Density and properties of matterIB MYP Chemistry: Revision notes
Section 1
Density
Density tells you how much mass is packed into a certain volume.
density = mass ÷ volume
The units depend on the units of mass and volume:
- mass in g and volume in cm³ gives density in g/cm³
- mass in kg and volume in m³ gives density in kg/m³
Worked example: a metal block has mass 120 g and volume 40 cm³. Density = 120 ÷ 40 = 3.0 g/cm³.
Rearranged: mass = density × volume, and volume = mass ÷ density.
Density does not depend on the size of the sample. A large and a small piece of the same pure metal have the same density, although their masses differ.
Section 2
Measuring mass and volume
Mass: use a balance. For a liquid, measure the mass of the empty container, then the container with the liquid, and subtract.
Volume of a regular solid: measure the lengths with a ruler and calculate, for example length × width × height for a cuboid.
Volume of an irregular solid: use displacement. Place the solid in water in a measuring cylinder. The volume of the solid is the rise in the water level (final volume − starting volume).
Volume of a liquid: read it from a measuring cylinder, with your eye level with the bottom of the curved surface (the meniscus).
Worked example: the water level rises from 50 cm³ to 82 cm³. The volume of the solid is 82 − 50 = 32 cm³.
Write the units in every line of working. They will tell you whether you have divided the right way round.
Section 3
SI units and standard form
The SI units are the standard units used in science: kilogram (kg) for mass, metre (m) for length, cubic metre (m³) for volume and kg/m³ for density.
Useful conversions:
- 1 m = 100 cm, so 1 m³ = 1 000 000 cm³
- 1 kg = 1000 g
- 1 cm³ = 1 mL
- 1 g/cm³ = 1000 kg/m³
Standard form writes a number as a number between 1 and 10 multiplied by a power of ten. For example, the density of iron is 7.9 g/cm³ = 7900 kg/m³ = 7.9 × 10³ kg/m³.
To convert mm to cm, divide by 10, so 4.0 mm = 0.40 cm.
Section 4
Physical properties
A physical property can be measured or observed without changing the substance into a new substance. Examples:
- Melting point and boiling point
- Density
- Solubility: how much of a substance dissolves in a solvent
- Electrical conductivity: whether it conducts electricity
- Hardness: how resistant it is to being scratched or dented
Physical properties help us choose materials. For example, aluminium is used for aircraft panels because it has a low density.
Section 5
Chemical properties
A chemical property describes how a substance reacts, or changes into new substances. Examples:
- Flammability: whether it burns
- Reactivity: how easily it reacts, for example with oxygen, water or acids
You can only observe a chemical property by carrying out a chemical change. For example, steel rusts because iron reacts with oxygen and water. Rusting is a chemical property of iron.
Physical properties and chemical properties are both used when choosing a material for a job.
Must Know
- Density = mass ÷ volume, in g/cm³ or kg/m³
- 1 g/cm³ = 1000 kg/m³
- Regular solids: calculate volume from lengths. Irregular solids: use displacement
- Standard form: a number from 1 to 10 × a power of ten
- Physical properties: melting and boiling point, density, solubility, conductivity, hardness
- Chemical properties: flammability and reactivity
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Density and properties of matter
- A jeweller in Jaipur wants to check whether a small shiny bar is pure gold or a cheaper metal that looks similar. The bar is a cuboid measuring 2.0 cm by 1.5 cm by 2.5 cm. She finds its mass is 96.5 g.Pure gold has a density of 19.3 g/cm³. Decide whether the bar is pure gold and justify your answer.2 marks
- A student in São Paulo wants to find the density of an irregular piece of rock. She finds that the rock has a mass of 84 g. She half-fills a measuring cylinder with water and reads 50 cm³. She lowers the rock into the water on a thread and the water level rises to 82 cm³.Calculate the density of the rock in g/cm³. Give your answer to 2 significant figures.2 marks
- A student in Cape Town wants to find out how adding salt changes the density of water. She dissolves different masses of salt in 100 cm³ of water to make four solutions, all at the same temperature. For each solution she measures 50 cm³ in a measuring cylinder and finds the mass of the 50 cm³ sample using a balance.Identify the independent variable, the dependent variable and one control variable.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).