3.1 Changes of state Flashcards
Density
Density is given by ρ=m/V, where the ρ is the Greek letter rho,
m is mass, V is volume.
Law of displacement
The Law of Displacement says that an object completely submerged in a fluid (like water) will replace an amount of fluid equal to its own volume.
Upthrust in fluids
An object floats in a fluid if the density of the object is lower than the density of the fluid.
The force that keeps the object afloat is called upthrust.
Physical changes
When a material changes state, it is able to recover its old properties if the change is reversed.
Some chemical reactions are also reversible.
However, some chemical reactions are irreversible.
In irreversible chemical reactions, the material would not be able to return to its original properties.
Conservation of mass
When a substance changes state, its mass is conserved (stays the same).
Melting
If we heat a solid, the solid particles vibrate more energetically, until they have enough energy to overcome the forces between them.
This is when the solid melts (becomes a liquid).
The temperature needed for this to happen is the melting point.
Melting is sometimes also called fusion.
Boiling
If we heat a liquid, the liquid particles move more energetically until they have enough energy to escape completely from the forces between them.
The particles become a gas and move completely freely.
The temperature needed for this to happen is the boiling point. Boiling can be called vaporisation.
Condensation
When the temperature of a gas decreases to the boiling point, the strength of the forces between particles increases and the particles condense to become a liquid.
For the same material, its boiling point is the same as its condensing point.
Freezing
When the temperature of a liquid decreases to the melting point, the strength of the forces between particles increases and the liquid solidifies to become a solid.
For the same material, its melting point is the same as its freezing point.
Change of state
When a substance reaches a certain temperature, energy will stop being transferred to the kinetic store of the molecules and will be transferred to their potential store instead.
This energy goes into overcoming the intermolecular forces of attraction between the molecules, causing them to move further apart from one another leading to a change of state.
When a substance changes its state:
The potential energy of the molecules increases, allowing them to overcome the intermolecular forces of attraction.
The kinetic energy remains the same, meaning that the temperature will remain the same, even though the substance is still being heated.
Practical 5 (determining densities of regular objects)
Aim: determine the densities of regular objects by using measurements of their dimensions.
Procedure - Place the object on a digital balance and note down its mass.
Use either the ruler, Vernier calipers or micrometer to measure the object’s dimensions (width, height, length, radius) – the apparatus will depend on the size of the object.
Repeat these measurements and take an average of these readings before calculating the density.
Practical 5 (determining densities of irregular objects)
Aim: determine the densities of irregular objects using a displacement technique.
Procedure - Place the object on a digital balance and note down its mass.
Fill the eureka can with water up to a point just below the spout.
Place an empty measuring cylinder below its spout.
Carefully lower the object into the eureka can.
Measure the volume of the displaced water in the measuring cylinder.
Repeat these measurements and take an average before calculating the density.
Practical 5 (determining densities of a liquid)
Aim: determine the density of a liquid by finding a difference in its mass.
Procedure - Place an empty measuring cylinder on a digital balance and note down the mass.
Fill the cylinder with the liquid and note down the volume.
Note down the new reading on the digital balance.
Repeat these measurements and take an average before calculating the density.
