Exemplar Questions and Solutions

Important CBSE Questions - 2 Mark Questions

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13) What is the difference between chemical and physical change?

Answer: A chemical change is a process where one or more substances are converted into new substances with different properties. On the other hand, a physical change is a process that does not change the chemical composition of a substance. For example, rusting of iron is a chemical change, while melting of ice is a physical change.

14) What is a chemical reaction? Give one example.

Answer: A chemical reaction is a process in which one or more substances undergo a chemical change to form new substances. For example, the reaction between iron and oxygen to form rust is a chemical reaction.

15) Define combination reaction. Give two examples.

Answer: In a combination reaction, two or more substances combine to form a single product. For example:

16) What is galvanised iron? How does galvanizing protect iron from rusting?

Answer: Galvanised iron is iron that has been coated with a layer of zinc to protect it from corrosion. Zinc acts as a sacrificial anode, preventing the iron from coming into contact with oxygen and moisture, which are necessary for rusting to occur.

17) State one characteristic each of a chemical reaction which take place when:

Answer:
- A new substance is formed
- Energy is absorbed or released

18) What is the difference between a displacement reaction and a double displacement reaction? Support your answers using examples.

Answer: In a displacement reaction, a more reactive element displaces a less reactive element from its compound. For example:

In a double displacement reaction, ions in two compounds exchange places to form two new compounds. For example:

19)Give two characteristics of chemical reaction which take place when dilute sulphuric acid is poured over Zinc granules:

Answer:
- Evolution of hydrogen gas
- Formation of zinc sulphate

20)When hydrogen burns in oxygen, water is formed and when water is electrolyzed, then hydrogen and oxygen are produced. What type of reaction takes place:

Answer:
- In the first case?
- In the second case?
- First case: Combination reaction
- Second case: Decomposition reaction

21) List 5 Observations to Determine Chemical Reactions

Answer:

• Change in Color: One of the most common indicators of a chemical reaction is a noticeable change in color. For example, when iron rusts, it forms a reddish-brown color.
• Formation of a Precipitate: When two solutions are mixed, and a solid substance forms, it indicates a chemical reaction. An example is when silver nitrate reacts with sodium chloride to form a white precipitate of silver chloride.
• Evolution of Gas: If bubbles or gas is released during a reaction, it signifies a chemical change. For instance, when hydrochloric acid reacts with calcium carbonate, carbon dioxide gas is evolved.
• Change in Temperature: Endothermic or exothermic reactions can be identified by a change in temperature. Heating magnesium in the presence of oxygen results in a temperature increase, indicating a chemical reaction.
• Change in State: When a substance changes from a solid to a liquid or gas, or vice versa, it points towards a chemical reaction. An example is when ice melts to form water.

22) Why Dilute Sulphuric Acid is Added during Electrolysis of Water?

Answer: During the electrolysis of water, dilute sulphuric acid is added to facilitate the process. This is because pure water is a poor conductor of electricity due to the absence of ions. By adding dilute sulphuric acid, it helps in the dissociation of water molecules into ions, allowing the flow of electricity to occur more efficiently.

23) Skeletal Equation and its Importance

Answer: A skeletal equation is a chemical equation that represents the reactants and products of a reaction without indicating the exact numbers of atoms involved. It provides a simplified representation of the reaction. For example, the skeletal equation for the combustion of methane is:

CH4 + O2 → CO2 + H2O

Balancing the skeletal equation is crucial to ensure the conservation of mass. It helps in determining the correct stoichiometry of the reaction by ensuring that the number of atoms of each element is the same on both sides of the equation.

24) Importance of Balancing a Skeletal Equation

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