Description:
Q1: What happens in the reaction flask?
Ans: When the solutions of X and Y are mixed, a chemical reaction occurs, often forming a precipitate. For example:
Q2: Do you think a chemical reaction has taken place?
Ans: Yes, a chemical reaction has occurred, as evidenced by the formation of a precipitate or other visible changes, indicating the formation of new substances.
Q3: Why should we put a cork on the mouth of the flask?
Ans: The cork prevents any gaseous products or volatile substances from escaping, ensuring that the total mass of the system (reactants and products) remains unchanged during the reaction.
Q4: Does the mass of the flask and its contents change?
Ans: No, the mass of the flask and its contents does not change after mixing. The weight before and after the reaction is the same.
Explanation: The activity demonstrates the Law of Conservation of Mass, which states that mass is neither created nor destroyed in a chemical reaction. The total mass of the reactants (solutions X and Y) equals the total mass of the products (precipitate and remaining solution). The cork ensures no mass is lost as gas, and the formation of a precipitate confirms a chemical reaction. For example, in the reaction of copper sulphate and sodium carbonate, the precipitate (copper carbonate) forms, but the total mass remains constant, supporting the law.
Description:
Refer to Table 3.4 for the ratio by mass of atoms present in molecules and Table 3.2 for atomic masses of elements.
Find the ratio by number of atoms of elements in the molecules of compounds given in Table 3.4 (water, ammonia, carbon dioxide).
Example provided for water:
Ans: To calculate the ratio by number of atoms for the compounds listed in Table 3.4: water (H₂O), ammonia (NH₃), and carbon dioxide (CO₂).
Calculations:
Water (H₂O):
Ammonia (NH₃):
Carbon Dioxide (CO₂):
Description:
Example 1: Make playcards with symbols and valencies of elements separately. Each student holds two placards: one with the symbol in the right hand and one with the valency in the left hand. Students criss-cross their valencies to form the formula of a compound while keeping symbols in place.
Example 2: Use empty blister packs of medicines, cut into groups according to the valency of the element. Make formulae by fixing one type of ion into another.
Ans: Formula for Sodium Phosphate:
Ions involved:
Criss-cross method:
Explanation using blister pack model:
General Explanation: The activity teaches students to write chemical formulae by applying the concept of valency and the criss-cross method. The placard game visually represents how atoms combine based on their valencies, while the blister pack model simulates the “locking” of ions to form neutral compounds. For sodium phosphate, the formula Na₃PO₄ reflects the combination of three sodium ions with one phosphate ion to achieve charge neutrality, aligning with the rules for writing chemical formulae.
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1. What are the basic rules for writing chemical formulae? | ![]() |
2. How do you determine the valency of an element when writing a formula? | ![]() |
3. What is the significance of using subscripts in chemical formulae? | ![]() |
4. Can you explain the difference between empirical and molecular formulae? | ![]() |
5. What strategies can be used to help students learn to write chemical formulae effectively? | ![]() |