Basic Science Solutions for Class 7 Science Chapter 3 Changes And Reactions are provided here with simple step-by-step explanations. These solutions for Changes And Reactions are extremely popular among class 7 students for Science Changes And Reactions Solutions come handy for quickly completing your homework and preparing for exams. All questions and answers from the Basic Science Book of class 7 Science Chapter 3 are provided here for you for free. You will also love the ad-free experience on Meritnation’s Basic Science Solutions. All Basic Science Solutions for class 7 Science are prepared by experts and are 100% accurate.

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A generally reversible change, in which no new substances are formed, is called a physical change.

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An irreversible change in which new substances are formed under specific conditions is called a chemical change.

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(a) Chemical change
(b) Physical change
(c) Chemical change
(d) Chemical change
(e) Physical change

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(a) A reaction in which two or more reactants add up to form a product is called a combination reaction.
(b) A reaction in which one substance breaks down into two or more simpler substances is called a decomposition reaction.
(c) A reaction in which one element displaces another from a compound and takes its place in the compound is called a displacement reaction
(d) A reaction in which an acid reacts with a base to form salt and water is called a neutralisation reaction.



Page No 27:

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In physical changes, generally no new substances are formed, whereas, in chemical changes, new kind of molecules or new substances are formed.

Few characteristics are mentioned below:

Physical Changes Chemical Changes
No new substances are formed; only physical state changes. New substances are formed.
These changes are generally reversible. These changes are irreversible.

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Once salt is dissolved in water, it can be recovered from the salt solution by evaporating the water. This dissolution of salt in the fluid is reversible. Thus, it is a physical change.

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A burning candle is an example of both physical and chemical changes. When a candle burns, the wax melts; on cooling, it solidifies. So, this is a physical change. However, burning of wax in candle is a chemical change.

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Energy is an important prerequisite for any change to take place. Both physical and chemical changes are accompanied with change in energy level. While energy is released during some changes, it is also absorbed during some other changes.
When a solid is converted into a liquid or gas, heat is absorbed. Similarly, when a liquid evaporates, it absorbs heat from the surroundings. Some solutes such as glucose, while dissolving, absorb heat.
Some chemical changes require release of energy. Photosynthesis is an example of such a change. Even cooking of food is a chemical change that requires heat.
On the other hand, there are processes that release heat during the process itself.
Physical changes such as conversion of a liquid into solid or condensation of a gas into liquid release heat into the environment. Similarly, certain chemical reactions (e.g., respiration) are accompanied with release of heat.

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An activity to determine that moist air is important for rusting has been described below:

• Take a few rust-free iron nails in test tube A. Pour some water on the nails, so that the nails are submerged.
• Boil some distilled water over the burner in test tube B, so that no dissolved air is left in the water. Put some iron nails and cork the test tube after pouring some oil on the water surface to completely cut off oxygen supply.
• Put some anhydrous calcium chloride in test tube C, so that the air inside the test tube is moisture-free. Put a perforated disc in the test tube, put some nails over the disc and cork the test tube.
Observation: The nails in test tube A get rusted because of presence of both moisture and air. The nails in test tube B remain rust-free because of the absence of air. The nails in test tube C are rust-free because of the absence of moisture. Therefore, there is no rusting in test tubes B and C.

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(c) the burning of kerosene
Burning of kerosene is a chemical change. Thus, it is irreversible.

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(b) the melting of wax   
The melting of wax is a reversible change because on reversing the conditions, molten wax solidifies.

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(d) rusting
Rusting is a chemical change in which the metal surface gets eaten away and forms rust due to the presence of moisture and air.

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(c) moist air
Both air and water are important for rusting to take place.

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(d) photosynthesis
Photosynthesis takes place by absorption of light.

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A B
(a) Neutralisation reaction (iv) Formation of sodium chloride and water when hydrochloric acid reacts with sodium hydroxide
(b) Displacement reaction (iii) Deposition of copper when an iron nail is placed in a solution of copper(II)sulphate
(c) Decomposition reaction (ii) Formation of carbon dioxide when baking soda is heated
(d) Combination reaction (i) Burning of carbon



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