Properties of materials Dissolving What is a solution? • A solution is a mixture of two or more substances (solid and liquid). • A solution is formed by a solute and a solvent. • A solute is a substance that dissolves • A solvent is a substance in which a solute dissolves. Examples • Sugar and water • When you place sugar in water, the sugar seems to disappear. • It does not disappear. It is still there. • The sugar particles have just spread out among the water particles Cont • All solutions are transparent, means you can see through • Transparent does not mean colorless. • Colored salts such as copper sulfate, form colored solutions • These colored solutions are transparent • A liquid such as milk is not transparent. • You cannot see through. • It is opaque. This means that milk is not a solution. Dissolving and melting • Dissolving is a complete mixing of solid particles in a liquid to form a solution • Melting is a change of state (a physical process) where a substance change from a solid to a liquid. • These two terms are completely different • In dissolving we need two substances (a solute and a solvent) while in melting only one substance. • In melting there is change in temperature while in dissolving it is not a must. Examples Dissolving and mass • A salt in water does not disappear, therefore the mass of solution is equal to the total mass of solute and solvent. • This is true for any solution • Mass of solute + mass of solvent = mass of solution • No mass has been lost. • The mass has been conserved(stay the same) Questions 1. In a solution of sugar and water, which is the solvent and which is the solute? 2. What is the difference between melting and dissolving? 3. What mass of salt solution is made when 9g of salt is dissolved in50g of water? Explain how you worked out your answer 4. A green powder was placed in a beaker of water. After it was stirred the water looked cloudy and lumps of powder could still be seen. Has a solution been formed? Explain your answer 5. When measuring the volume of a liquid, what should you do in order to make your measurements as accurate as possible? Solutions and solubility • A solution is said to be concentrated when there is a large mass of solute dissolved and diluted when there is a small mass of solute dissolved. Solubility • A solid that dissolves in a liquid is said to be soluble. • Sugar, copper sulfate and sodium chloride are examples. • A solid that does not dissolve in a solvent is said to be insoluble. • Example Iron filings are insoluble in water • Saturated solution is the one with a maximum amount of a solute (no more solute can dissolve) • Some substances are more soluble than others in the same solvent and same volume. • They are said to have greater solubility Cont • Example • If you have 100cm3 of water, you would be able to dissolve a lot of sodium chloride but little lead chloride. • Solubility of different solutes can be compared by measuring ow much of each can dissolve in a known amount of solvent Comparing solubility Questions 1. 2. 3. 4. 5. What is a saturated solution? How much copper sulfate will dissolve in 100g water at 20oC? How much potassium chlorate will dissolve in 200g water at 20oC? How much sodium chloride will dissolve in 50g water at 20oC? Use the data in the table to draw a bar chart to show the solubility of the various solutes in 100g water at 20oC. Temperature and solubility • Temperature affects solubility of salts • Most solutes dissolve more quickly in hot water than in cold water • In hot water particles receive more energy, vibrate more and start to move • You can dissolve greater mass of solute in hot water than in the same volume of cold water • This means that the solubility of most solutes increases with temperature Examples 1. If 204g of sugar dissolves in 100g of water at 20oC, in hot water (about 80oC), 362g of sugar can dissolve. 2. 204g of sugar dissolves in 100g of water at 20oC. a) how much sugar will dissolve in 200g of water at 20oC? b) how much sugar will dissolve in 50g of water at 20oC? Answers a) 204 x 2 = 408g b) 204g = 100g x = 50g Solubility of three salts Questions 1. 2. 3. 4. 5. 6. How much sugar can be dissolved in 250g of water at 20oC? How much more sugar can be dissolved if 250g of water is in 80oC? What is the general trend for the solubility of all three salts? What is the solubility of potassium nitrate at 45oC? Which of the three salts are the most soluble? Which salt is the most soluble at 80oC? Planning a solubility • When panning for an investigation, you must consider all the variables • A variable is a factor that can be changed in an investigation. • A variable can affect the result of an investigation • There are three types of variables. 1. Independent variable - the one you can change. 2. Dependent variable – the variable you measure 3. Control variable - the variable you keep the same Example • When you dissolve a salt in water at different temperatures, there are factors to consider (variables) • The factor to be changed is temperature • The factor to be measured is the amount of salt dissolves because this depends on temperature. • The factor to be controlled is the amount of water, they must use the same volume of water in all temperatures. • This will make the test fair. NOTE; in graphs, the independent variable appears on the horizontal axis (X- axis) and the dependent goes vertically (Y- axis) Planning for an investigation (solutions) 1. With a partner, discuss and write the differences between accurate results and reliable results. 2. Write the meaning of the following words; Variable, dependent variable, independent variable and control variables 3. Read the story of the three students on page 54 and explain in short what you understood. 4. Do questions 1 to 6 on pages 54 and 55.
