Laws of chemical combination

The governing concept is the law of multiple proportions. CO and CO₂ are two different compounds formed by the same two elements, carbon and oxygen. In CO, one fixed amount of carbon combines with a certain amount of oxygen; in CO₂, the same amount of carbon combines with twice that amount of oxygen. The oxygen masses therefore occur in a simple whole-number ratio of 1:2, demonstrating multiple proportions. Option A is correct. Each compound separately also has a definite composition, but comparing the two compounds reveals the stronger and more specific evidence for the law of multiple proportions. They are not mixtures.

Multiple proportions applies to pure compounds not arbitrary mixtures. Exam tip: eliminate mixture option.

The mass ratio is 6:12. Dividing both terms by their greatest common factor, 6, gives 1:2. The ratio 2:1 is the reverse ratio, so it is not correct. Exam tip: always reduce a ratio to the smallest whole numbers.

The mass ratio is 8:24. Dividing both terms by their greatest common factor, 8, gives 1:3. Therefore, 1:3 is correct. The ratio 3:1 is the reverse ratio, while 2:3 is not equivalent to 8:24. Exam tip: simplify a ratio by dividing both terms by the highest common factor.

Phosphorus and chlorine form two different compounds. Exam tip: look for same two elements different formulas.

The direct answer is A. The clues are ‘litres’ and the same temperature and pressure. Gay-Lussac's law of gaseous volumes states that reacting gas volumes, measured at the same temperature and pressure, bear simple whole-number ratios. A is correct because it directly matches both the quantity and the conditions. B, definite proportions, concerns the fixed mass ratio of elements in a pure compound, not specifically gas volumes. C, multiple proportions, compares different compounds made from the same elements. D, reciprocal proportions, concerns combining ratios among elements and compounds. The exam method is: gas volume plus same temperature and pressure → Gay-Lussac. Memory cue: same T and P, compare gas litres.

According to Gay-Lussac's law of combining volumes, at the same temperature and pressure, the volumes of reacting gases and gaseous products are in simple whole-number ratios. A ratio of 1:1:2 fits this law; for example, \(\mathrm{H_2 + Cl_2 \rightarrow 2HCl}\) has volumes in the ratio 1:1:2. The law of definite proportions concerns mass ratios, not gaseous volume ratios. Exam tip: a simple gas-volume ratio at the same temperature and pressure indicates the law of combining volumes.

The governing idea is that gas-volume comparisons require controlled conditions. In the law of gaseous volumes, reacting gas volumes are compared at the same temperature and pressure; otherwise heating, cooling, compression, or expansion can change the measured volume independently of the reaction. Thus the student’s mistake is ignoring variables that affect gas volume, making the comparison unreliable. Option A is correct. Option B is false because gases are highly compressible and expand on heating, option C denies the relevant physical dependence, and option D confuses a gas law with the behaviour of solids. The condition words are therefore essential.

The ratio of the given volumes is 2:4. Dividing both terms by 2 gives the simplest ratio, 1:2; therefore, option A is correct. The ratio 2:1 results from reversing the order, while 1:4 does not represent the given volumes. Exam tip: keep the stated order of quantities and reduce the ratio to its simplest whole-number form.

The ratio of the volumes is 6:9. Dividing both terms by their highest common factor, 3, gives 2:3. The ratio 3:2 reverses the order, while 1:3 and 2:9 are not equivalent to the given volumes. Exam tip: first ensure that quantities have the same unit, then divide both terms by their HCF to obtain the simplest ratio.

The reaction is \(2H_2 + O_2 \rightarrow 2H_2O\). At the same temperature and pressure, reacting gas volumes are in the ratio of their coefficients. Therefore, hydrogen : oxygen is \(2:1\). The ratio \(1:2\) represents the reverse order. Exam tip: always follow the order of substances stated in the question when writing a ratio.

The direct answer is option A: 1:3. A ratio compares quantities in the same order in which they are stated. Here the first quantity is nitrogen, with 1 volume, and the second is hydrogen, with 3 volumes. Step 1: write nitrogen volume = 1. Step 2: write hydrogen volume = 3. Step 3: form nitrogen : hydrogen = 1:3. Option A is therefore correct. Option B, 3:1, reverses the order and would mean hydrogen:nitrogen rather than nitrogen:hydrogen. Option C, 1:1, incorrectly says the volumes are equal. Option D, 2:3, changes the given nitrogen amount without any reason. This question asks for the stated volume ratio, not a balanced chemical equation or the volume of product formed. Memory cue: keep the names in the question order, then attach their numbers.

