chemical change for Class 7 students?

Key signs include gas evolution, colour change, heat or light production, and precipitate formation. Students can spot these in reactions like vinegar with baking soda (gas) or iron in copper sulphate (colour shift and deposit). Comparing before and after helps confirm new substances with altered properties, unlike physical changes.

Why is burning paper a chemical change but tearing it physical?

Burning produces ash, smoke, and gases: new substances with different properties from paper. Tearing keeps the same paper material. Experiments showing ash does not reform paper, while torn pieces do when taped, build clear distinctions through evidence-based justification.

How does active learning help teach chemical changes?

Active methods like station rotations or pair experiments let students observe gas, colour shifts, or solids firsthand, making signs memorable. Collaborative recording and discussions correct misconceptions instantly, as peers challenge ideas. This hands-on approach increases retention over lectures, with safe CBSE-aligned setups ensuring engagement.

What daily life examples show chemical changes?

Rusting of iron gates, milk turning to curd with lemon, or baking roti releasing heat and aroma all form new substances. Class activities linking these to school demos, like nail rusting, help students connect curriculum to home, analysing properties to classify changes accurately.