Matter and its changes (grade 8) · lesson

Pure substances and mixtures

💡 Elements and compounds are pure substances, while a mixture holds substances that are not joined and can be separated.

An element has only one kind of atom, like oxygen gas or iron. A compound has two or more elements chemically joined, like water or carbon dioxide.

A mixture has two or more substances that are not chemically joined. Salt water and air are mixtures.

You can separate a mixture using differences in properties. Salt dissolves in water, but sand does not.

🧠 Let's work it out

Plan a separation by property.
Add water, and the salt dissolves.
Filter, and the sand stays on the paper.
Heat the salty water, and the water leaves.
Dry salt is left behind.

What do you think comes next?

compound: two or more elements chemically joined
mixture: substances mixed together but not chemically joined

🔗 Khan Academy: Classifying matter · Khan Academy ↗

Properties identify a substance

💡 A pure substance has set properties, like one exact melting point, so matching properties identify it.

Each pure substance has its own properties. Its melting point, boiling point, and density do not change.

A pure substance melts at one exact temperature. A mixture usually melts slowly over a range.

If two samples share all the same properties, they are most likely the same substance.

🧠 Let's work it out

Compare samples side by side.
X and Y are both solids that melt at 801 °C and dissolve in water.
Z is a liquid that boils at 78 °C.

What do you think comes next?

melting point: the temperature at which a solid melts

Density

💡 Density is mass divided by volume, and it can identify a material or tell if it will float.

Density tells how much mass is packed into a space. Density = mass ÷ volume.

Each pure material has its own density. Iron is 7.9 g/cm³. Aluminum is 2.7 g/cm³.

Water's density is 1 g/cm³. Things less dense than water float. Things more dense sink.

🧠 Let's work it out

Find the density, then compare.
A block is 79 g and 10 cm³.
79 ÷ 10 = 7.9 g/cm³. That matches iron.
A wood block of 30 g and 50 cm³ is 0.6 g/cm³, less than water, so it floats.

What do you think comes next?

density: how much mass is in a certain volume
volume: how much space something takes up

🔗 PhET simulation: Density · PhET Interactive Simulations ↗

Synthetic materials

💡 Synthetic materials are made by chemical reactions from natural resources, and they bring both benefits and costs.

A synthetic material is made by people through chemical reactions. Nylon and plastic are made from chemicals in oil. Wool, cotton, and granite are natural.

The new material has new properties, like a new melting point. That shows a new substance was made.

Synthetic materials can help. Medicines made in factories give a steady supply without picking many wild plants. They also have costs. Oil runs out, and plastic litter can harm sea animals.

🧠 Let's work it out

Weigh benefits and costs.
Ask where the material comes from, what it does well, and where it ends up.
A plastic bag is cheap and handy.
But it uses up oil and lasts a long time as litter.

What do you think comes next?

synthetic: made by people through chemical reactions
natural resource: something from nature that people use, like oil or wood