Chapter 8: Solutions and Suspensions

Before we move on,

We have… Representing the

video clips available

Lesson Outline• distinguish among solute, solvent and solution

• show an awareness of the importance of factors that affect the solubility and rate of dissolving of substances in homes and industries

• show an understanding that indicators are substances that change colour when an acid or alkali is added to them

• deduce the nature of solutions and suspensions by simple laboratory tests

• investigate the factors affecting the solubility and rate of dissolving of substances

• investigate the effect of a variety of acidic, alkaline and neutral solutions on Universal Indicator paper and natural indicators

• investigate the effect on Universal Indicator paper when acidic and alkaline solutions are mixed

• investigate the properties of acidic and alkaline solutions

• show an appreciation of scientific attitudes (accuracy) in investigations on solubility and pH

8.1 Solutions

A solution contains both solute and solvent.

solute

substance that the solute dissolves in

Generally, there are three types of solutions:1. Dilute solution – contains small amount of solute dissolved in

large amount of solvent2. Concentrated solution – contains large amount of solute

dissolved in small amount of solvent3. Saturated solution – contains maximum amount of dissolved

solute in a given amount of solvent at a given temperature

substance that dissolves

solvent= =+ solution=

mixture in which its components are evenly mixed

When a substance dissolves in a solvent, it is soluble in that solvent.Water is known as the universal solvent as many substances can dissolve in it.Aqueous solution is thus a solution which has water as the solvent.Nature of solutionsWhen a solution is formed, solute particles are evenly and completely mixed with the solvent particles. evenly

mixedCan solutions be separated by physical methods?

solvent solute

Alcoholic drinks

Homogeneous (solution) = physical (e.g. colour) and chemical properties are the same in every part

The solute does not settle to the bottom or float to the top when the iodine in alcohol solution is left to stand

Light passes through the solution when light is shone through it.Reason : The tiny solute particles are too small to block any light passing through the solution.Filtration cannot be used to separate solutes from its solvent.Reason : The tiny solute particles can pass through the filter paper.

8.2 Suspensions

A suspension = mixture containing insoluble solid particles suspended in a liquidA suspension can also consist of liquid droplets or fine solids in a gasSome example of suspensions

chalk dust and water

calamine lotionhazy

environmentmedicine

Nature of suspensionsWhen a suspension is formed, solute particles remain as large visible insoluble solid particles

Heterogeneous (suspension) = physical (e.g. colour) and chemical properties are different in every partThe insoluble particles will settle to the bottom or float to the top when the suspension is left to stand

Light cannot pass through the suspension when light is shone through itReason : large insoluble particles are large enough to prevent light passing through the suspension

Filtration can be used to separate solute from its solvent and the residue collected is the soluteReason : The insoluble particles are too large to pass through the filter paper

8.3 Factors Affecting the Solubility of

SubstancesSolubility = maximum amount of a solute which can dissolve in 100 g of solvent at a fixed temperatureFor example: 200 g of salt in 100 g of water at 30 °C Factors affecting the solubility of a solute in a solvent:• nature of the solute• nature of the solvent• temperature of the solutionSolubility of solids and liquids when temperature

Solubility of gases when temperature

8.4 Factors Affecting the Rate of Dissolving of Substances Rate of dissolving = time taken for solute to completely dissolve when it is added to a fixed amount of solvent

Factors affecting the rate of dissolving a solute in a given volume of solvent:

• surface area of the solute

• temperature of the solution

• rate of stirring

Faster rate of dissolving

85 °C

15 °C

Faster rate of dissolving

Applications at Home and in the Industry Many substances that we use are in the form of solutes, solvents

and solutions. Hence, it is important to know about factors affecting the solubility and rate of dissolving of substances

At home: washing machine uses hot water and rotating drum to increase the solubility and rate of dissolving of detergent in water In fizzy drinks industry:

Temperature of water,

Solubility and rate of dissolving extracts

BUT,Temperature of water,

Solubility and rate of dissolving of carbon dioxide gas

So how do we increase the solubility and rate of dissolving of the extracts and gas at the same time?

8.5 Acids and Alkalis

acid alkali

Acids and alkalis are two common types of aqueous solutions we use every day.AcidsAcids can be found all around us:

Citrus fruitsContains: Citric acid

TeaContains: Tannic acid

Stomach secretionsContains: Hydrochloric acid

General Properties of Acidic Solutions• Acids are generally corrosive – they can burn our skin and react with many materials• All acids have sour taste• Acids turn blue litmus paper red

• Dilute acids react with reactive metals to produce hydrogen gas

• Aqueous solutions of acids are good electrical conductors

Test for hydrogen gas: a ‘pop’ sound is

heard when a burning splint comes into contact with the hydrogen gas

Sodium + dilute hydrochloric acid sodium chloride + hydrogen(metal)

(acid)

(salt)

(gas)

Word equation:

Go to Video 8.1

Test of carbon dioxide: Clear limewater will turn chalky when carbon dioxide passes through it (white precipitate formed)

• Dilute acids react with carbonates to produce carbon dioxide gas

calcium carbonate + dilute nitric acid calcium nitrate + carbon dioxide + water(carbonat

e)(acid)

(salt)

(gas)

AlkalisAlkalis can also be found all around us:

Household cleanersContains: Ammonia solution

SoapContains: sodium hydroxide

Go to Video 8.2

General Properties of Alkaline Solutions• Alkalis are generally corrosive – they can burn our skin and can cause bleaching• All alkalis taste bitter and feel soapy• Alkalis turn red litmus paper blue• Alkalis are good electrical conductors

• Alkalis react with acid to form salts can water onlyPotassium hydroxide + nitric acid potassium nitrate + water (alkali

)(acid)

(salt)

Go to Video 8.3

8.6 Indicators

To determine how acidic or alkaline a solution is, we use a suitable indicator (e.g. litmus paper, Universal Indicator)

Acidity increases

Alkalinity increases

Indicator = substance which changes colour according to whether the solution is acidic, alkaline or neutralUniversal Indicator• mixture of several types of indicators • shows a variety of colours according to the different degrees of acidity or alkalinity of the solution• comes with a colour chart for comparing the colour shown on it (representing degree of acidity or alkalinity) with colour chart

Compare the colour changes of the different indicators

Go to Video 8.4