Class 8 Science Chapter 9: The Amazing World of Solutes, Solvents, and Solutions Notes

What is this chapter about?

Homemade ORS tastes the same whether you take the first sip or the last; it is never salty in one mouthful and sweet in the next. That happens because sugar and salt spread evenly through the water. The chapter explores such mixtures: what dissolves in what, how much can dissolve, and what heat does to it. Its second half asks why husk floats while rice sinks when rice is washed, which leads to density and to measuring mass and volume carefully.

Solutes, solvents and solutions

Mix salt or sugar with water and you get a uniform mixture, called a solution. Stir chalk powder, sand or sawdust into water and the parts stay visible, so those are non-uniform mixtures, not solutions. When a solid dissolves in a liquid, the solid is the solute and the liquid is the solvent: solute + solvent → solution. The chashni of gulab jamun holds far more sugar than water, yet sugar is still the solute. When two liquids mix, the one present in the smaller amount is called the solute. Air, a uniform mixture of gases, is a solution too.

How much can dissolve?

Add salt to half a glass of water one spoon at a time (Activity 9.1). At first every spoonful disappears; after a few, some salt stays at the bottom however much you stir. Water can hold only a limited amount. A solution that can still dissolve more is unsaturated; once extra solute settles, it is saturated at that temperature. The amount of solute in a fixed amount of solution is its concentration: less solute makes a dilute solution and more makes a concentrated one, words that only compare. The most solute that a fixed amount of solvent can take is its solubility. Read solubility.

Temperature and solubility

In a demonstration (Activity 9.2), baking soda is stirred into 50 mL of water at 20 °C until some stays undissolved. Warmed to 50 °C, the leftover dissolves and more can be added; at 70 °C even more goes in. For most solids, solubility rises with temperature, so a saturated solution becomes unsaturated when heated. Gases behave the other way. Oxygen dissolves in water only slightly, yet that dissolved oxygen keeps fish and water plants alive, and cold water holds more of it than warm water.

Water has long been the main solvent in Ayurveda, Siddha and other Indian systems of medicine, with oils, ghee and milk also used to carry medicines. The chemist Asima Chatterjee used solvents to extract compounds from medicinal plants and developed anti-epileptic and anti-malarial drugs.

What is density?

A wooden stick and an iron rod of the same size feel very different in the hand. A crowded bus and a nearly empty one, or a dense forest and a sparse one, show the idea: how tightly matter is packed. Density is the mass in one unit of volume, density = mass ÷ volume. It does not depend on an object’s shape or size. Its SI unit is kg/m³, and g/mL or g/cm³ are handy for small amounts. Since 1 mL of water has a mass of about 1 g, water’s density is about 1 g/mL. Relative density compares a substance with water: aluminium, 27 g in 10 cm³, is 2.7 g/cm³, so its relative density is 2.7. Read density.

Measuring mass and volume

Mass is measured with a balance. On a digital balance, check that the display shows zero, put a clean watch glass on the pan, press tare to return to zero, and then place the object; a stone might read 16.400 g. Mass is the quantity of matter, in grams or kilograms; weight is the Earth’s pull, in newtons. Volume of a liquid is measured with a measuring cylinder: find the smallest reading by dividing the gap between two numbered marks by the number of small divisions, keep your eye level with the meniscus, and read its bottom for water. For a cuboid, volume = length × width × height. For a stone, use displacement. Read how to measure mass and volume.

Temperature, pressure and density

Heating makes particles move apart, so a substance takes up more space with the same mass, and its density falls. That is why hot air rises above cooler air and how a hot air balloon lifts off. Pressure squeezes gases into a smaller volume and raises their density, but barely affects liquids and solids. Water is unusual: it is densest at 4 °C and expands as it freezes, so ice floats and shelters the water and fish below. Inside the Earth the layers grow denser from the light crust to the inner core.

Before we move on

Long before ships of steel, people crossed rivers on rafts of hollow bamboo and wooden logs, which float easily. In Ningel village in Manipur, families still boil salty well water in metal pans until salt crystals form, then shape them into salt cakes. Answers to every question are in the NCERT solutions for Chapter 9.