LIWA

Elements Compounds and Mixtures

ICSE Class 8 · Chemistry · Chapter 3

What this chapter covers

Key definitions

Iron sulphide is a compound of iron and sulphur in which the two elements are chemically combined in a fixed proportion.

Sieving is a method of separating particles of different sizes by passing the mixture through a mesh that lets only the smaller particles through.

Sedimentation is the settling of heavier, insoluble solid particles at the bottom of a liquid when the mixture is left undisturbed.

Decantation is the careful pouring off of the clear liquid from above a sediment without disturbing it.

Filtration is the process of separating an insoluble solid from a liquid by passing the mixture through a filter, which holds back the solid.

Evaporation is the process of heating a solution so that the liquid changes into vapour and escapes, leaving the dissolved solid behind.

Magnetic separation is a method that uses a magnet to remove a magnetic substance, such as iron, from a mixture with non-magnetic substances.

Sublimation is the change of a solid directly into vapour on heating, without first becoming a liquid; on cooling, the vapour changes directly back into the solid.

Distillation is the process of heating a liquid to change it into vapour and then cooling the vapour to get back the pure liquid.

Fractional distillation is a method of separating two or more liquids that mix with each other, by heating the mixture and collecting each liquid separately as it boils off at its own boiling point.

Miscible liquids are liquids that mix completely with each other to form a single layer, such as alcohol and water.

Immiscible liquids are liquids that do not mix with each other and form separate layers when left to stand.

Key points

Questions answered

Define an element and a compound, and give two examples of each.
An element is a pure substance made of only one kind of atom that cannot be broken down into simpler substances by chemical methods, such as iron and oxygen. A compound is a pure substance formed when two or more elements combine chemically in a fixed proportion by mass, such as water and carbon dioxide.
Give three differences between a mixture and a compound.
The parts of a mixture are present in any proportion, but the elements in a compound are in a fixed proportion. The parts of a mixture keep their own properties, but a compound has new properties. A mixture can be separated by physical methods, but a compound needs chemical methods.
Classify each as an element, a compound or a mixture: brass, sodium chloride, sulphur, milk, carbon dioxide, air.
Elements: sulphur. Compounds: sodium chloride, carbon dioxide. Mixtures: brass, milk, air.
Iron filings and sulphur are mixed, and part of the mixture is heated strongly. Give two observations that show a new substance forms on heating.
The heated part forms a black solid that a magnet does not attract, and it does not show separate yellow and grey specks or float on water. The reaction also glows and gives out heat.
What is meant by the principle of a separation technique? State the principle of filtration.
The principle is the difference in a property of the parts of a mixture that the technique uses. Filtration works because the insoluble solid particles are too large to pass through the pores of the filter paper, while the liquid passes through.
Why is a separating funnel used for kerosene and water but not for alcohol and water?
Kerosene and water are immiscible and form two layers, which the funnel can run off one at a time. Alcohol and water are miscible and form a single layer, so they must be separated by fractional distillation using their different boiling points.
How would you obtain pure water from salt water? Name the technique and state its principle.
By distillation. Water boils at a much lower temperature than salt, so the water turns into steam, which is cooled in a condenser and collected as pure water, while the salt stays behind.
Describe how to separate calcium carbonate from common salt.
Stir the mixture in water so the salt dissolves, filter to collect the insoluble calcium carbonate as residue, and evaporate the filtrate to get the salt.