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NCERT Class 7 Science Chapter 5 on Physical and Chemical Changes walks you through the physical and chemical properties of substances and definitions of both changes. In the process of learning these changes, we learn about various scientific experiments that help us understand these concepts practically. Also, we explore daily life phenomena like rusting and crystallisation. You can walk through all these concepts in our Class 7 Science notes in simplified language accompanied by infographics. Additionally, the notes also include NCERT solutions to Science Chapter 5 on Physical and Chemical Changes.
Download NCERT Science Class 7 Science Chapter 5 Important Questions and Answers PDF
Explore all the Chapters of Class 7 Science:
| Chapter 1 | Chapter 2 | Chapter 3 | Chapter 4 | Chapter 5 | Chapter 6 |
| Chapter 7 | Chapter 8 | Chapter 9 | Chapter 10 | Chapter 11 | Chapter 12 |
| Chapter 13 |
Explore: Science Projects for Class 8 & Working Models
Table of Contents
Physical Changes
To understand physical changes, the authors of NCERT Class 7 Science Chapter 5 have included a few scientific experiments. Now, let us view some examples of physical changes:
- Cutting a paper into pieces.
- Crushing a piece of chalk and mixing the crushed powder into water to form a paste. In the next step, you can use the paste to a new piece of chalk in the desired shape.
- Turning ice into water by taking ice cubes from the freezer and putting them in a tumbler at room temperature.
- Boiling water on a stove. In this process, water turns into vapour through the process of evaporation.
We observe that in all these experiments no new substance is formed but the properties of the objects change. To understand it better, let us now delve into the definitions of physical properties and physical changes. Let’s go!!
Physical Properties
Characteristics of an object or a substance such as its state, shape, colour, and size are known as its “physical properties”.
Physical Changes
A transformation in which a substance changes its state, colour, shape, or size is known as “physical change”. It is an irreversible change. Also, in this type of change, no new substance is formed.
Read More: 10 Scientific Experiments That Were Ahead Of Their Time
Chemical Changes
Just like the above section of NCERT Class 7 Science Chapter 5 notes, this part also simplifies the activities and definitions related to chemical changes. Here are interesting scientific experiments for students of Class 8, which should be carried out under the guidance of teachers:
- Get a ribbon of magnesium and clean its tip with sandpaper. Now, you need to bring the clean edge near a candle flame using a tong. You will observe that the tip burns with a white light. After the ribbon is burnt down completely, it leaves a powdery ash.
Unlike physical changes, in this experiment, the substance changes its form. This change is depicted by the following equation:
| Magnesium (Mg) + Oxygen (O2)—————> Magnesium oxide (MgO) |
Now, take the ash or MgO and mix it in water. Now, it forms a new substance. This is explained by the following equation:
| Magnesium oxide (MgO) + Water(H2O)—————->Magnesium hydroxide [Mg(OH)2] |
- To get a better understanding of chemical change, let us perform one more experiment with vinegar and baking soda. Place approximately a teaspoon of vinegar into a test tube. Sprinkle a small amount of baking soda into the vinegar. You will observe a fizzing reaction and hear a hissing sound as gas bubbles are produced. Direct this gas through recently made lime water.
The following changes take place in the test tube:
| Vinegar (Acetic acid) + Baking soda(Sodium hydrogen carbonate) ————> Carbon dioxide (CO2)+ other substances |
Now, the CO2 reacts with lime water to emit water and calcium carbonate. The formation of calcium carbonate makes the lime water milky. This can be depicted through the following equation:
| Carbon dioxide (CO2) + Lime water [Ca(OH) 2 ]——–> Calcium Carbonate (CaCO3) + Water (H2O) |
Chemical Properties
A chemical property refers to a distinct attribute of a substance, such as an element or a compound, which pertains to how it can undergo transformations through reactions.
Chemical Changes
A chemical change is a process in which one or more new substances are generated. It is also termed a chemical equation. In this change, a new substance is formed.
In addition to the formation of new substances, a chemical change or a chemical equation is accompanied by the following:
- Emission or absorption of heat.
