Chemistry is the study of matter and how we can change matter chemically and physically. What is matter? Matter is everything around us that has mass and volume. Matter can be any phase - solid, liquid, or gas. In this unit, we explore the properties, phases, and how we measure matter. We review the standard units of measurement and how to report our measurements using significant figures.
Completing this unit should take you approximately 3 hours.
Matter is any substance that has volume and mass. In this section, we explore different properties of matter and the need for standard ways to describe them.
First, let's differentiate between extensive and intensive properties. Extensive properties, such as volume and mass, rely on the amount of the substance. Intensive properties, such as density and viscosity, are independent of the amount of matter and inherent to the matter.
Read this text, which gives examples of each type of property.
Scientists classify properties of matter as physical or chemical. Physical properties are those we can observe without altering the identity of the substance. For example, the melting or boiling point of a substance is a physical property because they do not alter the identity of the substance.
We can only observe chemical properties when we alter the identity of the substance. Rusting is an example of a chemical property because it is a chemical reaction that changes the composition of the substance.
Read this text. Pay attention to the chart at the end of the section, which gives examples of some physical and chemical properties of the element sodium (Na).
The three phases of matter are solid, liquid, and gas. Note the differences between these three phases of matter on the microscopic and macroscopic levels.
Read this text. Pay attention to the first image which shows the microscopic differences between the phases of matter.
Density is an intrinsic property of matter. We define density (d) as the mass or volume of a substance at a given temperature. We write d = m/v where d is density, m is mass, and v is volume. If we know two of the variables in this equation, we can solve for the third algebraically. The units for density are a mass unit divided by a volume unit. The units used to describe density often differ for the phases of matter: solids (g/cm3), liquids (g/mL), and gases (g/L).
After you read this section, try the practice problem examples 1 and 2.
Watch this video for additional practice with density problems as the instructor works through problems using the d = m/v formula.
Chemists classify matter as a pure substance or a mixture. A pure substance consists of only one type of matter, while a mixture consists of multiple types of matter. Pure substances are further categorized as single-element or compound. Mixtures are further categorized as homogeneous (single-phase) or heterogeneous (multiple phases). The distinction between homogeneous and heterogeneous mixtures presented here is dependent on phase, or physical, boundaries. Mixtures, whether homogeneous or heterogeneous, can be separated by physical means into pure substances.
Read this text. Pay attention to the flowcharts that classify types of matter.
In chemistry, we often study changes in matter. Two types of changes can occur in matter: physical and chemical.
To determine whether you are dealing with a physical or chemical change, ask yourself if you can reverse the process to recover the original material. Physical changes can be reversed, but chemical changes generally cannot. For example, ice melting is a physical change because you can re-freeze the water. However, cooking a steak is a chemical change because you cannot recover the raw meat. Note that we discuss the energetics of chemical change more thoroughly in Unit 6.
Watch this video to see examples of physical and chemical changes, and how we can observe a change to classify it.
We need to perform measurements when we observe properties of matter. We express these measurements in standard notations so we can communicate them consistently and easily to others. In this section, we explore how to express measurements in chemistry.
We use a standard set of units, based on the metric system, to perform scientific measurements. Most standard units are their base units (meter) for their property. Note that the base unit for mass (kg) is the only "base" unit with a prefix.
Read this section. Be sure to memorize the SI units of measure listed in the text and the metric prefixes listed in the section "The SI Decimal Prefixes". We will use these units of measure throughout this course, so you need to commit them to memory.
Watch this video which explains the basics of scientific notation. Note that we will discuss Avogadro's Number in more detail in Unit 2.
Uncertainty exists in any measured quantity because measurements are always performed by a person or instrument. For example, if you are using a ruler to measure length, it is necessary to interpolate between gradations given on the ruler. This gives the uncertain digit in the measured length. While there may not be much deviation, what you estimate to be the last digit may not be the same as someone else's estimation. We need to account for this uncertainty when we report measured values.
When measurements are repeated, we can gauge their accuracy and precision. Accuracy tells us how close a measurement is to a known value. Precision tells us how close repeat measurements are to each other. Imagine accuracy as hitting the bullseye on a dartboard every time, while precision corresponds to hitting the "triple 20" consistently. Another example is to consider an analytical balance with a calibration error so that it reads 0.24 grams too high. Although measuring identical mass readings of a single sample would mean excellent precision, the accuracy of the measurement would be poor.
Read this text, which text describes how uncertainty comes about in measurements. It uses the example of a dartboard to differentiate between accuracy and precision.
To account for the uncertainty inherent in any measured quantity, we report measured quantities using significant figures or sig figs, which are the number of digits in a measurement you report based on how certain you are of your measurement. Reporting sig figs properly is important, and we need to account for sig figs when performing mathematical calculations using measured quantities. There are rules for determining the number of sig figs in a given measured quantity. There are also rules for carrying sig figs through mathematical calculations.
Watch these two videos to learn how to count sig figs for a given quantity.
Next, watch these two videos. Note that the rules for sig figs for addition and subtraction are different from the rules for sig figs for multiplication and division.
After you watch the videos, complete this practice set.
Take this assessment to see how well you understood this unit.