Chemistry Solutions

7H Solutions Introducing solutions Separating mixtures Chromatography Contents 7H Solutions Introducing solutions Separating mixtures Chromatography More about solubility Summary activities

Mixtures and solutions A mixture is two or more substances that are mixed together but not chemically joined. A solution is a special type of mixture that is made when a solid dissolves and mixes a liquid. For example, a cup of instant coffee is a solution. The solid that dissolves (e.g. coffee granules) is called the solute. The liquid that does the dissolving (e.g. hot water) is called the solvent. How many other examples of solutions can you think of?

Solute + solvent

Solubility experiment Points to note are that: when a solute dissolves, mass is conserved. when a solute dissolves, the solute and solvent particles intermingle. increasing the temperature increases the rate of dissolving.

7H Solutions Introducing solutions Separating mixtures Chromatography Contents 7H Solutions Introducing solutions Separating mixtures Chromatography More about solubility Summary activities

Salty water Where does the salt around the Dead Sea come from?

Separating mixtures Sea water is a salty mixture that contains water, salt, sand and other substances. The Dead Sea is a salty lake that contains some of the saltiest water in the world! The Dead Sea is almost six times more salty than the ocean, so nothing is able to live in it and that’s why it is called “dead”. The substances in a mixture, such as salty water, are not chemically joined which means they can be separated. How is salt separated from salty water?

How are solids separated out of mixtures? Separating an insoluble solid and a liquid If a solid is insoluble (e.g. sand in water) then it is easy to separate it by filtering the mixture. The insoluble solid cannot pass through the filter paper but the water can. Separating a soluble solid and a liquid To separate a soluble solid from a liquid (e.g. salt and water), evaporation can be used. The solution is heated so that the water evaporates and leaves the dissolved solid behind.

Filtering

Evaporation

Separating salt from sea water The main salt dissolved in sea water is “common salt”. In hot areas, how can the Sun be used to separate the salt from sea water? The heat of the Sun evaporates the water, the salt is left behind and collected in salt beds. Salt can also be obtained from ‘rock salt’ found in layers under the ground. How are dissolving, filtering and evaporating used to separate the salt from rock salt?

Separating solvents Evaporation can be use to separate and collect the solute in a solution. How can the solvent be collected? The technique use to obtain a solvent from its solution is called distillation. Distillation has three steps: evaporating; condensing; collecting. The solution is heated so that the solvent (a liquid) evaporates and is turned into a gas. The solute is left behind. The gas cools in the condenser and turns back into a liquid. This liquid is collected and is pure solvent.

Distillation

Which separation technique?

7H Solutions Introducing solutions Separating mixtures Chromatography Contents 7H Solutions Introducing solutions Separating mixtures Chromatography More about solubility Summary activities

What is chromatography? Chromatography is used to separate mixtures of coloured or non-coloured substances that are soluble in the same solvent. spot of mixture Method A solvent Method B solvent pipette spot of mixture A spot of the mixture is placed on some filter paper. In method A, the solvent is soaked up the paper. In method B, the solvent or is slowly dripped onto the paper. The substances in the mixture get separated because more soluble substances spread along the paper faster.

Chromatography experiment

Identifying dyes in a mixture Chromatography can be used to find out if a dye is made up of a single coloured substance or a mixture. 1 2 3 4 5 6 mixture 1 2 3 4 5 6 mixture The mixture consists of dyes 3 and 5. Dots of known single dyes are placed alongside the dot for the unknown mixture. After the solvent washes through the paper, the pattern of the dyes in the mixture is compared with the single dyes. Which dyes does the mixture consist of?

Identifying dyes in a mixture Which dyes are in the mixture? 2 3 4 5 6 mixture 2 3 4 5 6 mixture The mixture consists of dyes 1 and 4.

7H Solutions Introducing solutions Separating mixtures Chromatography Contents 7H Solutions Introducing solutions Separating mixtures Chromatography More about solubility Summary activities

Conservation of mass experiment

10 g 50 g 60 g ? Conservation of mass If 10 g of salt is added to 50 g of water, what is the mass of the solution? 10 g 50 g ? 60 g How much salt will be recovered if the mixture is separated by evaporation?

Conservation of mass – extension If 10 g of salt is added to 50 g of sea water, what is the mass of the solution? 10 g 50 g ? 60 g How much salt will be recovered if the mixture is separated by evaporation?

Does a solid keep dissolving? Points to note are that: when a solid is added to a liquid, eventually no more will dissolve; different masses of different solids dissolve in the same volume of a particular solvent. that solids can dissolve in liquids other than water.

How does temperature affect solubility? Does sugar dissolve in a cup of cold tea? The sugar does dissolve but not as much as in a cup of hot tea. The sugar is more soluble at higher temperatures. The amount of a solute that can dissolve at a given temperature is called its solubility. How does the solubility of a substance change with temperature? The solubility of a substance usually increases as the temperature increases.

7H Solutions Introducing solutions Separating mixtures Chromatography Contents 7H Solutions Introducing solutions Separating mixtures Chromatography More about solubility Summary activities

dissolving – The mixing of a solid with a liquid to make a Glossary dissolving – The mixing of a solid with a liquid to make a solution. mixture – Two or more substances that are mixed together but are not chemically joined. saturated – A solution containing the maximum amount of solute that it can hold. soluble – A substance that can dissolve in a solvent. solubility – A measure of how much solute can dissolve in a solvent at a given temperature. solute – Solid that dissolves in a solvent to make a solution. solution – Mixture made when a solute dissolves in a solvent. solvent – Liquid in which a solid dissolves to make a solution.

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