Elements Compounds and Mixtures
ICSE Class 7 · Chemistry · Chapter 3
What this chapter covers
- Pure substances and impure ones
- Elements and compounds
- Mixtures
- Mixtures versus compounds
- Types of mixtures
- Why and how mixtures are separated
- Separation techniques
- Choosing the right order of methods
- Paper chromatography
Key points
- Matter is classified as elements, compounds and mixtures; elements and compounds are pure substances.
- An element contains only one kind of atom and is written with a single symbol, such as Fe, Cu or Al.
- A compound contains two or more elements combined chemically in a fixed proportion; it has a formula, such as H2O, NaCl or CaO, and properties different from its elements.
- A mixture contains two or more substances in any proportion; each keeps its own properties, and no chemical change occurs.
- Heating iron and sulphur in a fixed proportion forms iron sulphide, a compound; simply stirring them forms a mixture.
- Homogeneous mixtures have the same composition throughout (sugar solution, alloys, vinegar); heterogeneous mixtures do not (sand and salt, kerosene and water).
- By state, mixtures may be solid–solid, solid–liquid or liquid–liquid.
- Every separation method uses a difference in a physical property of the components.
- Evaporation recovers a dissolved solid; distillation recovers a pure liquid; a separating funnel separates immiscible liquids; sublimation removes a solid that changes directly into vapour (iodine, camphor, naphthalene); ammonium chloride only appears to sublime, as it splits into two gases that join again on cooling, so heating still separates it from common salt; fractional distillation separates miscible liquids with different boiling points.
- In a mixture of several components, the order of methods matters; a step must not dissolve or spoil a component needed in a later step.
- Paper chromatography separates coloured substances in a mixture using a stationary phase (paper) and a mobile phase (solvent); components move at different speeds.
Questions answered
Classify as element, compound or mixture: copper, sea water, quicklime, brass, plaster of Paris, aluminium.
Elements: copper, aluminium. Compounds: quicklime (calcium oxide), plaster of Paris. Mixtures: sea water, brass (an alloy).
Write the symbol of iron and of sodium, and explain why neither begins with the first letter of the English name.
Iron is Fe and sodium is Na. Their symbols come from their Latin names, ferrum and natrium.
Give two reasons why a heated iron–sulphur product is a compound and not a mixture.
A magnet cannot separate the iron from it, and it is black, with properties unlike either grey iron or yellow sulphur. It also forms only in a fixed proportion and gives out heat while forming.
Is air homogeneous or heterogeneous? Is sand in water homogeneous or heterogeneous?
Clean air is homogeneous; its gases are evenly spread. Sand in water is heterogeneous; the sand can be seen and settles at the bottom.
Name the method and its principle for separating oil from water.
A separating funnel. Oil and water are immiscible and of different densities, so they form two layers and the lower one can be run off through the tap.
Why can alcohol and water not be separated with a separating funnel? Which method is used instead?
They are miscible and form a single layer. Fractional distillation is used, because alcohol (about 78 °C) boils at a lower temperature than water (100 °C).
Why is distillation preferred over evaporation for obtaining drinking water from salty water?
Evaporation lets the water escape as vapour, so only the salt is kept. Distillation condenses the vapour and collects the pure water.
Describe how you would separate a mixture of iodine and sand.
Heat it gently in a china dish covered with an inverted funnel. Iodine sublimes and collects on the cool funnel; sand stays in the dish.