Chapter 7: Solutions Air is a Gaseous Solution Sodium Chloride Liquid Solution Section 1: Solutions VS Mixtures What is a mixture? • Mixtures of different substances exist all around us. – Many of these mixtures are invisible, and we never notice them. – However, some mixtures can easily be identified. What is a mixture? • All matter is either a pure substance or a mixture of other substances. – Definition: pure substance – matter that has a fixed chemical composition. • For example: Water is a pure substance. Kool-Aid is a mixture of It is ALWAYS H2O. water and other substances. Types of Mixtures • Mixtures can be categorized into two groups: – Heterogeneous and Homogeneous – Definition: heterogeneous mixture – a mixture made of visibly different substances. • The particles in a het. Mixture are not spread evenly. • Orange juice with pulp is an example of a heterogeneous mixture. Heterogeneous Mixture Clear Orange Juice Orange Juice Pulp Heterogeneous Mixtures Different fragments make up this rock. This rock is a heterogeneous mixture. Homogeneous Mixtures • Homogeneous mixtures look the same all over. – Definition: homogeneous mixture – a mixture in which the particles are spread evenly. – Sweet tea is an example of a homogenous mixture. – There are several different substances (tea, water, and sugar) but you cannot see them. The tea looks uniform (the same) throughout. + + = The tea looks the same throughout the pitcher. Solutions • We can call homogeneous mixtures by another name: solutions. – Definition: solution - a homogenous mixture of 2 or more substances that are evenly dispersed. Solutions • Many solutions are formed by dissolving one substance into another substance. – These tablets are dissolving in the water to form a solution. What is in a Solution? • It is important to know what makes up a solution. • All solutions are made of solutes and solvents. – Definition: solute - a substance that dissolves into another substance. – Definition: solvent - a substance that a solute is dissolved into. Solvents • For example: – When you stir sugar into water, the sugar dissolves. – The water is the solvent. – The sugar is the solute. + = Solute Solvent Solution The Universal Solvent • Water is very good at dissolving things. – Water is known as “The Universal Solvent” Other types of Solutions • Not all solutions contain water! • Other states of matter can be solutions. – Gases & Solids can form solutions also. – Air is an example of a gaseous solution. – Air is composed of lots of different gases that we cannot see. Metal Alloys • 2 or more solids can form solutions also. – Metal alloys are homogenous mixtures that contain a metal mixed with another substance. – Some examples are: • Steel – iron and carbon • Brass – copper and zinc • Bronze – copper and tin Metal Alloys • In order to make an alloy, the metals must be melted. – While melted, the metals are mixed to form a solution. Bronze + Tin Copper Chapter 7: Solutions Section 2: How Substances Dissolve How do things dissolve? • Water can dissolve ionic compounds because of its structure. – The electrons in the hydrogen atoms are pulled toward the oxygen atom. – This gives the oxygen atom a slight negative charge. – The hydrogen atoms gain a slight positive charge. The Water Molecule H H Oxygen Polar Compound • Because water’s + and – charges are not spread out evenly, it becomes “polar”. – Definition: polar compound – a molecule that has a positive side and a negative side. •Because water is a polar compound, it is a good solvent. Like dissolves Like – In chemistry, a rule of thumb is that “like dissolves like.” – Water is a polar compound, so it can dissolve other polar compounds. – If water cannot dissolve a substance, then that substance is “nonpolar”. Nonpolar Compound – Definition: nonpolar compound – a compound that has no charge on its molecules.. •Nonpolar compounds can only dissolve other nonpolar substances. •Example – oil-based paint will not dissolve in water. A nonpolar solvent must be used. Polar vs. Nonpolar Oil is Nonpolar Water is Polar They cannot mix. The Dissolving Process • We have all seen solutes dissolve into solvents before. • And we all *probably* know some ways to speed up the process. The Dissolving Process • Making a solute smaller makes it dissolve faster. – By crushing up a solute, you increase the surface area. Crushed Salt Rock Salt The Dissolving Process • Stirring or shaking will make a solute dissolve faster. The Dissolving Process • Increasing the solvent’s temperature will make the solute dissolve faster. Which one will Dissolve sugar Fastest? The Dissolving Process: Gases • Did you know that liquids can dissolve gases? • Fish and other aquatic life breath oxygen that has dissolved into water. • Liquids dissolve gases best when they are cold. The Dissolving Process: Gases • Don’t believe it? Well, consider this… – Which makes a louder “whoosh” sound when opened… a hot soda or a cold one? Concentration • The amount of solute dissolved in a solvent affects its concentration. – Think of concentration as being how “strong” a solution is. – Definition: concentration – the amount of a substance in a certain amount of solution. Concentration • “Concentrated” substances have lots of solute. • “Diluted” substances only have a little solute. Saturated / Unsaturated • When a solvent can no longer hold any more solute, we called it “saturated”. – Definition: saturated solution – a solution that cannot dissolve any more of a given solute. – Definition: unsaturated – a solution that CAN hold more solute. Super Saturated • Sometimes, a solvent can be made to hold more solute than normal. – The solution is called “supersaturated”. – Definition: supersaturated – a solution that has more solute than normal. Super Saturated • Supersaturated solutions are unstable. – The extra solute can “fall out” at any time.
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