Question

Judge the following statements as true or false. If you believe a statement to be false, provide a corrected version. (a) Air and water are both elements. (b) All mixtures contain at least one element and one compound. (c) Compounds can be decomposed into two or more other substances; elements cannot. (d) Elements can exist in any of the three states of matter. (e) When yellow stains in a kitchen sink are treated with bleach water, the disappearance of the stains is due to a physical change. (f) A hypothesis is more weakly supported by experimental evidence than a theory. (g) The number \(0.0033\) has more significant figures than \(0.033 .\) (h) Conversion factors used in converting units always have a numerical value of one. (i) Compounds always contain at least two different elements.

Short answer

(a) False. Air and water are not elements, but compounds. (b) False. Mixtures can consist of any combination of elements or compounds. (c) True. Compounds can be broken down into simpler substances, while elements cannot. (d) True. Elements can exist in solid, liquid, or gaseous states. (e) False. The disappearance of the stains is due to a chemical change. (f) True. A hypothesis is less supported by experimental evidence than a theory. (g) False. The number \(0.0033\) has two significant figures, while the number \(0.033\) has three. (h) True. Conversion factors used in converting units have a numerical value of one. (i) True. Compounds always contain at least two different elements.

Step-by-step solution

(a) Air and water are both elements.

This statement is false. Air and water are not elements, but compounds. Air is a mixture of various gases, mainly nitrogen and oxygen, and water has a molecular formula of \(H_2O\), which is a compound formed by two hydrogen atoms and one oxygen atom.

(b) All mixtures contain at least one element and one compound.

This statement is false. Mixtures can consist of any combination of elements or compounds, but they don't necessarily have to contain an element and a compound. For example, a mixture of two different elements is still a mixture and contains no compound.

(c) Compounds can be decomposed into two or more other substances; elements cannot.

This statement is true. Compounds can be broken down into simpler substances (elements or other compounds) through chemical reactions, while elements are the simplest form of matter and cannot be decomposed further.

(d) Elements can exist in any of the three states of matter.

This statement is true. Elements can exist in solid, liquid, or gaseous states, depending on the temperature and pressure conditions. For example, oxygen exists as a gas at room temperature but can be converted to a liquid or solid under different conditions.

(e) When yellow stains in a kitchen sink are treated with bleach water, the disappearance of the stains is due to a physical change.

This statement is false. The disappearance of the stains results from a chemical change, as the bleach reacts chemically with the stain components, breaking them down and removing them from the surface.

(f) A hypothesis is more weakly supported by experimental evidence than a theory.

This statement is true. A hypothesis is an initial explanation or prediction based on available evidence, while a theory is a well-established explanation that has been tested and confirmed many times through experimental evidence.

(g) The number \(0.0033\) has more significant figures than \(0.033\).

This statement is false. The number \(0.0033\) has two significant figures, while the number \(0.033\) has three significant figures.

(h) Conversion factors used in converting units always have a numerical value of one.

This statement is true. Conversion factors are used to convert from one unit to another and represent the same quantity in different units. Therefore, when used in a calculation, they are equivalent to multiplying by one, and they do not change the value of the quantity being converted.

(i) Compounds always contain at least two different elements.

This statement is true. Compounds are formed when two or more elements are chemically bonded together in fixed proportions. Therefore, by definition, a compound must contain at least two different elements.

Key concepts

Compounds and Elements

Understanding the distinction between compounds and elements is fundamental in chemistry. Elements are pure substances that cannot be broken down into simpler substances by chemical means. The periodic table is a comprehensive chart that lists all known elements, each with a unique atomic number that represents the number of protons in its nucleus.

Compounds, on the other hand, are substances formed when two or more elements chemically bond together. The properties of a compound are distinct from those of the individual elements that comprise it. For instance, sodium is a reactive metal and chlorine is a toxic gas, but when combined, they form sodium chloride (table salt), which is safe to consume. Water, with the chemical formula \( H_2O \), is a classic example of a compound formed by the elements hydrogen and oxygen.

It is crucial for students to recognize that air is not an element but a mixture of gases, mainly nitrogen and oxygen, and that water is a compound made of hydrogen and oxygen. This knowledge helps correctly categorize substances encountered in chemistry.

Physical and Chemical Changes

When studying physical and chemical changes, students often need to determine whether a substance has changed its chemical composition or merely its physical form. Physical changes involve alterations in the appearance or state of a substance without changing its chemical identity. For example, melting ice changes from solid to liquid, but it remains water, chemically speaking.

Chemical changes, however, result in the formation of new substances with new chemical properties. This occurs through chemical reactions, where the original substances (reactants) are transformed into different substances (products). The treatment of stains with bleach leading to their removal is an example of a chemical change, where the bleach reacts with the stains causing a change at the molecular level that alters the stain's chemical structure.

Recognizing this distinction is essential as it impacts understanding reactions, predicting products of reactions, and grasping conservation of mass during chemical processes.

Significant Figures in Chemistry

Significant figures are used in chemistry to express the precision of measurements. They include all the digits in a number that are known with certainty plus the first uncertain digit. Understanding how to count significant figures is vital for correctly reporting and interpreting data.

For example, the number 0.0033 has two significant figures because the leading zeroes only serve as placeholders and do not count towards the significant figures. Conversely, 0.033 has three significant figures, with the '3's being significant and the leading zero being a placeholder. Misunderstanding significant figures can lead to inaccuracies in calculations and the reporting of scientific data.

Students must practice identifying the number of significant figures in various numbers and learn the rules for arithmetic operations with them to maintain the correct level of precision throughout their scientific work.