Laws of chemical combination

According to the law of conservation of mass, the total mass in a closed system remains the same before and after a chemical reaction. Hence, the total mass of reactants equals the total mass of products. In an open system, measured mass may appear lower if a gas escapes, but mass is still conserved when the escaped gas is included. Exam tip: look carefully for the terms “total mass” and “closed system.”

When a chemistry question gives volumes of reacting gases in litres and specifies the same temperature and pressure, the first relevant idea is Gay-Lussac’s law of combining volumes. This law states that gaseous reactants and gaseous products combine in simple whole-number volume ratios when measured at the same temperature and pressure. For example, two volumes of hydrogen combine with one volume of oxygen to form two volumes of steam under suitable conditions. Option A is therefore the best first choice. Definite proportions concerns mass composition, reciprocal proportions compares elements, and conservation of mass concerns total mass.

The correct answer is A. The law of definite proportions states that a pure compound always contains the same elements in the same mass proportions, so its element mass percentages are the same in different samples.

The law of multiple proportions is tested by comparing two different compounds made from the same two elements. Carbon and oxygen form several compounds, including carbon monoxide and carbon dioxide. For a fixed mass of carbon, the masses of oxygen in these compounds are in a simple whole-number ratio, such as 1:2, which is the defining observation of the law. Thus option A provides the required comparison. Testing the purity of one water sample concerns composition or analysis, not multiple proportions. The mass of a vessel and the lustre of a metal do not involve two compounds formed by the same pair of elements.

The direct answer is option A: total mass is equal, not the mass of each individual substance. Conservation of mass means that in a closed chemical reaction, the sum of the masses of all reactants equals the sum of the masses of all products. It does not mean that every product has the same mass as every reactant. For example, several reactants may combine to make one product, so the product mass can equal their combined mass while differing from each separate reactant mass. Option A correctly identifies the student's mistake: the word total is essential. Option B, no mass is equal, is wrong because total mass is conserved. Option C says only colour is equal; colour is not what the law conserves. Option D says gases should be removed, but removing a gas can make a system appear to lose mass and would violate the closed-system condition. Thus the student confused individual masses with the sum of all masses. Exam cue: conservation means total before equals total after, not each item before equals each item after.

The correct answer is A. Salt water is a mixture, not a pure compound. The amount of salt dissolved can vary, so salt water does not have a fixed composition as required by the law of definite proportions.

The correct answer is A. In a chemical reaction, the law of conservation of mass says the total mass remains unchanged. The law of definite proportions instead says that a pure compound has a fixed composition by mass.

The correct answer is A. Definite proportions describes the fixed mass composition of one pure compound. Multiple proportions compares different compounds made from the same two elements: the masses of one element that combine with a fixed mass of the other are in a simple whole-number ratio.

A is correct: the law of multiple proportions compares the masses of one element that combine with a fixed mass of another in different compounds. Gay-Lussac’s law of gaseous volumes compares volumes of reacting gases at the same temperature and pressure, which are in simple whole-number ratios.

A is correct. The law of definite proportions states that a pure compound always contains the same elements in the same fixed proportion by mass, regardless of the sample’s source or size.

A is correct. Gay-Lussac’s law of gaseous volumes states that reacting gas volumes, and the volumes of gaseous products, are in simple whole-number ratios when measured at the same temperature and pressure.

A is correct. In an open vessel, a gaseous product may escape and be left out of the measurement, making the remaining material seem lighter. If the escaped gas is included, total mass is conserved; in a closed system, all reactants and products must be accounted for.

Dividing both terms of the second ratio, 4:14, by 2 gives 2:7. Thus, the mass ratio of the elements is the same in both samples, in accordance with the law of definite proportions. Option C is incorrect because 4:14 is not a new ratio; it is a multiple form of the same ratio. Exam tip: reduce ratios to their simplest form before comparing them.

This is a direct numerical test of the law of multiple proportions. The mass of A is fixed at 4 g in both compounds, so only the masses of B need to be compared. Those masses are 5 g and 10 g. Their ratio is 5:10, which simplifies to 1:2, a simple whole-number ratio. Therefore, the data support the law of multiple proportions, and option A is correct. The law of definite proportions would concern the fixed composition of one particular compound. Gay-Lussac’s law compares gaseous volumes, not these masses, and option D is not a scientific law.

