Mixtures

Announcements We will have a short quiz on Tuesday morning. This will be on everything that we have covered so far up to the end of this week. Previous Question: : In which ways can we tell if a chemical change takes place?

Goal of the class To understand mixtures, compounds and molecules. Question of the day: How can we separate a mixture? Previous answer: There are a number of ways such as a change in colour, a precipitate formed, a change in temperature, smell or gas given off. Previous Question: : In which ways can we tell if a chemical change takes place?

Molecules Atoms of most elements have the ability to combine with other atoms. When atoms combine they form a chemical bond, this chemical bond is a force of attraction between two atoms. Atoms can sometimes form small groups called molecules. In fact, it’s rare to find atoms on their own with the exception of the noble gases.

Molecule Examples Examples of molecules include: Water (H2O) Carbon dioxide (CO2) Oxygen (O2) Octane (C8H18) Glucose (C6H12O6) Models of molecules have particular colours. Oxygen = red Hydrogen = white Carbon = black Sulphur = ??? Nitrogen = blue Chlorine = green

Compounds A compound is a pure substance made from two or more elements chemically combined in a set ratio. This ratio is shown by a compound’s chemical formula. When elements form compounds their properties are different. Test knowledge of ratios Example 9 boys and 15 girls Ratio of 3:5 as you can divide by 3 The ratio doesn’t tell you have much you have. Making custard, 1 spoon sugar and 1 spoon of custard powder but you can add as much as you like. Use water as an example O, h = gas H2O = liquid Sodium chloride also

Mixtures Elements and compounds are pure substances, but most of the materials you see everyday are not. A mixture is made of two or more substances that are together but not chemically combined. Example a handful of soil. Bits of clay, sand, rocks, plants and water. Different amounts – no fixed ratio. Different from place to place

Heterogeneous Mixtures When you can see the different parts of the mixture. E.g. A salad

Homogeneous Mixtures You can’t see the different parts of the mixture. E.g. Sugar dissolved in water. Or oxygen in the air. Can also be solids Brass is a mixture of copper and zinc (an alloy)

Separating Mixtures Suppose you work at a recycling factory and you have to separate steel cans from aluminium cans, how would you do it? Use a big magnet to attract the steel cans, leaving the aluminium ones behind.

Separating Mixtures Other ways in which you can separate mixtures: Distillation, e.g. crude oil and alcohol Evaporation, e.g. sea salt and sugar in water Filtering, e.g. coffee and stones in water Ask about how to separate large balls and small balls Comment on coin slots how they sort by size and mass Coffee filter paper Fish use filtering in their gills to filter oxygen from the water.

Vocabulary Molecule – Small groups of atoms chemically bound together. Distillation – Separating fluids by their boiling points. Chemical Formula – The ratio of atoms in a substance. What does that tell you about water? It’s not an element.

Homework Please complete Chemical Building Blocks workbook page 11