Changes Around Us: Physical and Chemical is a chapter in the CBSE Class 7 Science syllabus from Curiosity. This chapter hub brings together revision notes, practice questions, worksheets, flashcards, formula sheet to help students learn, practice, and revise Changes Around Us: Physical and Chemical effectively.
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NCERT Class 7 Science Chapter 5: Changes Around Us: Physical and Chemical (Pages 57–72)
CBSE
Class 7
Science
Curiosity
5
57–72
7 study resources
Download the Changes Around Us: Physical and Chemical revision guide with key points, summaries, and quick revision notes for CBSE Class 7 Science.
Key Points
Physical Change: Definition & Example.
Physical changes alter appearance without changing the substance. E.g., melting ice.
Chemical Change: Definition & Example.
Chemical changes form new substances. E.g., rusting of iron creates iron oxide.
Ice to Water: Physical Change.
Melting ice changes its state from solid to liquid, no new substance formed.
Heating Water: Physical Change.
Boiling water vaporizes it; water remains H₂O, showcasing a physical change.
Cutting Paper: Physical Change.
Chopping paper changes its shape but retains its chemical identity.
Combustion Process: Key Points.
Combustion involves a fuel reacting with oxygen, yielding heat/light. E.g. burning wood.
Rusting: A Chemical Change.
Iron reacts with oxygen to form rust (iron oxide), a distinct new substance.
Ignition Temperature Defined.
The minimum heat needed for a substance to combust. Essential for fire initiation.
Lime Water Test for CO₂.
Exhaled CO₂ turns lime water milky. Indicates a chemical reaction has occurred.
Vinegar and Baking Soda Reaction.
Combining them produces CO₂ gas, showcasing chemical change with visible bubbles.
Weathering Explained.
Natural processes break rocks into sediments, illustrating physical and chemical changes.
Erosion: Definition & Process.
Erosion shifts sediments via wind/water. A long-term physical change in the landscape.
Candle Burning: Mixed Changes.
Wax melting (physical) and vapor burning (chemical) represent simultaneous changes.
Reversible Changes: Examples.
Melting ice can refreeze. Boiling water can condense back, illustrating reversibility.
Irreversible Changes: Examples.
Chopping vegetables, baking bread are irreversible; original form cannot be restored.
Combustible Substances Defined.
Materials that can catch fire. E.g., wood, paper. Require heat and oxygen to ignite.
Desirable vs. Undesirable Changes.
Changes like cooking food are desirable, while rusting and decay are often undesirable.
Observing Changes: Senses Involved.
Sight, smell, touch, hearing, and taste help us perceive changes in our environment.
Chemical Reaction: Process Understanding.
A reaction produces new substances. E.g., addition of water to lime results in calcium carbonate.
Role of Oxygen in Combustion.
Oxygen supports combustion. Without it, fires extinguish, crucial understanding in fire safety.
Natural Changes Over Time.
Changes like erosion can take thousands of years, emphasizing the slow nature of some processes.
Practice important questions and exam-style problems from Changes Around Us: Physical and Chemical. These questions cover key topics from the CBSE Class 7 Science syllabus.
How to practice: Start with the questions below to test your understanding of Changes Around Us: Physical and Chemical. Use the revision guide to review concepts you find difficult, then come back and retry the questions for better retention.
Which of the following is a physical change?
What happens when air is blown into lime water?
Which process shows that a substance can change its appearance?
When ice melts, what type of change occurs?
If two gases react and produce liquid water, what type of change is this?
Why can a cut fruit still show signs of a physical change?
What indicates a chemical change?
Which of the following statements is true?
What does the process of rusting demonstrate?
If you burn wood, what happens to its appearance?
Why might dissolving salt in water still be a physical change?
Which step is essential in confirming a chemical change?
Why does blowing air into water leave it unchanged?
What is a physical change?
When you crush chalk into powder, what type of change are you observing?
After inflating a balloon and then deflating it, what happens to the original balloon?
Which of the following is NOT an example of a physical change?
If you fold a piece of paper, what happens to the paper?
Which of these changes demonstrates a change in state of matter?
Why is crushing a can considered a physical change?
Which process can turn ice back into water?
What is the primary characteristic of physical changes?
Which of the following actions would NOT result in a physical change?
