Density is a fundamental property of physical objects. Chemists sometimes use it to help in the identification of materials. The effects of density are often demonstrated by considering whether an object will float in water. Water has a density very close to exactly 1 g/cm3. When an object has a density greater than 1 g/cm3 then it will not float in water. When its density is less than 1 g/cm3 then it floats. For example, common vegetable oil has a density of around 0.92 g/cm3, which is why Italian salad dressing looks the way it does.
Density is defined as the amount of mass per unit volume and mathematically by the equation D = m/V. The letter D stands for density, m for mass, and V for volume. The SI unit of density is kg/m3 but it will suffice in the lab for us to use g/cm3 (remember, 1 cm3 = 1 mL). The measurement of density requires two pieces of data and therefore requires great care in the making of both measurements so as not to lose precision.
In this lab you will measure the density of several substances. In addition you will use the known density of aluminum (2.70 g/cm3) to measure the thickness of aluminum foil. Successful completion of this investigation will include:
Density is what chemists call an intensive property. That means that the density of a material does not depend on how much of it you have. Other examples of intensive properties include temperature, concentration, color, odor and molar mass. You cannot add intensive properties together. If you have 2.70 g of aluminum it has a volume of 1 cm3 (at ordinary room temperature and pressure). If you have 2,700 g of aluminum it has a volume of 1,000 cm3. If you have 1 m3 (1 × 106 cm3) of aluminum it has a mass of 2,700 kg. Work all of these out using D = m/V and you will always get a result of 2.70 g/cm3. No matter how much aluminum you add, the density, an intensive property, does not change. An intensive property can be understood in this way: there is a ratio of two measurements, which together have a constant value. Just like the slope of a line in an algebraic equation: the slope is the same no matter what the values of x and y. With density, this can be shown in a simple graph, as seen at right. The slope of the line in the graph is the density of aluminum, according to the data.
Contrast intensive properties with extensive properties: measurements like mass, volume, height, width, and depth all depend on the size of a particular object. These measurements are extensive properties. You can always add more volume or mass.
Density is often confused with weight. We often say that lead is heavy (ask your teacher if there is a piece of lead around to use as an example). But which is heavier, one pound of lead or one pound of feathers? … So weight is not the difference between them. Try another question: which is heavier, 1 liter of lead or 1 liter of feathers? Clearly, even if you compressed the feathers to the maximum degree possible, they would probably never weigh as much as the lead, assuming you have the same volume of each. So the substance lead is not heavy (an extensive property), it is dense (an intensive property). Still not convinced? Answer this: is 0.0001 g of lead heavy?
You must find the density of two substances. The procedure for this lab is completely up to you. After your teacher gives you an introduction and leads a brief class discussion it is your decision how to proceed.
Here are a few things you need to know how to do in order to be successful in answering the objective questions.
Do multiple trials for each type of substance. At least four trials should be made in which you measure the volume and mass of the sample. Vary the size of the sample from one to the next: start with a small sample and make each of the others bigger than the last. Try 10 g, 20 g and 30 g, for example.
When something is too oddly shaped to make simple measurements of its size you must resort to the method of Archimedes: immerse it in water. The amount of water displaced by an object is equal to its volume. When using this method beware of bubbles! Do everything you can to reduce the number of air bubbles before reading the volume. Why do you think you should do this?
If you use water to help find the volume of a certain amount of material be sure to weight the material first, before putting it into water. This saves you from making it wet and then waiting for it to dry.
Due to the fact that dense materials have a smaller volume for the same mass than less dense materials you will find that small samples are not useful for measuring density. For example, a substance with a density of around 11 g/mL will only change the volume in a graduated cylinder by 1 mL if the mass used is 11 g. This is a small difference and small differences are hard to measure precisely. To obtain more significant figures, and higher precision, use larger samples for the denser materials.
In your lab notebook describe, step-by-step, how you intend to measure the density of each material. You will find the density of four substances, including one liquid. Some of the substances are available as ingots or lumps and these should be measured individually, one measurement per substance. Water, salt-water, or other liquids should be measured multiple times with a different amount each time.
When you have completed your work in this section check in with your teacher. This is a required part of the lab and your teacher’s initials are required before you can move on to the next section. Initials will be given for a good procedure.
Collect your data in a data table like the one you see here. Do this in your lab notebook. All of your data points should have very different masses from one another. Collect at least five data points.
|Mass (g)||Volume (mL)||Density (g/mL)|
When you have completed collecting data for one substance check in with your teacher. This is a required part of the lab and your teacher’s initials are required before you can move on to the next substance. Initials will be given for good data collection and calculations.
Obtain or manufacture three smooth, perfectly rectangular pieces of household aluminum foil. Here are the data you need to collect to calculate the thickness of aluminum foil:
Create a data table below to collect these data. No averages are required.
Calculate the thickness of the aluminum foil using the density.
First, calculate the volume of material in the sheet:
m m D = --- so V = --- the density of aluminum of 2.70 g/cm3 V D your answer is the volume in cm3
Next, calculate the thickness, or height, of the aluminum foil:
V = L × W × H and L × W = Area so V = A × H Therefore V (cm3) H (cm) = --- A (cm2)
Make a data table for the volume in cm3, the area in cm2, the height in cm, and the height in μm. Calculate the average height (in μm) and the range, ± amount, result, and percent error and include them in your data table.