Chemistry Scientific Measurements and Matter

Density is a physical property that describes how much mass is contained in a given volume. The formula D = mass ÷ volume indicates that density (D) is obtained by dividing the mass of an object by its volume. This relationship helps to determine how compact or spread out the material is. For example, if two objects have the same volume but different masses, the one with the greater mass will have a higher density. Understanding this concept is essential in fields such as physics, chemistry, and engineering.

Compounds are substances formed when two or more elements chemically bond together in fixed proportions. Water (H₂O) consists of hydrogen and oxygen, carbon dioxide (CO₂) is made of carbon and oxygen, and salt (NaCl) is composed of sodium and chlorine. Each of these examples has distinct properties that differ from the individual elements, demonstrating the characteristics of a compound. In contrast, gold (Au) is a pure element and does not qualify as a compound.

Dissolving salt in water is a physical change, not a chemical one. When salt (sodium chloride) dissolves, its ionic components separate and disperse in the water, but the chemical structure of the salt remains unchanged. This process is reversible; evaporating the water will allow the salt to crystallize again. In contrast, a chemical change involves the formation of new substances with different properties, which does not occur when salt dissolves in water.

The measurements are consistent with each other, showing little variation (100.1 g, 100.2 g, 100.1 g), indicating high precision. However, they are consistently higher than the actual mass of 95 g, demonstrating a lack of accuracy. Therefore, while the measurements are precise due to their close proximity to one another, they do not accurately reflect the true value, leading to the conclusion that they are precise but not accurate.

An electronic balance is specifically designed to measure the mass of objects with high precision. It uses electronic sensors to detect the weight of the sample placed on it, providing accurate readings that are displayed digitally. In contrast, a graduated cylinder measures volume, a meter stick measures length, and a vernier caliper measures dimensions, making them unsuitable for mass measurement. Therefore, the electronic balance is the most appropriate instrument for determining mass in a laboratory setting.

J.J. Thomson discovered the electron in 1897 through his experiments with cathode rays. He proposed the plum pudding model, which suggested that atoms are composed of a positive "pudding" with negatively charged electrons embedded within it, resembling a plum in a pudding. This model was significant as it challenged the existing notion of atomic structure and laid the groundwork for future atomic theories, despite being later revised by Rutherford's nuclear model. Thomson's work was pivotal in advancing the understanding of atomic particles.

A homogeneous mixture is a combination of substances that are uniformly distributed, resulting in a consistent appearance throughout. This means that any sample taken from the mixture will have the same composition and properties as any other sample. Examples include saltwater and air, where the individual components are not distinguishable. In contrast, heterogeneous mixtures have visibly different parts. Thus, the defining characteristic of a homogeneous mixture is its uniformity, making it look the same throughout.

Density is an intensive property because it does not depend on the amount of substance present. It is defined as mass per unit volume (mass/volume), meaning that regardless of how much of a material you have, its density remains constant. In contrast, properties like mass, volume, and weight are extensive properties, as they vary with the quantity of the substance. Thus, density uniquely characterizes a material's intrinsic properties, making it independent of size or shape.

In the number 0.00450, the leading zeros (0.00) do not count as significant figures; they merely indicate the decimal place. The digits 4, 5, and the trailing zero after 5 are significant. Therefore, the number has three significant figures: 4, 5, and the trailing zero, which indicates precision in measurement.

Liquids have a fixed volume, meaning they occupy a specific amount of space, but they do not have a fixed shape. Instead, they take the shape of their container. This property arises because the molecules in a liquid are close together, allowing them to maintain a constant volume, while still being able to move freely past one another, enabling them to adapt to different shapes. In contrast, solids have both fixed volume and shape, gases have neither, and plasmas are ionized gases with unique properties.

A cation is formed when an atom loses one or more electrons, resulting in a net positive charge due to the imbalance between the number of protons (positively charged) and electrons (negatively charged). This loss of negatively charged electrons makes the overall charge of the ion positive, distinguishing cations from anions, which gain electrons and carry a negative charge. Thus, the statement accurately describes the nature of cations.

An atom is the smallest unit of an element that retains its chemical properties. It consists of a nucleus made up of protons and neutrons, surrounded by electrons in various energy levels. While molecules are formed by the chemical bonding of two or more atoms, and ions are charged atoms or molecules, the atom itself is the fundamental building block of matter. Electrons, although smaller than atoms, do not represent the smallest part of an element as they are subatomic particles within an atom.

Light is not considered matter because it does not possess mass or occupy space. Matter is defined as anything that has mass and volume, which includes substances like water, air, and a chair. Light, on the other hand, is a form of energy that travels in waves and can exhibit properties of both waves and particles, but it does not meet the criteria for matter.

Matter is defined as anything that has mass and occupies space. Mass refers to the amount of matter in an object, which contributes to its weight and influences how it interacts with other forces. By having mass, matter can be quantified and measured, distinguishing it from energy or other non-material entities. Additionally, occupying space means that matter has a physical presence, which can be observed and interacted with in the natural world. This dual characteristic of mass and volume is fundamental to the study of physics and chemistry.

To express 0.0045 in scientific notation, we first move the decimal point three places to the right, transforming it into 4.5. This movement corresponds to a negative exponent of 10, specifically \(10^{-3}\), because we are dividing by 1000. Therefore, 0.0045 can be accurately represented as \(4.5 \times 10^{-3}\), which adheres to the standard form of scientific notation where the coefficient is between 1 and 10.

To express 7,200 in scientific notation, we first need to write it as a number between 1 and 10 multiplied by a power of 10. The number 7,200 can be rewritten as 7.2 (which is between 1 and 10) multiplied by 10 raised to the power of 3, since moving the decimal point two places to the left gives us 7.2, and we account for that shift by adding 3 to the exponent. Thus, 7,200 is correctly represented as 7.2 × 10³.

The number 2.300 has four significant figures because all non-zero digits are significant, and any zeros between significant digits or trailing zeros in the decimal portion are also counted. In this case, the digits '2', '3', and the two trailing zeros after the decimal point contribute to the total of four significant figures. In contrast, the other numbers either have fewer significant figures or do not meet the criteria due to the absence of a decimal point.