Gay-Lussac's law is linked with gas volumes and same conditions. Exam tip: check both volume and condition.

The law of conservation of mass states that the total mass of reactants equals the total mass of products in a chemical reaction; mass is neither created nor destroyed. Gay-Lussac's law of combining volumes states that, at the same temperature and pressure, volumes of reacting gases and gaseous products are in simple whole-number ratios. Option B refers to the law of definite proportions, not either of these laws. Exam tip: associate equality of mass with conservation of mass and gas-volume ratios with Gay-Lussac's law.

Definite proportions relates to compound composition and conservation relates to total reaction mass. Exam tip: keep bases separate.

Multiple proportions relates to mass and Gay-Lussac relates to gas volume. Exam tip: identify grams and litres.

Fixed percentage composition of a pure compound indicates definite proportions. Exam tip: look for pure and percentage clues.

The direct answer is A, conservation of mass. This law states that in a chemical reaction matter is not created or destroyed. At the level of a closed reaction, the total mass of all reactants equals the total mass of all products. The reasoning is simple: reactants are the substances present before the reaction, and products are the substances formed after it. If the question specifically compares their total amounts or masses and says they are equal, it is pointing to conservation of mass. Option A is correct because equality of total reactant and product mass is its central idea. Option B, definite proportions, deals with the fixed ratio of elements inside one pure compound, not with total reactant and product mass. Option C, multiple proportions, compares different compounds made by the same two elements, where masses combine in small whole-number ratios. Option D, gaseous volumes, concerns volume relations of reacting gases at the same temperature and pressure. Do not confuse equal mass with equal composition. Exam cue: words such as total mass, before reaction, after reaction, neither created nor destroyed, and reactants equal products usually indicate conservation of mass.

Different compounds of the same two elements are the main clue for multiple proportions. Exam tip: compare formulas.

The governing concept is the law of gaseous volumes, commonly associated with Gay-Lussac. If a problem gives volumes in litres and specifically states that gases are measured at the same temperature and pressure, it is signalling a volume relationship in a gas reaction. Under those identical conditions, reacting gas volumes and product gas volumes can be compared in simple whole-number ratios. Therefore, option A is correct. The law of definite proportions concerns fixed composition by mass, the law of multiple proportions compares mass ratios in different compounds, and conservation of mass concerns total mass rather than gas volume. The units and conditions are the key clues.

Sugar dissolved in tea forms a mixture, and its amount can be varied according to choice. Hence, the composition is not fixed. In contrast, pure \(CO_2\) has a fixed ratio of carbon and oxygen; comparing CO and \(CO_2\) relates to the law of multiple proportions, while equal total mass in a closed vessel illustrates the law of conservation of mass. Exam tip: variable composition is a key feature of a mixture.

The direct answer is A, the fixed composition of pure HCl. A pure compound is a substance formed when different elements are chemically joined in a definite ratio. Its particles and chemical formula remain the same throughout the sample. Pure HCl contains hydrogen and chlorine chemically combined in a fixed composition, so it clearly illustrates a compound with definite composition. Option A is correct because it gives both the idea of purity and the idea of fixed composition. Option B, adding water to milk, is mixing substances; the amount of water can be changed, so it is a variable mixture, not a pure compound. Option C, soil, contains many substances such as minerals, sand, clay and organic matter in varying amounts; it is not one compound with one formula. Option D, sea water, contains water and dissolved salts whose amounts vary from place to place and after evaporation, so it is a mixture. The word pure is important, but it must be connected with a chemical formula and fixed composition. Remember: a compound has chemically combined elements and a fixed ratio; a mixture has physically combined substances and a variable ratio.

Lavoisier proposed the law of conservation of mass: in a chemical reaction, total mass is neither created nor destroyed. Proust is associated with definite proportions, while Gay-Lussac is associated with gaseous volumes. Exam tip: link each scientist to a key term such as mass, definite proportion, or gaseous volume.

In a closed system matter does not leave so the law is observed correctly. Exam tip: remember closed system phrase.

The direct answer is A. The law of definite proportions applies to a pure chemical compound: its elements are present in a fixed ratio by mass, regardless of where the pure sample comes from. A is correct because mixtures can have changing composition, so the law is not applied to mixtures as mixtures. B is wrong because a drink may be a mixture and its ingredients can vary. C is wrong because the law is not limited to gas litres; it concerns composition by mass. D is wrong because an open vessel is not the defining condition. To solve such questions, first identify pure compound versus mixture. Memory cue: pure compound has fixed composition; mixture can vary.