Laws of chemical combination

Antoine Lavoisier is commonly credited with formulating this law: in a chemical reaction, the total mass remains constant when the reacting system is considered as a whole.

In a closed system, no matter enters or leaves. Therefore, the total mass of the reactants equals the total mass of the products; mass is neither created nor destroyed in the reaction.

A closed system does not exchange matter with its surroundings. Thus, the total mass of the reacting sample remains constant; in an open system, matter such as a gas may escape or enter and affect the measured mass.

Joseph Proust proposed the law of definite proportions: a pure chemical compound contains its constituent elements in a fixed proportion by mass, regardless of its source or preparation.

The law of definite proportions states that a given pure compound always contains the same elements in the same fixed ratio by mass, regardless of its source or preparation.

Water is a pure compound, H₂O, so its hydrogen-to-oxygen mass ratio is fixed. From the formula, the mass ratio is 2 × 1 : 16 = 1 : 8.

The governing concept is the law of multiple proportions, one of the laws of chemical combination. Dalton stated that when two elements form more than one compound, the masses of one element that combine with a fixed mass of the other occur in simple whole-number ratios. For example, carbon and oxygen form carbon monoxide and carbon dioxide; for a fixed amount of carbon, the oxygen masses combine in a simple 1:2 ratio. Therefore, option C, Dalton, is correct. Proust is associated with the law of definite proportions, Lavoisier with conservation of mass, and Rutherford with the nuclear model of the atom. Distinguishing these scientists by their laws prevents confusion in basic chemistry.

The law applies when the same two elements form different compounds. For a fixed mass of one element, the masses of the other element in those compounds are in a simple whole-number ratio.

For a fixed mass of one element, the different masses of the other element that combine with it in different compounds are in a simple whole-number ratio. This is the defining relationship in the law of multiple proportions.

The direct answer is A: Richter. The law of reciprocal proportions is associated with Jeremias Benjamin Richter. To understand the idea, imagine three elements, X, Y and Z. If a fixed mass of X combines separately with fixed masses of Y and Z, then the masses of Y and Z are related in a simple whole-number ratio to the masses in which Y and Z combine with each other. It is a law about quantitative mass relationships in chemical combinations. Option A is correct because Richter proposed this law. Option B, Proust, is associated with the law of definite proportions, which says that a pure compound always contains its elements in a fixed mass ratio. Option C, Gay-Lussac, is associated with combining volumes of gases, especially when gases react in simple volume ratios. Option D, Boyle, is associated mainly with the pressure-volume relationship of gases, expressed as Boyle's law. These scientists may all appear in chemistry questions, so attach each name to its distinctive law. Memory cue: Richter = reciprocal proportions; Proust = definite proportions; Gay-Lussac = gas volumes; Boyle = pressure and volume.

The law involves three elements: two elements combine separately with a fixed mass of a third, and their masses are compared with the ratio in which they combine with each other.

The governing concept is the law of combining volumes, also called Gay-Lussac’s law of gaseous volumes. It states that when gases react at the same temperature and pressure, their volumes and the volume of any gaseous product are in simple whole-number ratios. For example, two volumes of hydrogen combine with one volume of oxygen to form two volumes of water vapour under suitable conditions. This law was proposed by Joseph Louis Gay-Lussac, so option C is correct. Dalton is associated with atomic theory, Lavoisier with conservation of mass and oxygen-related studies, and Proust with the law of definite proportions; those contributions explain why the other options are distractors.

Gay-Lussac's law compares the volumes of reacting gases and gaseous products when they are measured at the same temperature and pressure. Under these conditions, the volumes are in simple whole-number ratios.

At the same temperature and pressure, the volumes of reacting gases and gaseous products are in simple whole-number ratios. For example, 2 volumes of hydrogen react with 1 volume of oxygen to form 2 volumes of water vapour.

According to the law of conservation of mass, the total mass of reactants equals the total mass of products in a chemical reaction. Thus, mass is neither created nor destroyed. Option B states the law of definite proportions, while option C refers to the law of multiple proportions. Exam tip: compare the total mass before and after a reaction to identify this law.

The law of definite proportions states that a given pure compound always contains its elements in the same fixed mass ratio, regardless of its source or method of preparation.

CO and CO₂ are two compounds made from the same two elements, carbon and oxygen. For a fixed mass of carbon, the masses of oxygen in the two compounds are in a simple whole-number ratio (16:32 = 1:2), illustrating the law of multiple proportions.

The law of definite proportions states that a pure compound always contains the same elements in the same fixed ratio by mass. In water, hydrogen and oxygen have a mass ratio of 1:8.

The law of conservation of mass states that mass is neither created nor destroyed in a chemical reaction. Therefore, in a closed system, the total mass of the reactants equals the total mass of the products.

The governing concept is the law of conservation of mass: in a chemical reaction, matter is neither created nor destroyed, although it may change form. If a gas escapes from an open container, the balance measures only the material left inside, so the observed mass may decrease even though the total mass of the reacting system has remained constant. A closed vessel keeps the gaseous products within the measured system. Therefore, option B is correct. An open vessel allows matter to leave, while stopping temperature measurement or observing colour does not prevent mass loss and cannot test conservation quantitatively. The comparison should also be made using the total mass before and after reaction, with the same sealed apparatus.

Dalton used the laws of chemical combination, such as conservation of mass and definite proportions, to develop his atomic theory. These laws helped explain chemical reactions in terms of atoms combining and rearranging.

The direct answer is A: the law of definite proportions is valid for a pure compound. A compound is a substance in which elements are chemically combined in a fixed way. Therefore, every pure sample of a particular compound contains the same elements in the same fixed mass ratio. For example, pure water contains hydrogen and oxygen in a constant mass ratio of 1:8, even if the sample is large or small. A is correct. B is wrong because a mixture contains substances physically combined in variable amounts, so its composition can change; the law does not apply to any mixture. C is wrong because the law is not limited to metal mixtures, and a metal mixture is often an alloy or mixture rather than a definite compound. D is wrong because soil is a heterogeneous mixture with changing composition. The key is the word pure: fixed composition belongs to a pure compound, not to a mixture. Memory cue: compound means constant composition; mixture means variable composition.

Salt water is a mixture, not a pure compound. The amount of salt dissolved in a given amount of water can vary, whereas the law of definite proportions applies to the fixed composition of a pure compound.

Pure water is a compound with the formula H₂O. Its hydrogen-to-oxygen mass ratio is always 2:16, or 1:8, which illustrates the law of definite proportions.

The law of multiple proportions applies when the same two elements form more than one compound. For a fixed mass of one element, the masses of the other element in those compounds are in a simple whole-number ratio.