01 · Explore
The Law of Conservation of Mass
In any chemical or physical change, the total mass of the substances involved remains constant. This fundamental principle was proposed by Antoine Lavoisier in 1789.
The Law of Conservation of Mass states that matter can neither be created nor destroyed in a chemical reaction. This means the total mass of the reactants (the starting substances) must equal the total mass of the products (the substances formed).
In a physical change, such as dissolving salt in water, the mass of the solution is exactly equal to the sum of the masses of the water and the salt. In chemical reactions, this is also true, though it can be harder to measure if gases are involved. If a gas is produced and allowed to escape, the final mass may appear lower, but if the reaction is contained in a closed system, the mass remains unchanged.
A classic demonstration involves the reaction between sodium sulfate and barium chloride. When these two solutions are mixed, they form a white precipitate of barium sulfate and sodium chloride. If the total mass is measured before and after mixing, it remains identical because no atoms have been lost or gained; they have simply rearranged.
Verifying Mass Conservation
Calcium Carbonate (4.0 g) + Hydrochloric Acid (2.92 g) → Calcium Chloride (4.44 g) + Water (0.72 g) + Carbon Dioxide (1.76 g)
Total mass of reactants = 4.0 g + 2.92 g = 6.92 g. Total mass of products = 4.44 g + 0.72 g + 1.76 g = 6.92 g. Since the mass of reactants equals the mass of products, the law is obeyed.
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Question
If 12 g of carbon reacts with 32 g of oxygen to form carbon dioxide, what is the mass of the carbon dioxide produced?
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02 · Explore
The Law of Constant Proportions
Also known as the Law of Definite Proportions, this principle states that a chemical compound always contains the same elements in a fixed ratio by mass.
Proposed by Joseph Proust, this law explains that the composition of a pure chemical compound is independent of its source or method of preparation. Whether water is taken from a river, a well, or synthesized in a lab, it will always consist of hydrogen and oxygen in a mass ratio of 1:8.
This means that if you decompose 9 g of water, you will always obtain 1 g of hydrogen and 8 g of oxygen. Similarly, Proust studied copper carbonate and found it always contained copper, carbon, and oxygen in the same proportions by mass, regardless of whether it was found in nature or prepared in a laboratory.
If the ratio of elements provided does not match this fixed proportion, the excess amount of one element will remain unreacted. This distinguishes compounds from mixtures, where components can be mixed in any ratio.
Calculating Reactant Mass
Condition: The ratio 23:35.5 must be maintained.
Sodium : Chlorine = 23 : 35.5 (for Sodium Chloride)
If 46 g of sodium is reacted to form NaCl, the mass of chlorine needed is calculated by the ratio. Since 23 parts sodium require 35.5 parts chlorine, 46 g sodium (23 × 2) requires 71 g chlorine (35.5 × 2).
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Question
Two students prepare copper oxide. Student A uses a 4:1 ratio of copper to oxygen, and Student B uses an 8:2 ratio. Do they follow the Law of Constant Proportions?
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03 · Explore
Dalton's Atomic Theory
John Dalton provided a theoretical framework in 1808 to explain the laws of chemical combination using the concept of atoms.
Dalton proposed that all matter is made of tiny, indivisible particles called atoms. These atoms cannot be created or destroyed in a chemical reaction, which explains the Law of Conservation of Mass. He also stated that atoms of a given element are identical in mass and properties, while atoms of different elements differ in these aspects.
The theory further explains the Law of Constant Proportions by postulating that atoms combine in the ratio of simple whole numbers to form compounds. In any given compound, the relative number and kinds of atoms are constant. This ensures that the mass ratio of the elements remains fixed.
A postulate is a fundamental assumption accepted as truth without formal proof. Dalton's postulates served as the basis for modern scientific theory, transforming chemistry from a qualitative study into a quantitative science.
Dalton's Logic for Chemical Laws
- 1
Indivisible Atoms
Atoms are the smallest units and cannot be broken down.
- 2
Rearrangement
Chemical reactions involve only the separation, union, or rearrangement of atoms.
- 3
Mass Conservation
Since atoms are not created or destroyed, total mass remains constant.
How Dalton's postulates provide a physical explanation for experimental chemical laws.
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Which postulate of Dalton's theory explains the Law of Constant Proportions?
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04 · Explore
Covalent Bonding: Sharing Electrons
Atoms often achieve stability by sharing electrons with other atoms, forming what is known as a covalent bond.
A molecule is an electrically neutral group of two or more atoms held together by chemical bonds. Atoms with fewer than eight electrons in their valence shell (or two for the K-shell) are unstable. To reach a stable 'octet' or 'duplet' configuration, they share electrons.
When two atoms share one pair of electrons, a single covalent bond is formed (represented as a single line, e.g., H—H). If they share two pairs, a double bond is formed (O=O). This sharing allows each atom to 'count' the shared electrons toward its own stable outer shell.
Covalent bonds typically form between non-metal atoms. For example, in a water molecule (H₂O), one oxygen atom shares electrons with two separate hydrogen atoms so that oxygen completes its octet and each hydrogen completes its duplet.
Formation of a Covalent Bond
- 1
Individual Atoms
Two atoms (like Hydrogen) each have an incomplete outer shell with one valence electron.
- 2
Electron Sharing
The atoms move close together so their outer shells overlap, sharing a pair of electrons.
- 3
Stable Molecule
The shared pair attracts both nuclei, and both atoms now count the shared electrons toward a full outer shell.
Covalent bonding involves the sharing of electron pairs between atoms to achieve stability.
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How many electrons are shared in a double covalent bond?
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05 · Explore
Naming Covalent Compounds
Covalent compounds are named using a systematic prefix system that indicates the number of atoms of each element present in the molecule.
To name a binary covalent compound, the first element's name is kept as is, and the second element's name is modified to end in '-ide'. Prefixes like mono- (1), di- (2), tri- (3), tetra- (4), penta- (5), and hexa- (6) are used to specify the quantity of atoms.
There are specific rules for these prefixes: 'mono-' is usually omitted for the first element. If a prefix ends in 'a' or 'o' and the element starts with a vowel, the last letter of the prefix is dropped (e.g., 'monoxide' instead of 'monooxide'). However, if the prefix ends in 'i' (like di- or tri-), it is retained.
When hydrogen is the first element, prefixes are generally not used (e.g., H₂S is hydrogen sulfide). Some compounds are known by common names, such as water (H₂O) and ammonia (NH₃). For example, SF₆ is named as sulfur hexafluoride, showing six fluorine atoms, and N₂O₄ is named as dinitrogen tetroxide.
| Formula | Systematic Name | Common Name |
|---|---|---|
| CO | Carbon monoxide | n/a |
| CO₂ | Carbon dioxide | n/a |
| CS₂ | Carbon disulfide | n/a |
| PCl₃ | Phosphorus trichloride | n/a |
| SF₆ | Sulfur hexafluoride | n/a |
| N₂O₄ | Dinitrogen tetroxide | n/a |
| N₂O₅ | Dinitrogen pentoxide | n/a |
| NH₃ | Nitrogen trihydride | Ammonia |
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What is the systematic name for SF₆?
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