What will happen to a balloon if it is pricked with a pin?
Which change indicates a transformation within the states of solids, liquids, or gases?
What happens when you dissolve salt in water?
Which substance undergoes a physical change when it freezes?
Why is the change from solid to liquid classified as a physical change?
What is rust primarily made up of?
Which condition is essential for rusting to occur?
Which of the following does NOT prevent rusting?
What type of change is rusting categorized as?
Rust formation can be accelerated by which of the following factors?
Why is rusting considered a undesirable change?
Which element is primarily responsible for rusting?
What is the chemical equation for the rusting of iron?
How can the reaction rate of rusting be reduced?
Rust can often be described as which of the following?
Which scenario best illustrates rusting?
What preventive measure does galvanization involve?
Which of these is NOT a common effect of rusting?
What can be a long-term impact of rusting on machinery?
What is essential for combustion to occur?
Which of the following is NOT a combustible substance?
What happens to a combustible substance when it reaches its ignition temperature?
Which of the following describes combustion best?
Which factor is MOST critical for starting the combustion process?
Which of the following statements about combustion is FALSE?
What is the role of oxygen in the combustion process?
How can you extinguish a fire effectively?
What happens when a combustible material burns in a limited supply of oxygen?
Which of the following is a common product of combustion?
During combustion, energy is released. This is an example of which type of change?
In which of the following scenarios would combustion NOT happen?
Which fuel has a higher ignition temperature?
What kind of reaction is combustion characterized as?
What could happen if there is an excess of oxygen during combustion?
What happens to the wax when a candle is lit?
Which of the following is a physical change occurring in a burning candle?
What type of change occurs when the vaporized wax burns?
In the context of a burning candle, which of the following represents both changes occurring?
Why is the melting of wax considered a physical change?
What kind of substances are produced during candle burning?
When analyzing a burning candle, what happens first?
Which of these statements about physical and chemical changes is true?
Which factor is crucial for the melting wax to change into vapor?
If a candle burns for a long time, what will be the final result of the changes?
What role does oxygen play in the chemical changes during the candle burning?
Which of the following best describes the overall process of burning wax?
If ice melts to form water, can it be returned to ice?
What happens to the wax after it has been burned?
When paper burns, which type of change occurs?
Which of the following is an example of a reversible change?
Which change is described as permanent?
What happens during a physical change?
Which change can NOT be reversed?
What physical changes can generally be reversed?
When a balloon is inflated and then deflated, what type of change occurs?
Why is rusting of iron considered a permanent change?
Which of the following represents a chemical change?
Which process involves a permanent change?
Which of the following is true about physical changes?
If water is boiled and turned into steam, what type of change is this?
What is an irreversible change?
Which describes a temporary change?
What process involves the breaking down of large rocks into smaller pieces?
Which natural force is primarily responsible for erosion?
Which of the following contributes to the weathering of rocks?
What is a primary result of chemical weathering?
What is erosion during a landslide classified as?
How do tree roots contribute to weathering?
What is the primary role of water in chemical weathering?
In which scenario would you observe sedimentation?
What type of rock changes color due to oxidation?
What does not happen during the erosion process?
How long does erosion typically take to significantly change a landscape?
What causes pebbles in a river to become smoother over time?
Which of the following is an effect of physical weathering?
Which of the following is considered a desirable change?
Why is the rusting of iron considered an undesirable change?
Which of the following describes decomposition?
What type of change is the process of curdling milk?
Which change may be considered desirable in agricultural waste management?
How does increased carbon dioxide from human activities affect the environment?
What is an example of a slow natural change?
Why is the drying of paint considered a chemical change?
What can be concluded about undesirable changes?
Which of the following is a benefit of the weathering process?
What type of change occurs when bread becomes stale?
In which situation is food decay considered a desirable change?
Why is it important to understand desirable and undesirable changes?
How does the rusting of iron demonstrate both a physical and chemical change?
Which food modification is a result of desirable changes due to human intervention?
Download and practice Changes Around Us: Physical and Chemical worksheets to improve problem-solving accuracy and speed for CBSE Class 7 Science exams.
This worksheet covers essential long-answer questions to help you build confidence in Changes Around Us: Physical and Chemical from Curiosity for Class 7 (Science).
