While chemists can measure many different things, there are five principle measurable quantities in chemistry. These fundamental measurements are applicable to a wide variety of chemicals and can be combined with one another to create additional units.
Mass is defined as the quantity of substance that is contained in an object. The word "mass" represents the full term for the measurable quantity and can be written with its first letter being either capitalized or lower-case. Every quantity in science can also be represented by a variable, which is an abbreviation that can be used in equations and other contexts. The capitalization of variables is important! There are many terms that start with the letter "m" in chemistry, so both a capitalized "M" and a lower-case "m" are used by scientists, so as to be able to represent as many terms as possible. The variable for mass is a lower-case m.
Note that "weight" is not a term that should be substituted for "mass". "Weight" depends on gravity; therefore, as the gravitational force around an object changes, the weight of the object changes. This inconsistency is not very scientific. However, "mass" does not depend on gravity and so can be more consistently-measured, regardless of the location of an object.
Finally, each system of measurement has a primary unit for measuring quantities. The metric unit for measuring mass is the gram, which can be abbreviated by a lower-case g, and the kilogram (kg) is the main unit used in the SI system.
Length (l) is defined as the distance between two points. The meter (m) is the unit used by both the metric system and the SI system for measuring length. However, a lower-case "m" has now been used for two different quantities - the variable "mass" and the unit "meter". When determining the meaning of an abbreviation, context is important. Remember that "mass" is a measurable quantity, so its "m" is a variable, which is often used in equations. The "meter" is a unit of measurement, so its "m" must follow a number.
Volume (V) is defined as the amount of space that is occupied by an object. The metric unit for measuring volume is the liter (L), and the cubic meter (m3) is the main unit used in the SI system. Note that the meter, a unit for measuring length, can be used to create a new unit - in this case, the cubic meter. While liquids and gases are often measured in liters, solids are not; therefore, scientists adopted a different unit that could be used not only for liquids and gases, but also for solids.
Temperature (T) is defined as the measure of how hot an object is. The metric unit for measuring temperature is degrees Celsius (°C), and citizens in the United States primarily use degrees Fahrenheit (°F) for recording temperatures. However, both the Fahrenheit and Celsius scales allow for the existence of negative values, which is problematic in some chemical applications. Therefore, the SI system uses Kelvin (K) for temperature measurements, as the Kelvin scale is based on a quantity called absolute zero, which is defined as the temperature at which all motion stops. Put simply, this is the lowest possible temperature that can be theoretically achieved and is defined to have a value of 0 Kelvin. As this is the lowest possible temperature that can be recorded, the Kelvin scale does not allow for negative temperatures and is most appropriate for chemical applications.
Time (t) is defined as the period over which an event occurs. The second (s) is the unit used by both the metric system and the SI system for measuring time.