- Emission or absorption of light
- Production of sound
- Production of a smell
- Colour change
- Formation of gas/es
Also Read: Class 10 Science Chemical Reactions and Equations
Rusting of Iron
The NCERT Class 7 Science Chapter 5 also helps us learn about rusting. It is a chemical change or process in which if an iron piece is left in an open space a layer of brownish material is formed on the iron. This brown substance is known as “rust”.
Let us understand rusting through the following equations:
| Iron (Fe) + Oxygen (O2, from the air)+ Water (H2O)———–>Rust (iron oxide Fe2O3) |
Rusting requires the presence of both oxygen and water (or water vapour). In fact, when the atmospheric moisture level is elevated, indicating higher humidity, the process of rusting occurs more rapidly.
Ways to Prevent Rusting?
As rust destroys iron, it is essential to prevent rusting. Here are some ways to prevent rust formation:
- Apply a coat of grease or paint over the iron.
- Apply a layer of rust-resistant metals like zinc (Zn) or chromium (Cr) on iron. This process of depositing a layer of Zn on iron is known as “Galvanisation”.
Why Stainless Steel is Rust Proof?
Stainless steel does not rust because it is created by mixing iron (Fe) with carbon and rust-resistant metals such as chromium (Cr), manganese (Mn), and nickel (Ni).
Also Read: All Branches of Chemistry with Examples, PDF, PPT
Crystallisation
It is a type of physical change mentioned in NCERT Class 7 Chapter 5 titled Physical and Chemical Changes. In this process, crystals are formed after the evaporation of liquid. For example, salt crystals are obtained by the evaporation of seawater.
Also Read: 5 STEM Activities Ideas for School Students
Explore all the Chapters of Class 7 Science:
| Chapter 1 | Chapter 2 | Chapter 3 | Chapter 4 | Chapter 5 | Chapter 6 |
| Chapter 7 | Chapter 8 | Chapter 9 | Chapter 10 | Chapter 11 | Chapter 12 |
| Chapter 13 |
NCERT Solutions Class 7 Science Chapter 5: Physical and Chemical Changes
In this section, we come across solutions to questions following the NCERT Class 7 Science Chapter 5.
Ques 1. Classify the changes involved in the following processes as physical or chemical changes:
a. Photosynthesis
b. Dissolving sugar in water
c. Burning of coal
d. Melting of wax
e. Beating aluminium to make aluminium foil
f. Digestion of food
Ans:
| a. Photosynthesis | Chemical change |
| b. Dissolving sugar in water | Physical change |
| c. Burning of coal | Chemical change |
| d. Melting of wax | Physical change |
| e. Beating aluminium to make aluminium foil | Physical change |
| f. Digestion of food | Chemical change |
Ques 2. State whether the following statements are True or False. In case a statement is false, write the corrected statement in your notebook.
a. Cutting a log of wood into pieces is a chemical change.
b. Formation of manure from leaves is a physical change.
c. Iron pipes coated with zinc do not get rusted easily.
d. Iron and rust are the same substances.
e. Condensation of steam is not a chemical change.
Ans:
a. False.
Correct statement: Cutting a log of wood into pieces is a physical change.
b. False
Correct statement: Formation of manure from leaves is a chemical change.
c. True
d. False.
Correct statement: Iron is a metal. Whereas, the brownish substance formed on iron when the metal comes in contact with moisture and air is known as rust.
e. True
Ques 3. Fill in the blanks in the following statements:
a. When carbon dioxide is passed through lime water, it turns milky
due to the formation of________________.
b. The chemical name of baking soda is_______________.
c. Two methods by which rusting of iron can be prevented are_(1)______________ and _(2)___________.
d. Changes in which only_________________ properties of a substance change are called physical changes.
e. Changes in which new substances are formed are called_______________ changes.
Ans:
a. Calcium Carbonate
b. Sodium hydrogen carbonate
c. (1) Painting (2) Galvanisation
d. Physical
e. Chemical
Ques 4. When baking soda is mixed with lemon juice, bubbles are formed with the evolution of a gas. What type of change is it? Explain.
Ans: When you mix baking soda (sodium hydrogen carbonate) with lemon juice (citric acid), it produces bubbles. These bubbles are a result of the release of carbon dioxide gas. This process constitutes a chemical change.