Questions
Define physical changes and provide three examples from everyday life. Explain the characteristics that distinguish physical changes from chemical changes.
Physical changes are alterations that affect one or more physical properties of a substance without modifying its chemical composition. Examples: melting ice, boiling water, and dissolving sugar in water. Physical changes are typically reversible, and the original substance retains its chemical identity. Characteristics include changes in state, shape, size, or appearance without a new substance being formed.
What are chemical changes? Discuss the process and characteristics of a chemical change, giving two examples.
Chemical changes involve a transformation that produces one or more new substances with distinct properties. Examples: burning wood, which produces ash and gases; rusting of iron, which results in iron oxide. Characteristics include changes that are often irreversible and the formation of new substances, evidenced by changes in color, temperature, or gas production.
Explain how combustion is a chemical change. Describe an experiment that demonstrates this process.
Combustion is a rapid reaction involving oxygen that releases energy in the form of heat and light. An example of an experiment is burning a magnesium ribbon, which reacts with oxygen to form magnesium oxide, releasing heat and light. The chemical equation is: Magnesium + Oxygen → Magnesium oxide + Heat + Light. This demonstrates a chemical change as a new substance is formed and energy is released.
Choose two examples of changes (one physical and one chemical) and compare them by discussing their properties, reversibility, and real-world applications.
Example 1: Melting ice (physical change) - it can be refrozen, and the substance (water) remains the same. Example 2: Baking a cake (chemical change) - once baked, the ingredients create new substances that cannot revert to original components. Physical changes are often reversible, while chemical changes result in new substances with different properties. Real-world applications include using melting ice in drinks and baking cakes for celebrations.
What are the effects of rusting on metals, and how can it be prevented? Discuss the chemical process involved.
Rusting is a chemical change where iron reacts with oxygen and moisture to form iron oxide, a reddish-brown substance. To prevent rusting, methods include painting, galvanizing, or using stainless steel. The chemical reaction can be represented as: Iron + Oxygen + Water → Iron oxide (rust). Preventing rusting extends the lifespan of metal objects.
Describe the process of making curd from milk. What kind of change occurs here, and how is it significant in our daily life?
The process of making curd from milk involves adding a small amount of curd (containing live bacteria) to warm milk. The bacteria ferment the lactose in milk, transforming it into lactic acid, leading to curd formation. This is a chemical change because new substances (curd) with different properties are formed. Curd is significant in our diet for nutrition and as a common ingredient in various dishes.
Investigate the process of making popcorn from corn. Identify the changes occurring and classify them.
Making popcorn involves heating corn kernels, causing the moisture inside to turn into steam, building pressure until the kernel bursts. This results in the transformation of the kernel to fluffy popcorn, which is a physical change during popping. However, the heating of the kernel and moisture alters its structure, thus creating a slightly chemical change. This process demonstrates principles of transformations in cooking.
What role does oxygen play in combustion, and why is it essential? Discuss with examples.
Oxygen plays a critical role in combustion as it reacts with fuel (combustible substances) to release energy through heat and light. For example, candle wax combusts in the presence of oxygen, producing flame and heat. Without oxygen, combustion cannot occur, as demonstrated in experiments showing that covered flames extinguish due to lack of air supply.
How can changes be classified into reversible and irreversible? Provide examples of each type.
Changes can be classified based on whether the original state can be restored. Reversible changes allow returning to the original state (e.g., melting ice), while irreversible changes permanently alter the substance (e.g., baking bread). This classification helps in understanding processes in nature and everyday life, demonstrating alterations in materials.
Explain the concept of weathering and erosion, highlighting differences and factors contributing to these changes.
Weathering involves the breakdown of rocks through physical or chemical processes, while erosion is the movement of these fragments due to wind, water, or ice. Weathering can be caused by temperature changes, freezing and thawing, while erosion often occurs in flowing water or wind. Both processes shape landscapes and contribute to soil formation.
This worksheet challenges you with deeper, multi-concept long-answer questions from Changes Around Us: Physical and Chemical to prepare for higher-weightage questions in Class 7.
Questions
Describe the processes involved in the melting of ice and boiling of water. How do both relate to the concepts of physical changes?
Melting of ice (solid to liquid) and boiling of water (liquid to gas) are both physical changes. No new substances are formed; only the state changes. Melting occurs at 0°C, while boiling happens at 100°C. Both changes are reversible.
