In this article, we will study if there is any change in the density of a substance with temperature or not. So, does density change with temperature? Yes, the density of a substance is inversely related to its temperature which means that density increases with the decrease in temperature and vice versa. When the temperature of a substance increases, its molecules push away from each other due to their kinetic energy gained. As a result, the volume of substance increases, and density decreases. As the temperature of a substance increases it results in an increase in its volume due to the increased kinetic energy of its molecules that begin to move at higher speeds owing to which there are more frequent collisions between these molecules making them move away from each other. This instance is especially true when a substance is present in the liquid and gaseous form of matter due to more availability of space allowing the molecules to expand the volume of the substance, and, thus, decreasing its density. However, in the case of solids, the molecules are very closely packed and do not have enough space for their movement. Due to which there is negligible change in the volume of a solid substance even at high temperature unless there is a phase change i.e. a solid substance changes into liquid or gas phase where the volume increases significantly resulting in the decrease of density.
What is Density?
Density is the measure of the number of molecules of a substance present in a specific volume. It is the characteristic property of a substance and as the name indicates it gauges how dense an object is or how compactly are the molecules of a substance arranged in space. Actually, it is the density of a substance that decides whether it will float or sink or rise high in the atmosphere. It is given by the formula mass by volume and is written as under: Density = Mass / Volume This means that a denser object will always have a smaller volume in comparison to an object with a lower density. For example, one kilogram of wood will have more volume in comparison to one kilogram of iron as the density of wood is quite lower in comparison to iron. Density is expressed in gram per centimeter cube or kilogram per cubic meter.
Relation between Temperature and Density for different states of matter
As discussed earlier the change in temperature has different effects on a substance when applied in different phases viz. solid, liquid or gas. Therefore, there are specific formulas for expressing the relation between the temperature and density of the substance in different phases. For ideal gas: P = ρRT Where, P = Pressure in pascal ρ = Density of gas R = Universal gas constant T = Temperature For non-ideal gas: PV = ZRT Where, P = Pressure in pascal V = Volume of gas Z = Compressibility factor R = Universal gas constant T = Temperature For liquids: PV = nRT Where, P = Pressure in pascal V = Volume of gas n = Number of moles R = Universal gas constant T = Temperature As the change in temperature has minimal effect on solids, there is no specific formula. Also, based on the above formulas volume of the substance is calculated and then density is measured by dividing the mass of a substance by its density.
Why Does Temperature Affect Density?
The temperature of a substance is the measure of its heat i.e. it is the property used to explain how hot or cold a substance is using a number of scales viz. Fahrenheit, Kelvin, Celsius, etc. It is an illustration of the thermal energy of a substance. Usually, the temperature of a substance is inversely proportional to its density, and the change in any of these properties is reflected in the other. Usually, solids are the densest substances as they have a closely packed structure with more number of atoms or molecules in a specific volume of substance while gases are least dense as the molecules have enough space for their movement.
The reason why temperature affects density is that increase in temperature provides heat to the system which is a form of energy. As this heat energy is absorbed by the atoms or molecules of a substance it changes into kinetic energy due to which they start vibrating at higher speeds. This increase in speed causes more collisions than usual due to which they have pushed apart from each other. This movement of atoms and molecules away from each other causes the expansion of the substance thus increasing its volume. As the volume of a substance is inversely proportional to its density, an increase in temperature results in a decrease in density.
What is Relative Density
Relative density is also known as specific gravity. It is actually the density of a substance relative to another substance that is being used as reference material. It is used to check which of the given substances is denser. If the ratio of the density of a substance and its reference material is less than one it means that the given substance is less dense in comparison to the reference material and if the ratio is greater than one, this means that the given substance is denser than the reference material. It is a dimensionless quantity and is given by the formula: Relative Density = Density of sample substance / Density of reference substance
As density is directly affected by a change in temperature, therefore, it is necessary that the density of both substances is measured at the same temperature while calculating the relative density. Mostly water is taken as the reference material to make the calculations easier and also because it is readily available.
Graph of Density Vs. Temperature
The density of a substance decreases as the temperature increases and vice versa. This indicates that these two physical properties of a substance are inversely related to each other. When a density vs. temperature graph is drawn is relation becomes rather more clear. Let us look at the graph for density of air at 1 atm pressure as a function of temperature.
In the above graph, it is clear that the density of air decreases as a function of temperature, at constant pressure. Also, it is seen that the density of air at standard temperature and pressure is 1.18 kg/m3. Now, to understand the relationship between density and temperature in the case of liquids let us have a look at the graph between the density of water at 1 atm pressure as a function of temperature.
The above graph indicates that the density of water also decreases with an increase in temperature. Further, it states that the density of water at standard temperature and pressure is 997 kg/m3.
Archimedes Principle of Density
It is named after the Greek mathematician Archimedes of Syracuse who first used and described it. As per the Archimedes principle when an object is immersed in a fluid the weight of the fluid displaced is equal to the buoyant force exerted by the fluid on that object. The mass of the substance submerged appears lower than its actual mass. When this principle is applied for water it can be used for determining the volume of a substance which in turn can be used for establishing the density of any substance with an irregular shape. The following formula is used for determining density through Archimedes Principle: Mass of Object – Apparent mass when submerged = Density of water X Volume of the object In this equation water is used instead of any other fluid as the density of water is 1 gm/cm3 which makes the calculations easier.
Conclusion
The density of a substance is inversely related to its temperature i.e. it decreases with the increase in temperature. The increase in the volume of a substance resulting in a decrease of its density due to the rise in temperature is most prominent in gases enough space for movement of atoms and molecules, while it is almost negligible in solids due to closely packed structure. As the temperature of a substance increases it provides energy to its atoms and molecules that cause them to vibrate at faster rates than under normal conditions. These vibration movements cause more collisions between these particles due to which they have pushed away from each other in space. This means that now fewer molecules are present in a given space than earlier indicating a decrease in density. The density of a substance is given by the mass of a substance divided by its volume. The relative density of a substance is the density of that substance relative to the density of another substance that is being used as the reference material. It does not have any units. Usually, water is chosen as the reference material as the density of water is 1 gm/cm3 which makes the calculations easier.