The volumes of gas A, gas B, and gas C are given as 2, 5, and 2 respectively. Therefore, the correct ratio is 2:5:2. The ratio 1:5:2 is incorrect because only the first term has been divided by 2; all terms must be divided by the same number to simplify a ratio. Exam tip: retain the given order of reactants and product while writing the ratio.

According to Gay-Lussac’s law of gaseous volumes, the volumes of reacting gases and gaseous products, measured at the same temperature and pressure, are in simple whole-number ratios. Therefore, volumes—not masses in grams—must be compared. Comparing masses at the same temperature and pressure is the closest distractor, but mass is not the basis of this law. Exam tip: whenever “gaseous volumes” is mentioned, check that temperature and pressure are the same.

The direct answer is option A: different compounds made from the same two elements are needed. The law of multiple proportions compares two or more compounds containing the same pair of elements. A fixed mass of one element combines with different masses of the second element, and those masses show a simple whole-number ratio. Two samples of the same compound do not create this comparison; they should have the same composition and therefore illustrate definite proportions instead. Option A gives the correct correction. Option B, one compound is enough, is wrong because one compound cannot show how the elements combine differently in another compound. Option C, a mixture, is wrong because a mixture is not a single compound with a fixed chemical composition. Option D, only gas volumes, describes the type of data used for gaseous-volume law, not multiple proportions. The logical steps are: identify the same two elements, find at least two distinct compounds, fix one element's mass, compare the other element's masses, and simplify the ratio. Memory cue: one pure compound means definite proportions; different compounds of the same two elements means multiple proportions.

A is correct. Laws of chemical combination summarize quantitative patterns observed through experiments, such as fixed mass ratios in compounds and conservation of mass in chemical reactions.

A is correct. In an ordinary chemical reaction in a closed system, matter is conserved: the total mass of the reactants equals the total mass of the products. Any gases formed must also be included in the products.

A is correct. Soil is a mixture, and its composition can vary from sample to sample. Pure compounds such as water, sodium chloride, and carbon dioxide have fixed proportions of their constituent elements by mass.

The law of multiple proportions applies when the same two elements form two or more different compounds. If the mass of one element is fixed, the masses of the other element that combine with it are in a simple whole-number ratio. Two identical samples of pure water are both merely samples of H₂O, so they provide only one compound and cannot demonstrate multiple proportions. Therefore option A is the wrong example. CO and CO₂, SO₂ and SO₃, and H₂O with H₂O₂ are pairs of distinct compounds made from the same two elements, so each can be used for comparison. The issue is not the physical number of samples but the existence of different compounds and combining ratios.

The direct answer is option A: volume ratios of gases measured at the same temperature and pressure. The law of gaseous volumes concerns the volumes of reacting gases and gaseous products, not unrelated physical properties. To solve this, first look for the word gas; next look for volume or volume ratio; finally check that the measurements use common temperature and pressure. Option A contains all three essential ideas, so it is the best example. Option B, the melting point of a solid, is a property of a solid and gives no reacting-gas volume ratio. Option C, the amount of sand in a mixture, concerns a mixture and not gaseous reaction volumes. Option D, the lustre of a metal, is an appearance property and is unrelated to this law. The law does not say that all physical properties have fixed values; it specifically describes simple volume relationships in gas reactions under comparable conditions. Therefore A is correct. Exam reminder: gas volume plus same temperature and pressure is the identifying pattern for Gay-Lussac's law.

The law of conservation of mass states that in a chemical reaction, the total mass of the reactants equals the total mass of the products. The question gives this equality, not a composition ratio.

A pure compound has a fixed composition: its elements occur in a constant ratio by mass. Therefore, the mass percentages of its elements are related to the law of definite proportions.

A reliable strategy is to identify what is being compared before selecting a law. If the question compares the masses of one element in two compounds made from the same elements, use multiple proportions. If one pure compound is discussed, a fixed mass composition points to definite proportions. If the question asks whether total mass changes during a reaction, use conservation of mass. If volumes of gases are measured at the same temperature and pressure, consider the law of gaseous volumes. Thus option A is correct because it uses units and wording to identify the governing concept. The other choices apply one rule blindly or ignore essential data.