Problem 92 - Chemistry The Central Science

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.

Step-by-Step Solution

Answer

(a) False. Air is a mixture and water is a compound. (b) False. Mixtures can contain elements and/or compounds. (c) True. (d) True. (e) False. It is a chemical change. (f) True. (g) True. (h) True. (i) True.

Step 1: Evaluate the statement

Air and water are not elements; instead, they are mixtures and compounds, respectively. Air is a mixture of gases (mainly nitrogen, oxygen, and trace amounts of other gases), and water is a compound (H₂O) made up of hydrogen and oxygen elements. Statement (b):

Step 2: Evaluate the statement

This statement is false. Not all mixtures must contain an element and a compound. Mixtures can also contain two or more elements, two or more compounds, or a combination of elements and compounds. Statement (c):

Step 3: Evaluate the statement

This statement is true. Compounds can be decomposed into two or more other substances through chemical reactions, while elements cannot be further decomposed into different substances. Statement (d):

Step 4: Evaluate the statement

This statement is true. Elements can exist in any of the three states of matter: solid, liquid, or gas, depending on the temperature and pressure conditions. Statement (e):

Step 5: Evaluate the statement

This statement is false. In this case, a chemical reaction occurs between the bleach (sodium hypochlorite) and the stain, causing its disappearance. Thus, it is a chemical change, not a physical one. Statement (f):

Step 6: Evaluate the statement

This statement is true. A hypothesis is an initial explanation made based on observations, which needs further testing and evidence to support it. In contrast, a theory has been thoroughly tested and confirmed by multiple lines of evidence. Statement (g):

Step 7: Evaluate the statement

This statement is true. The number 0.0033 has three significant figures, while 0.033 has only two significant figures. Statement (h):

Step 8: Evaluate the statement

This statement is true. Conversion factors are used to convert units and always have a numerical value of one. They represent equivalent quantities expressed in different units. Statement (i):

Step 9: Evaluate the statement

This statement is true. Compounds are formed when two or more different elements combine in fixed proportions.

Key Concepts

Elements and Compounds

Elements and compounds are fundamental to chemistry. An element is a pure substance that cannot be chemically broken down into simpler substances. Elements are made up of only one type of atom. For example, oxygen (O), hydrogen (H), and gold (Au) are elements.

Compounds, on the other hand, are pure substances formed when two or more different elements combine in fixed ratios. They can be broken down into those individual elements through chemical reactions. Water (H₂O) is a common example of a compound, consisting of two hydrogen atoms and one oxygen atom. Compounds have different properties from those of the elements that make them up, often dramatically so.

Understanding the differences between elements and compounds is crucial as it forms the basis for studying more complex chemical interactions.

Mixtures and Chemical Reactions

Mixtures and chemical reactions are key concepts in understanding matter and its changes. A mixture is a combination of two or more substances, where each retains its chemical identity, such as air or a salad. Mixtures can be homogeneous (uniform composition, like saltwater) or heterogeneous (different components are visible, like a trail mix).

Chemical reactions occur when substances interact to form new substances with different properties. For example, when bleach is used to clean yellow stains, a chemical reaction causes the stains to disappear, forming new compounds. Reactions can involve decomposing compounds, like breaking down water into hydrogen and oxygen, or forming compounds, like combining hydrogen and oxygen to make water.

Distinguishing between mixtures and chemical reactions helps in identifying how substances interact and change.

Significant Figures and Measurements

Significant figures are vital in scientific measurements as they reflect the precision of the data. Significant figures include all known digits plus one estimated digit. For example, 0.0033 has three significant figures, while 0.033 has two. This means 0.0033 is more precise than 0.033.

When performing calculations, it's crucial to retain the correct number of significant figures to maintain the precision of the data. This practice ensures that the reported results are as accurate as possible given the limitations of the measuring equipment used.

Understanding significant figures aids in evaluating the reliability and accuracy of scientific data.

Physical and Chemical Changes

Chemical and physical changes describe transformations in matter. A physical change involves a change in the physical properties but not in the chemical identity of a substance. Examples include changes in state, such as ice melting to water or water evaporating to gas.

A chemical change, on the other hand, results in the formation of new substances with different properties, usually through a chemical reaction. When bleach removes a stain from a sink, it is a chemical change, as a new substance is formed. Identifying whether a change is physical or chemical helps understand the processes that substances undergo in various conditions.

Recognizing the differences between these changes is fundamental to mastering chemical reactions.

Scientific Method and Hypothesis

The scientific method is an organized approach used to investigate and understand phenomena. It involves making observations, forming a hypothesis, conducting experiments, and developing theories.

A hypothesis is a tentative explanation for an observed phenomenon and acts as the starting point for further investigation. A well-formulated hypothesis can become a theory after extensive testing and validation. Theories provide comprehensive explanations that are strongly supported by evidence, such as the theory of evolution or the theory of relativity.

Understanding the scientific method and hypothesis formulation is essential for the advancement of scientific knowledge and helps in conducting effective research.