Laws of chemical combination

The laws of chemical combination are basic rules that describe how substances combine or react to form new substances. These laws were established in the 18th and 19th centuries to explain chemical reactions. Here’s a simple and clear explanation of each law:

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1. Law of Conservation of Mass

• What it says: Mass can neither be created nor destroyed during a chemical reaction. The total mass of reactants equals the total mass of products.
• Example:
  When you burn wood, the mass of the wood and oxygen equals the mass of ash, gases, and smoke produced. Mass of Reactants=Mass of Products

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2. Law of Definite Proportions (or Constant Composition)

• What it says: A chemical compound always contains the same elements in the same proportion by mass, regardless of how it is prepared or where it comes from.
• Example:
  Water (H2OH_2OH2​O) is always made up of 2 parts hydrogen and 16 parts oxygen by mass (ratio 1:8), whether it’s from a river or a lab.

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3. Law of Multiple Proportions

• What it says: If two elements combine to form more than one compound, the masses of one element that combine with a fixed mass of the other are in simple whole-number ratios.
• Example:
  Carbon and oxygen form two compounds:
  • Carbon monoxide (CO): 12 g of carbon combines with 16 g of oxygen (1:1 ratio).
  • Carbon dioxide (CO₂): 12 g of carbon combines with 32 g of oxygen (1:2 ratio).

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4. Gay-Lussac’s Law of Gaseous Volumes

• What it says: When gases combine, they do so in simple whole-number ratios by volume, provided all gases are at the same temperature and pressure.
• Example:
  • Hydrogen gas + Oxygen gas → Water vapor
    2 volumes of hydrogen react with 1 volume of oxygen to produce 2 volumes of water vapor (2:1:2 ratio).

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5. Avogadro’s Law

• What it says: Equal volumes of gases, at the same temperature and pressure, contain an equal number of molecules.
• Example:
  1 liter of oxygen gas and 1 liter of nitrogen gas (at the same conditions) have the same number of molecules.

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Summary Table:

Law	Key Idea
Conservation of Mass	Mass is neither created nor destroyed in a reaction.
Definite Proportions	A compound always has the same elements in the same ratio.
Multiple Proportions	Elements combine in whole-number ratios to form compounds.
Gaseous Volumes (Gay-Lussac)	Gases react in simple volume ratios under same conditions.
Avogadro’s Law	Equal volumes of gases = Equal number of molecules.

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These laws are the foundation of modern chemistry.