Class 7 Science
Chapter 6 – Physical and Chemical Changes
Very Important Questions Answers - Set 3
Q.1. What happens in a displacement reaction?
(a) A more reactive element replaces a less reactive element.
(b) A less reactive element replaces a more reactive element.
(c) An element replaces one which is equally reactive.
(d) Any element can replace any other element with no relation to reactivity.
Ans: (a) A more reactive element replaces a less reactive element.
In a displacement reaction, a more reactive element replaces a less reactive element from its compound.
Q.2. A neutralization reaction is a
(a) decomposition reaction.
(b) displacement reaction.
(c) combination reaction.
(d) double displacement reaction.
Ans: (d) double displacement reaction.
A neutralisation reaction is a double displacement reaction in which an acid reacts with a base, and the radicals are exchanged, to form salt and water.
Q.3. An oxidation reaction involves addition of
(a) oxygen only
(b) hydrogen only
(c) oxygen or removal of hydrogen
(d) hydrogen or removal of oxygen
Ans: (c) oxygen or removal of hydrogen
An oxidation reaction involves addition of oxygen or removal of hydrogen from a substance.
Q.4. If which of the following reaction is there only one reactant?
(a) combination reaction
(b) decomposition reaction
(c) single displacement reaction
(d) double displacement reaction
Ans: (b) decomposition reaction
In a decomposition reaction, a compound (reactant) breaks up into two or more substances (products).
Q.5. If A is more reactive than B, which of the following is true?
(a) A will replace B from a salt of B.
(b) B will replace A from a salt of A.
(c) Both can displace each other from their salts.
(d) Both cannot displace each other from their salts.
Ans: (a) A will replace B from a salt of B.
The given reaction is an example of displacement reaction in which a more reactive element replaces a less reactive element from its salt solution.
Q.6. Which of these is a chemical change?
(a) A substance with different chemical properties is formed.
(b) A substance in a different state but similar chemical properties, is formed.
(c) The molecules of the substances do not undergo any change.
(d) A substance with a different shape and size but similar chemical properties, is formed.
Ans: (a) A substance with different chemical properties is formed.
A chemical change occurs when two substances react chemically to form new substances with different chemical properties.
Q.7. Which of these is an irreversible physical change?
(a) rusting of iron
(b) freezing of water
(c) a double decomposition reaction
(d) breaking of a glass tumbler
Ans: (d) breaking of a glass tumbler
Breaking of a glass tumbler is an irreversible physical change. Rusting of iron is a chemical change. Freezing of water is a reversible physical change. A double decomposition reaction is a chemical change.
Q.8. An arrow pointing downwards in a chemical reaction shows
(a) evolution of gas
(b) formation of precipitate
(c) oxidation
(d) displacement
Ans: (b) formation of precipitate
In a chemical reaction, an arrow pointing downwards shows the formation of a precipitate and an arrow pointing upwards shows the evolution of a gas.
Q.9. A chemical reaction in which heat is given out is called
(a) exothermic reaction
(b) endothermic reaction
(c) decomposition reaction
(d) neutralization reaction
Ans: (a) exothermic reaction
Exothermic reactions are accompanied with release of heat. Endothermic reactions are accompanied with absorption of heat.
Q.10. In a combination reaction how many products are formed?
(a) two only
(b) one or two only
(c) one only
(d) number cannot be specified
Ans: (c) one only
In a combination reaction, a compound is formed from its elements. For example, water and oxygen combine to form water.
Q.11. In which changes do substances undergo chemical transformation-physical or chemical?
Ans: In chemical changes, substances undergo chemical transformation to form new substances.
Q.12. Rush is a mixture and does not have a chemical formula. True or false?
Ans: False.
Rust is a compound. Its chemical formula is Fe2O3.H2O.
Q.13. Salt is obtained from sea water by crystallization. True or false?
Ans: False.
Salt is obtained from sea water by evaporation.
Q.14. A supersaturated solution of a solid in a liquid crystallizes easily. True or false?
Ans: True.
A supersaturated solution contains more solute than it can remain dissolved in water. It is highly unstable and crystallises easily.
