Matter describes everything around us that has mass; chemistry is the academic discipline that studies matter. This includes solid objects, such as the table where we sit, the liquid water we drink, and the air (a gas) we breathe. Chemistry is part of everything we touch, feel, see, and smell.
Chemistry studies the properties and structure of matter, including chemical reactions, which describe the transformation of matter. Many people call chemistry the "central science" because we use it in most science and technology fields.
We can describe matter by its physical and chemical properties.
Physical properties describe our observations about the properties of a substance in which the substance itself does not change during or after our observation. Examples of physical properties we use to describe a substance include its boiling point, melting point, appearance, and density.
Chemical properties describe our observations about the properties of a substance during or after its chemical transformation. Examples of chemical properties we use to describe a substance include its acidity and the types of chemical reactions it can withstand or perform.
To review, see the table in section three that compares the physical and chemical properties of the element sodium.
Matter can undergo two types of changes: physical and chemical.
Physical changes do not alter or change the identity of a substance. Examples of physical transformations include freezing, melting, or boiling.
For example, when a solid ice cube melts to become liquid water, the water's chemical identity does not change. The transformation is physical because the identity of the initial and final substance remains the same, in this case, water. Likewise, water vapor (gas) that results from boiling is still comprised of water molecules.
Chemical changes, on the other hand, which we also call chemical "transformations" and chemical "reactions", do alter the identity of the substance.
For example, a nail that rusts represents a chemical change because the rusting process creates a new substance with a different chemical composition than the original nail. The transformation is chemical because the identity of the substance has changed. Similarly, burning represents a chemical change because the chemical composition of the substance changes during the burning process.
We can describe three states or phases of matter: solid, liquid, and gas.
Molecules in solid state matter are arranged in an "ordered" fashion. The particles touch each other and are close together. Solid materials have a definite shape.
Molecules in liquid state matter are close together but are not ordered like a solid. The particles can move or "slip" around each other. The liquids flow and move. Liquid materials take the shape of their container.
Molecules in gas state matter are far apart from one another and usually do not touch or interact. The molecules are completely disordered. Molecules in a gas take up the entire space of their container.
Review this simple diagram of the three states of matter.
Density measures the mass of an object per unit volume. In other words, a "denser" object has a higher mass than a second object which shares the same volume. Most liquids and solids have significantly higher densities than gases.
We consider molecules in a liquid or solid phase condensed phase matter. This means the molecules are in direct contact with their neighboring molecules.
In chemistry, we use a defined set of units when we make measurements – to ensure consistency, accuracy, and make comparisons. We base units of measurement on a commonly-accepted, standard scale, so we can describe and communicate the results of our measurements with other researchers.
Quantity describes the amount we measure in an experiment. For example, a quantity could describe the mass, volume, length, or another observable measure.
A unit relates to the standard measurement scale. A unit defines the amount of a quantity measured. For example, we use meters to measure length (50 meters), grams to measure mass (10 grams), and liters to measure volume (5 liters), according to the metric scale of measurement.
A measurement standard is a universally-agreed-upon object that defines a unit of measurement.
Measuring instruments are calibrated to match measurement standards scientists have agreed to follow. Think about the length of a ruler (how long is 12 inches or one meter?), a mutually-agreed-upon amount of water used to follow a recipe (how much is in a cup?), or a common temperature used to calibrate a thermometer (how cold is 20 degrees Celsius or Fahrenheit?).
Systeme Internationale (SI) units describe the internationally-recognized set of units of measurement that are standard in all scientific fields.
The base SI units for length, mass, time, and volume are as follows:
Volume (not actually SI)
Review the table of SI decimal prefixes in The SI Units.
We use SI prefixes to convert among units that have different orders of magnitude. For example, you should use millimeters to measure extremely short lengths, and centimeters, meters, or kilometers, to measure longer units of measurement or distances.
We use dimensional analysis to convert among units since it makes it easier to compare quantities in different units. For example, from the SI decimal prefixes table we see that one kilogram (1 kg) = 103 g, or 1 kg = 1,000 g.
You should use significant figures when reporting a measurement to convey your level of confidence in your measurement.
Chemists consider the last digit in a measurement uncertain because it is an approximation. Significant figures tell us "how good" or "confident" your measurement is, according to the equipment you used to make the measurement.
For example, you usually have to make an approximation when you measure the distance between the smallest markings on a ruler. You might say the coin in the image below measures 2.7 centimeters, but you really have to make an educated guess about the last figure since it is not exact. Scientists recognize this last digit is often uncertain (in other words, you have less confidence in its accuracy).
Review these general rules for determining the number of significant figures in a measured quantity:
Here is an example.
For the number 0.003900270, the leading zero (the single zero to the left of the decimal point) is not significant. The trailing zero is significant because it follows the decimal point. Therefore, this number has seven significant figures (indicated in red in the diagram below).
(Note that in this diagram, the "placeholder" zeros mean they are showing the order of magnitude of the number rather than an actual measured value. So, the leading zeros in this number indicate the order of magnitude of the number and are not significant measured numbers.)
For the number 9024000.0, all of the zeros are significant because all zeros are between nonzero numbers, between nonzero numbers and a decimal point, or trailing after a decimal point. This number has eight significant figures. The last example, 9024000, has ambiguous trailing zeros.
When working with measured quantities, you should round the numbers properly to avoid confusion about the confidence you have in the measurement.
Review the rules for rounding significant figures in the yellow boxes in Rules for Rounding.
When rounding, round down if the first insignificant digit is less than five. Round up if the first insignificant figure is greater than five.
For example, to round 45.556 to four significant figures, the number should become 45.56 because the first insignificant digit (the last digit, six, in this case) is greater than five.
When rounding the answer for a multi-step problem, it is important to keep track of significant figures, but you should not round your number until after you have completed all of your calculations.
Chemists often need to perform calculations on the quantities they have measured. They use significant figures to convey their level of confidence (or level of accuracy) in their measurements and follow specific rules for adding and subtracting, multiplying, and dividing quantities with significant figures.
For addition and subtraction, we determine the answer's number of significant figures by decimal places. Look at your input quantities and identify the quantity that has the fewest number of decimal places.
Line your addition or subtraction up vertically, according to the decimal point, to make this more clear. Your answer should have the same number of decimal places as the input quantity that had the fewest number of decimal places.
For multiplication and division, your answer should have the same number of significant figures as the input quantity that had the fewest number of significant figures.
For base 10 logarithms, the answer will have the same number of significant figures as the normalized form of the logarithm. Normalized means the logarithm is given in scientific notation a x 10b, where a is a number greater than 1 and less than 10.
Scientific notation allows scientists and mathematicians to express small and large numbers more succinctly because they do not include all of the zeros in their notations and conversions. For example, scientists frequently use scientific notation when making a dimensional analysis to convert measurements from one unit to another.
Scientific notation uses multipliers of a x 10n, where a represents the part of the number that includes non-zero numbers. The decimal point is moved to follow the first non-zero number, and n represents the number of zeros that precede or follow the first nonzero number.
For example, 15,000 in scientific notation is 1.5 x 104. In this case, we move the decimal point to follow the first non-zero digit. Then count the number of digits that follow the first nonzero digit to get 104.
Similarly, 0.0007005 in scientific notation is 7.005 x 10−4. In this case, we move the decimal point to follow the first nonzero digit to get 7.005. Count back to the original decimal point to determine the number of zero digits before the first nonzero number. Since you can count four digits until you hit the original decimal point, the multiplier is 10−4. Be sure to use a negative exponent when the original number is less than one.