How can you differentiate between physical and chemical changes based on the properties of substances? Provide examples.
Physical changes alter appearance but not chemical composition (e.g., melting ice). Chemical changes result in new substances (e.g., rusting iron). Observe if the same substance can be regained.
Explain why burning magnesium ribbon is a chemical change. Include the observations made during the reaction.
Burning magnesium results in magnesium oxide. It produces light and heat, indicating a chemical reaction. The original magnesium cannot be reformed, proving that a new substance is created.
What role does oxygen play in combustion? Discuss an experiment to demonstrate this.
Oxygen supports combustion. Experiments like burning a candle in a jar show that without oxygen, the flame extinguishes. It illustrates that combustion needs combustible material, heat, and oxygen.
In which situations can a chemical change be reversed? Provide examples and your reasoning.
Reversible chemical changes are rare. For instance, dissolving some gases is reversible, but rusting is not. Analyze how substances cannot return to their original form.
Discuss the significance of lime water turning milky when carbon dioxide is introduced. What does this indicate about gas reactions?
The reaction indicates carbon dioxide forms calcium carbonate in lime water. This demonstrates a chemical change, highlighting how gases interact with solutions. It is a qualitative test for carbon dioxide.
Using the example of food decomposition, explain how some chemical changes are beneficial and some are harmful.
Decomposition converts waste into compost (beneficial). In contrast, goods decaying lead to spoilage (harmful). Thus, chemical changes can have positive or negative impacts depending on context.
Analyze how the evaporation of water and the condensation of vapor are interrelated. Are these physical or chemical changes?
Both evaporation (liquid to gas) and condensation (gas to liquid) are physical changes. They demonstrate the water cycle where no new substances are created, only a state change occurs.
What is the process of rusting? Is it a physical or chemical change, and why is it significant to everyday life?
Rusting is a chemical change where iron reacts with oxygen and moisture to create rust (iron oxide). It's significant as it indicates material degradation, affecting structures and appliances.
Evaluate how weathering and erosion contribute to soil formation. What types of changes do they represent?
Weathering involves both physical (breaking rocks) and chemical changes (chemical weathering). Erosion physically transports particles. Together, they form soil, essential for ecosystems.
The final worksheet presents challenging long-answer questions that test your depth of understanding and exam-readiness for Changes Around Us: Physical and Chemical in Class 7.
Questions
Analyze how physical changes and chemical changes can be distinguished in everyday scenarios, such as cooking. Provide examples to support your evaluation.
Differentiate by examining the transformation and by checking if new substances are formed. Cooking involves both, such as boiling (physical) and caramelization (chemical).
Discuss the environmental impacts of combustion-based changes in our daily lives. How could these changes be perceived differently from a scientific and a social perspective?
The chemical reaction in combustion releases CO2, impacting air quality. While necessary for energy, it contributes to pollution, raising ethical concerns.
Evaluate the role of temperature in both physical and chemical changes using the example of water. How does changing temperature affect the state and composition of water?
Temperature can change water's state (solid, liquid, gas) without altering its chemical composition, while in reactions like boiling with added substances, new compounds may form.
Reflect on the implications of irreversible changes in food processing. How can understanding these processes contribute to better food preservation methods?
Irreversible changes like frying or baking change foods permanently. Recognizing these helps develop preservation techniques like freeze-drying to minimize waste.
Propose a creative project that could illustrate both physical and chemical changes using common kitchen ingredients. Outline the steps and expected outcomes.
Combine vinegar and baking soda to create gas (chemical) and examine how melting butter (physical) changes form. Document results and explain the science behind them.
Assess the statement: 'All physical changes are reversible and all chemical changes are irreversible.' Provide counterarguments and examples from real life.
Some physical changes, like melting and freezing, are reversible. Some chemical changes can be reversed, like rusting through electrolysis.
Interpret weathering and erosion as processes that involve physical and chemical changes. How do these processes shape our environment over time?
Weathering includes breaking down rocks chemically and physically, while erosion moves sediments. Both contribute to landscape transformation.
Critique the necessity of oxygen for combustion. What experiments can illustrate the relationship between oxygen presence and the occurrence of combustion?