In this chemical transformation, the citric acid present in lemon juice reacts with sodium hydrogen carbonate, leading to the production of carbon dioxide gas:
| Lemon juice (Citric acid) + Baking soda (Sodium hydrogen carbonate) → Carbon dioxide + Other substances |
Ques 5. When a candle burns, both physical and chemical changes take place. Identify these changes. Give another example of a familiar process in which chemical and physical changes occur.
Ans: A burning candle undergoes both chemical and physical changes. Here are the changes:
| Chemical Change | Burning of wax |
| Physical Change | Melting of wax |
Another example of this type of change is the eating and digestion of food. Here are the changes:
| Chemical Change | Digestion of food with the help of intestinal juices and enzymes in the digestive tract. |
| Physical Change | Breaking of larger food particles into smaller ones. |
Ques 6. How would you show that the setting of curd is a chemical change?
Ans: It is a chemical change because once curd is formed from milk, it cannot be reversed or turned back into milk. Furthermore, milk and curd exhibit distinct properties.
Ques 7. Explain why burning wood and cutting it into small pieces are considered two different types of changes.
Ans: Cutting wood is considered a physical change because it does not change the fundamental nature of the wood. It merely changes its physical shape or size.
On the other hand, the burning of wood is a chemical change because, during this process, wood is transformed into charcoal, and carbon dioxide (CO2) is released. This chemical transformation results in a change in the composition and properties of the wood, making it a chemical change.
Ques 8. Describe how crystals of copper sulphate are prepared.
Ans: Copper sulphate crystals can be prepared using the crystallization method, as outlined in the following steps:
1. Take a beaker and pour a cupful of water into it.
2. Add a few drops of dilute sulfuric acid to the water.
3. Heat the water, and when it begins to boil, slowly add copper sulfate powder into the hot water while stirring continuously.
4. Keep adding copper sulfate powder until the solution becomes saturated, which means it can no longer dissolve more copper sulfate.
5. Filter the saturated solution into a china dish and let it cool.
6. Ensure that the solution is undisturbed during cooling.
7. Over time, the crystals of copper sulfate will gradually separate from the solution as it cools.
Ques 9. Explain how the painting of an iron gate prevents it from rusting.
Ans: Rusting takes place in the presence of air and moisture. Thus, applying a layer of paint over an iron gate prevents it from rusting as it cuts the contact between moisture and iron.
Ques 10. Explain why rusting of iron objects is faster in coastal areas than in deserts.
Ans: Rusting of iron objects is faster in coastal areas than in deserts because the coastal area is humid and the desert is dry. Owing to the moisture in humid coastal areas, iron objects easily acquire rust.
Ques 11. The gas we use in the kitchen is called liquified petroleum gas (LPG). In the cylinder, it exists as a liquid. When it comes out from the cylinder it becomes a gas (Change – A) then it burns (Change – B). The following statements pertain to these changes. Choose the correct one.
a. Process – A is a chemical change.
b. Process – B is a chemical change.
c. Both processes A and B are chemical changes.
d. None of these processes is a chemical change.
Ans: Option b. Process – B is a chemical change.
Ques 12. Anaerobic bacteria digest animal waste and produce biogas (Change – A). The biogas is then burnt as fuel (Change – B). The following statements pertain to these changes. Choose the correct one.
a. Process – A is a chemical change.
b. Process – B is a chemical change.
c. Both processes A and B are chemical changes.
d. None of these processes is a chemical change.
Ans: Option c. Both processes A and B are chemical changes.
Also Read:
FAQs
Q1. Define Physical Changes.
Ans: A transformation in which a substance changes its state, colour, shape, or size is known as “physical change”. It is an irreversible change.
Q2. What are the changes accompanying a chemical equation?
Ans: A chemical change or a chemical equation is accompanied by the following:
- Emission or absorption of heat.
- Emission or absorption of light
- Production of sound
- Production of a smell
- Colour change
- Formation of gas/es
Q3. What do you understand by galvanisation?
Ans: This process of depositing a layer of Zinc (Zn) on Iron (Fe) is known as “Galvanisation”.
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