All questions of Physical and Chemical Changes for Class 7 Exam

Preventing rusting of iron articles:

Apply a coat of paint:
One of the most effective ways to prevent rusting of iron articles is by applying a coat of paint. This creates a barrier between the iron and moisture in the air, preventing oxidation from occurring.

How does it work:
When iron comes into contact with oxygen and moisture, a chemical reaction occurs, leading to the formation of rust. By applying a coat of paint, the iron is shielded from these elements, thus reducing the chances of rust formation.

Types of paint:
There are various types of paint available that are specifically designed for preventing rust on iron surfaces. These paints contain corrosion inhibitors that provide extra protection against rust.

Regular maintenance:
It is important to regularly inspect the painted surface for any signs of wear or damage. If any areas of the paint have chipped or peeled off, they should be immediately repainted to prevent rust from forming in those areas.

Conclusion:
In conclusion, applying a coat of paint is a simple yet effective way to prevent rusting of iron articles. By creating a barrier between the iron and moisture, paint helps to inhibit the oxidation process that leads to rust formation. Regular maintenance and inspection of the painted surface are also essential to ensure long-lasting protection against rust.

Chemical changes cannot be reversed. New substances are formed in chemical changes. Chemical changes can be endothermic or exothermic. Chemical changes either absorb energy or release energy.

Explanation:

False
- Paper undergoing cutting and reassembly does not represent a chemical change.
- A chemical change involves the formation of new substances with different chemical compositions.
- Cutting and reassembling paper only changes its physical appearance and shape, without altering its chemical composition.
- Therefore, the process of cutting and reassembling paper is a physical change rather than a chemical change.

B)Oxygen

Introduction
When carbon dioxide (CO₂) is bubbled through lime water, a chemical reaction occurs, resulting in the formation of calcium carbonate (CaCO₃). This process is a classic experiment used to demonstrate the presence of carbon dioxide.

Reaction Process
- Lime water is a dilute solution of calcium hydroxide (Ca(OH)₂).
- When CO₂ is introduced, it reacts with calcium hydroxide to form calcium carbonate:
    Ca(OH)₂ + CO₂ → CaCO₃ + H₂O

Formation of Calcium Carbonate
- Calcium carbonate is only slightly soluble in water, leading to its precipitation.
- The solid calcium carbonate appears as a white precipitate, making the lime water appear milky.

Significance of the Milky Appearance
- The cloudiness or "milky" effect indicates a positive test for carbon dioxide.
- This experiment is commonly used in educational settings to illustrate gas reactions and solubility concepts.

Conclusion
In summary, when carbon dioxide is passed through lime water, calcium carbonate is formed, which makes the lime water milky. This experiment is not only straightforward but also serves as a fundamental demonstration of chemical reactions involving gases and their products.

Understanding Physical Change
A physical change refers to a transformation that affects one or more physical properties of a substance without altering its chemical structure.
Characteristics of Physical Change:
- No New Substance Formed:
- During a physical change, the original substance remains the same. For example, when ice melts into water, it is still H2O.
- Reversible Nature:
- Many physical changes can be reversed. For instance, freezing water back into ice is a simple reversal of the melting process.
- Change in Physical Properties:
- Physical changes can involve changes in state (solid, liquid, gas), shape, size, or texture. For example, cutting a piece of paper changes its size but not its chemical composition.
Examples of Physical Change:
- Melting:
- Ice melting into water.
- Dissolving:
- Sugar dissolving in water; the sugar can be recovered by evaporating the water.
- Evaporation:
- Water turning into steam; again, it can return to liquid form.
Conclusion:
In summary, the correct answer is option 'C' because a physical change involves a modification in physical attributes without inducing a chemical transformation. Understanding physical changes is essential in distinguishing between different types of changes substances can undergo in science.

Mixing baking soda (sodium bicarbonate) and vinegar (acetic acid) causes a chemical reaction that produces a salt (sodium acetate) and water, as well as carbon dioxide gas. In this demonstration, baking soda is placed in a balloon that is attached to a flask holding vinegar. This creates a closed system.

• Sedimentation and crystallization are unrelated to rust prevention.

Explanation:

Iron pipes coated with zinc do not get rusted easily:
- Zinc coating on iron pipes acts as a protective layer that prevents direct contact between iron and oxygen or moisture in the air.
- This protective layer of zinc is sacrificial, meaning it corrodes instead of the iron, thus preventing rusting of the iron pipes.
- This process is known as galvanization, and it is commonly used to protect iron and steel from rusting.
Therefore, the statement "Iron pipes coated with zinc do not get rusted easily" is true because the zinc coating on the iron pipes prevents rusting by acting as a protective barrier.