Q.15. The process of depositing zinc on iron to prevent rusting is called__________
Ans: The process of depositing zinc on iron to prevent rusting is called galvanization.
Q.16. When two soluble substances in solution react to give an insoluble substance, a _________ is formed.
Ans: When two soluble substances in solution react to give an insoluble substance, a precipitate is formed.
Q.17. Heat is absorbed in an _________ reaction.
Ans: Heat is absorbed in an endothermic reaction.
Q.18. An oxidizing agent provides ________ or removes _________ from a substance.
Ans: An oxidising agent provides oxygen or removes hydrogen from a substance.
Q.19. In which of these do the molecules of substances undergo changes–physical change or chemical change?
Ans: In chemical change molecules of substances undergo changes, as in a chemical change molecules reacts with other molecules. Sometimes they combine with other molecules and sometimes they degenerate into smaller molecules. While in a physical change, only the shape size, and colour of the substance changes, which does not affect the molecular structure of the substance. Hence, we can say in a chemical change, molecules of the substance undergo changes.
Q.20. What kind of change does this represent?
Reactants → Products
Ans: The given representation shows a chemical reaction, in which reactants are converting into products.
Q.21. What kind of reaction is
(a) melting of wax
(b) burning of wax?
Ans:
(a) Melting of wax is an example of physical change.
In a physical change, the substance undergoes only change in physical properties such as shape size, colour, and state.
(b) Burning of wax is an example of chemical change.
In a chemical change, reactants get converted to give new products/substances.
Q.22. Two chemicals in solution form react, forming an insoluble solid that settles down. What is the settled solid known as?
Ans: When two chemicals react in their solution form, an insoluble solid is formed which settles down to the bottom. That insoluble solid is known as a 'precipitate'.
Q.23. In which reaction will heat be released–exothermic reaction or endothermic reaction?
Ans: Heat is released in exothermic reactions.
Exothermic reactions are those reactions in which heat is released.
e.g., C + O2 → CO2 + Heat
Endothermic reactions are those reactions in which heat is absorbed.
e.g., C + 2S + Heat → CS2
Q.24. Nitrogen + oxygen → nitrogen dioxide
What type of reaction is this?
Ans: Given reaction is an example of a combination reaction.
When two or more elements combine together to give a compound, then it is known as a combination reaction.
E.g., N2 + 2O2 → 2NO2
Q.25. What is the characteristic of the reaction:
MN + PQ → MQ + PN
What is the general name given to such reactions?
Ans: Given reaction is an example of double displacement reaction.
MN + PQ → MQ + PN
In double displacement reaction, compounds react by exchanging their elements or groups.
All neutralization reactions are an example of double displacement reaction.
Q.26. Which of the following is true for a substance that is oxidized in a chemical reaction–oxygen is added to it, hydrogen is removed from it, or both are true?
Ans: Oxidation can be defined as the addition of oxygen or removal of hydrogen from the substance. So, both are true for the oxidized substance in a chemical reaction.
Q.27. The atoms or molecules of a solid are always arranged in a regular pattern. Do you agree? Give reasons.
Ans: Yes, atoms or molecules are arranged in a regular pattern.
Solids have a regular arrangement of atoms; they are tightly arranged and are not free to move.
Q.28. You are given a solution of a solid and a liquid. How can you obtain the solid from it?
Ans: We can separate a solid from a liquid by the process of 'Evaporation'. This process is based on the fact that liquids vaporize easily while solid do not. In this process, a solution of a solid and a liquid is heated, molecules of liquid start evaporating and solid substance starts to settle down in the jar. After complete evaporation of liquid, the solid substance can easily be extracted.
Q.29. What is the process of obtaining crystals of a substance from its solution in water called?
Ans: The process of obtaining crystals of a substance from its solution in water is called crystallization.
Q.30. Which gas is given off when calcium carbonate is heated?
Ans: When calcium carbonate is heated, calcium oxide is formed and carbon dioxide is evolved.
CaCO3 → CaO + CO2
Online Tuitions & Self-Study Courses for Grade 6 to 12 & JEE / NEET