is density physical or chemical
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Chemical properties are those which can only be established by carrying out a chemical reaction (heat of combustion, flash point, enthalpies of formation, etc).
Density can be established simply by determining the mass and volume of substance, no reaction is involved, so its a physical property.
How can density be used to identify substances?
You can identify an unknown substance by measuring its density and comparing your result to a list of known densities.
Density = mass/volume. Assume that you have to identify an unknown metal.
A metal bolt with a mass of 99.7 g is dropped into a graduated cylinder containing 50.0 cm³ of water. The new volume reads
72.1 cm³. Identify the metal.
V = 72.1 cm³ – 50.0 cm³ = 22.1 cm³
D = mV=99.7g22.1cm³ = 4.51 g/cm³
Why Is Density Considered a Physical Property Rather Than a Chemical Property of Matter?
Learning to tell the difference between physical and chemical properties is a cornerstone of all basic chemistry courses. Some properties, however, are often difficult for beginning students to place in one category or the other. Density is one of those properties. With just a little clarification, students can gain the confidence to tackle not only density but other tricky cases as well.
EXPLORE THIS ARTICLE
Physical properties are those that do not change the chemical composition of the substance being described. In other words, measuring a physical property does not cause a chemical reaction. Length, mass, index of refraction and melting point are all examples of physical properties because none of these involves a chemical reaction. Measuring melting point involves changing the phase of a substance, but it does not change it into another substance. (In other words, melting ice still leaves you with H2O.)
Chemical properties are the opposite of physical properties. Measuring a chemical property involves a chemical reaction. Rate of oxidation, flammability and acidity (pH) are examples of chemical properties because measuring each of these properties involves a chemical reaction. To determine the rate of oxidation, for example, the substance must oxidize.
Density is defined as mass per unit volume. In other words, it’s how much the mass is confined or spread out in a substance. It helps demonstrate whether atoms are packed closely together or spread so far apart they don’t even interact with each other (as is the case for an ideal gas). To measure density, simply measure the mass on a balance, calculate volume from measured lengths and divide the two.
This process involves no chemical change; therefore, density is a physical property. Another way to measure density is by using its displacement of liquid. As a consequence of Archimedes’ principle, the volume of the liquid after the object of interest is submerged minus its original volume will give the volume of the object. Mass is again measured on a balance, and density is calculated. This process still involves no chemical change in the substance, so density is a physical property.
4Source of Possible Confusion
Every substance has its own density; this property changes when the chemical substance changes. Don’t confuse this with the definition of a chemical change, which is that the measurement itself alters the chemical nature of the substance. In the first case, you are simply identifying a chemical based on its properties (either physical or chemical). In the second case, a chemical reaction is taking place.
Physical and Chemical Properties
The main objective of this experiment is to experimentally determine several chemical and physical properties of two unknown compounds and based on the set of properties identify each substance as either ionic or molecular. Another objective is to compare chemical and physical properties of unknown substances with those of known substances in order to identify the unknown substances.
Characteristics such as melting point, boiling point, density, solubility, color, odor, etc. are physical properties. Properties that describe how a substance changes identity to produce a new substance are chemical properties. Reactions of a substance, either with itself or with another substance are chemical properties.
Ionic compounds usually have high melting points, in excess of 300oC, are often soluble in water, conduct electricity when in aqueous solution or molten, and do not burn in a flame. Molecular compounds consist of neutral molecules that are held together in the solid (or liquid) state by attractions between these molecules called intermolecular forces. Intermolecular forces are also electrostatic in nature, but are much weaker than the ionic bonds that hold ionic compounds together. Consequently, molecular compounds usually have low melting points, often less than 300oC, are usually less dense than ionic compounds, and are often less soluble in water than ionic compounds. When molecular compounds do dissolve in water, they do not conduct electricity.
There are exceptions to everyone of the “usual” properties associated with ionic and molecular compounds. Consequently, to characterize a substance as ionic or molecular several properties must be determined and the set of properties is used to characterize the substance as ionic or molecular.
Physical and chemical properties can sometimes be used to identify a substance. In order for you to do this you will be given a list of substances from which your substance was chosen. You will need to look up some of the properties of these substances in the literature. Resources that you can use include The Handbook of Chemistry and Physics, The Merck Index, or you may do a Google search. You can then compare the properties that you observed in lab with those that you found in the literature to identify your unknown substances.
Special Safety Precautions
Be careful not to burn yourself with either the Bunsen burner or the melting point apparatus.
Do not breathe the vapors of the solids as you attempt to burn them; burn them near or under a snorkel.
Obtain 2 unknowns from your instructor and record the code numbers of your unknowns in your record book.
- Solubility in Water
For each unknown determine the solubility in water as follows:
Place 0.25 g of the solid in a medium size test tube. Add 1 mL of water from a 10 mL graduated cylinder and stir the mixture with a stirring rod. If after 2 minutes the solid has not all dissolved, add another milliliter of water and repeat the stirring process. Continue adding water in 1 mL increments until all the solid has dissolved and then record the total volume of water required to dissolve each solid. If the solid does not dissolve after the addition of 20-mL of water, discontinue the addition of water and record the fact that the solid did not dissolve after the addition of 20 mL of water in your record book. Based on this data alone, classify your unknowns as either ionic or molecular.
Add 1 g of each solid in a 50 mL beaker along with about 20 mL of water. Stir the mixture thoroughly for 1 to 2 minutes and test the conductivity of the solution even though the sample may not completely dissolve. Your instructor will show you how to measure the conductivity. Record whether the solution resulted in the light shining brightly, dimly, or not at all. A bright light indicates that the solid is an electrolyte; no light at all indicates that the substance is a nonelectrolyte. Based on this data alone, indicate whether your substances are ionic or molecular.
- Melting Point
- Color and Odor
Record the color and odor of each of your unknowns. This is one more piece of data that along with several others may help you in identifying the unknown. Color or odor alone will not indicate whether a substance is ionic or molecular.
- Oxidation with Oxygen (Combustion
Place a small amount, about the size of half a pea, of each unknown on the end of a spatula and place this in the flame of a Bunsen burner that is near or under a snorkel. Be careful not to melt the snorkel hood. Record whether or not the unknown burns. Based on this data alone, indicate whether your substances are ionic or molecular.
C4A: Physical and Chemical Properties
C4B: Intensive and Extensive Properties