Oxygen is crucial for combustion; experiments using candles illustrate this. Covering a candle suffocates the flame due to lack of oxygen.
Investigate the transformation of a candle when lit, detailing both physical and chemical changes. How can these relate to broader scientific concepts?
Lighting a candle initiates melting (physical) and burning wax (chemical). This relates to energy transformations and states of matter.
Evaluate the desirability of certain changes in nature, like decomposition, versus those typically regarded as undesirable, such as rusting or decay.
Decomposition is beneficial for nutrient cycling, while rusting leads to material loss. Context defines desirability; it varies across scenarios.
Use this Class 7 Science Changes Around Us: Physical and Chemical Formula Sheet for quick revision before school exams and CBSE exams. It brings together the important formulas, key concepts, and worked examples in one place so students can revise faster and download a printable PDF for offline study.
Important Formulas
E = mc²
E represents energy (in joules), m is mass (in kg), and c is the speed of light (≈ 3 × 10⁸ m/s). This formula shows how mass can be converted into energy, a foundational idea in Einstein’s theory of relativity.
Q = mcΔT
Q indicates heat energy (in joules), m is mass (in kg), c is specific heat capacity (J/kg·°C), and ΔT is the change in temperature (in °C). This formula determines the amount of heat required to change the temperature of a substance.
p = m/V
p is density (kg/m³), m is mass (in kg), and V is volume (in m³). This formula relates mass and volume to calculate the density of a substance.
R = ρL/A
R is resistance (in ohms), ρ is resistivity (Ω·m), L is length of the conductor (in m), and A is cross-sectional area (in m²). This formula finds the resistance of a conductor based on its material and dimensions.
C = Q/V
C represents capacitance (in farads), Q is charge (in coulombs), and V is voltage (in volts). It is used to determine the ability of a system to store electrical energy.
V = IR
V is voltage (in volts), I is current (in amperes), and R is resistance (in ohms). This formula defines the relationship between current and voltage in a circuit.
PV = nRT
P is pressure (in pascals), V is volume (in m³), n is the amount of substance (in moles), R is the ideal gas constant, and T is temperature (in kelvins). This equation explains the relationship in an ideal gas.
ΔS = ΔH/T
ΔS is entropy change (J/K), ΔH is enthalpy change (in joules), and T is temperature (in kelvins). This formula relates the changes in entropy to enthalpy and temperature.
H₂ + O₂ → 2H₂O
This chemical equation represents the reaction between hydrogen and oxygen to form water. It shows the conservation of mass in chemical reactions.
Fe + O₂ → Fe₂O₃
This equation represents the rusting process where iron reacts with oxygen to produce iron oxide, commonly known as rust.
Worked Examples
Lime water test: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O
This equation represents the reaction in which lime water (calcium hydroxide) reacts with carbon dioxide to form calcium carbonate (a cloudy precipitate) and water.
Vinegar + Baking soda → CO₂ + Other substances
This reaction produces carbon dioxide gas when vinegar reacts with baking soda, resulting in bubbling and fizzing.
Mg + O₂ → MgO + Heat + Light
This equation shows the combustion of magnesium in oxygen, producing magnesium oxide along with the release of heat and light.
H₂O (l) → H₂O (g)
This equation signifies the physical change of water from liquid (l) to gas (g) through evaporation.
C₃H₈ + 5O₂ → 3CO₂ + 4H₂O + Heat
This is the combustion reaction of propane (C₃H₈) in oxygen that produces carbon dioxide and water, releasing energy in the form of heat.
4Fe + 3O₂ → 2Fe₂O₃
This equation describes the formation of rust, where iron reacts with oxygen to form iron oxide.
CaCO₃ → CaO + CO₂
This equation shows the thermal decomposition of calcium carbonate into calcium oxide and carbon dioxide upon heating.
2H₂ + O₂ → 2H₂O
This balanced reaction indicates the synthesis of water from hydrogen and oxygen, highlighting the conservation of mass.
N₂ + 3H₂ → 2NH₃
This equation represents the synthesis of ammonia, a crucial process in chemistry, emphasizing stoichiometry in reactions.
C₁₂H₂₂O₁₁ + H₂O → 2C₆H₁₂O₆
This equation describes the hydrolysis of sucrose (table sugar) into glucose, demonstrating the chemical change during digestion.
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