We know that when elements combine to form a compound, the new substance often has completely different properties from the original elements.
Hydrogen and oxygen are both gases. Hydrogen serves as an energy-rich fuel that can power various applications, while oxygen plays a crucial role in supporting combustion, allowing fires to burn brightly. Interestingly, when these two gases combine, they form water - a compound that acts as an effective fire extinguisher.
Water as fire extinguisher
Even though properties change, the total mass of the substances remains the same before and after the reaction. Does mass remain unchanged during physical as well as chemical changes?
Let us explore whether the mass remains unchanged during physical and chemical changes, which led to important laws of chemical combination.
i. Physical change and mass conservation:
- A physical reaction does not produce any new substances.
- Although usually reversible in many aspects, it may be a slow process or even irreversible, like breaking a glass object or cutting paper into pieces.
- The changes generally occur only in the physical properties, such as size, shape, colour and state. The changes are involved in the form, not in the substance identity.
Activity:
Take \(5\ g\) of salt and dissolve in \(100\ g\) of water. Stir untill the salt dissolves completely. The salt is still present in the solution even though it is not visible.
If the total mass is measured:
Mass before mixing = \(5\ g + 100\ g = 105\ g\)
Mass after mixing = \(105\ g\)
Conclusion:
Even though the appearance changes, no new substance is formed. Also, the total mass remains the same. This shows that the mass is conserved in physical changes.
ii. Chemical change and mass conservation:
- A chemical reaction forms a new substance and it does not destroy matter.
- Chemical changes are irreversible.
- There are several indicators of a chemical change, including a change in chemical properties, a colour change, the evolution of gas, and the absorption or release of heat or light during the reaction.
Activity:
A chemical reaction is carried out between vinegar and baking soda. When vinegar is added to baking soda, a chemical reaction takes place immediately and following changes are observed,
- Rapid bubbling or effervescence
- Formation of a gas (carbon dioxide)
- Formation of new substances
\(\text{Baking soda} + \text{Vinegar} \to \text{Salt} + \text{Water} + \text{Carbon dioxide}\)
\(NaHCO_3 + CH_3COOH \to CH_3COONa + H_2O + CO_2\)
Case 1: Open system:
The reaction is carried out in an open container like conical flask.
Observation:
- Before reaction: total mass is measured.
- After reaction: mass appears to decrease.
Conclusion:
The decrease in mass is not due to loss of matter. It is because,
- The reaction produces carbon dioxide gas. This gas is released into the air.
- Since it escapes, it is not included in the final measurement.
Case 1: Closed system:
The same reaction is repeted in a closed setup, the flask is sealed with a balloon is attached to collect the gas.
Observation:
Mass before reaction = Mass after reaction
Conclusion:
The experiment demonstrates that,
- Total mass remains constant in a closed system
- Mass can neither be created nor destroyed in a chemical reaction.
Law of conservation of mass:
Scientist observed the chemical experiments and stated that the mass of the reactants at the beginning of a reaction would be equal to the mass of the products at the end of the reaction. Antoine Lavoisier is known as the Father of Modern Chemistry. He proposed the Law of Conservation of Mass. This law applies to every chemical reaction.
“The mass in an isolated system can neither be created nor destroyed but can be transformed from one form to another”
Law of conservation of mass
Example: In a group activity, students place \(4.0\ g\) of calcium carbonate with \(2.92\ g\) of hydrochloric acid in a closed container. After the reaction is over, they measured \(1.76\ g\) of carbon dioxide, \(0.72\ g\) of water, and \(4.44\ g\) of calcium chloride. Verify whether the Law of Conservation of Mass is obeyed or not.
Solution: Mass of calcium carbonate = \(4.0\ g\)
Mass of hydrochloric acid = \(2.92\ g\)
Total mass of reactants: \(4.0\ g + 2.92\ g = 6.92\ g\)
Mass of carbon dioxide = \(1.76\ g\)
Mass of water = \(0.72\ g\)
Mass of calcium chloride = \(4.44\ g\)
Total mass of products: \(1.76\ g + 0.72\ g + 4.44\ g = 6.92\ g\)
Mass of hydrochloric acid = \(2.92\ g\)
Total mass of reactants: \(4.0\ g + 2.92\ g = 6.92\ g\)
Mass of carbon dioxide = \(1.76\ g\)
Mass of water = \(0.72\ g\)
Mass of calcium chloride = \(4.44\ g\)
Total mass of products: \(1.76\ g + 0.72\ g + 4.44\ g = 6.92\ g\)
Law of constant (definite) proportion:
The elements are always present in definite proportions by mass in a chemical substance.
The great scientist Lavoisier, along with other scientists, observed that compounds are composed of two or more elements; and each such compound had the same elements in the same proportions, irrespective of where the compound came from or who prepared it.
In water (\(H_2O\)), the ratio of the mass of hydrogen to oxygen is always \(1:8\), whether it is tap water, rainwater, or distilled water.
Law of constant proportion
Example: Sodium chloride (\(NaCl\)) contains sodium and chlorine in the mass ratio of \(23:35.5\). If \(46\ g\) of sodium reacts completely, how much chlorine is needed to form \(NaCl\)?
Solution: Mass of chlorine required = \((35.5 ÷ 23) × 46\) = \(71\ g\)
This is known as the Law of Constant Proportions,or the Law of Definite Proportions, or sometimes as Proust’s Law.