Unit 6M.1: Dissolving in water. Different solvents dissolve different substances. Science skills: Classifying Observing Predicting By the end of this unit you should: List factors affecting dissolving Give examples of solvents 75
6.11.1, 6.11.2, 6.11.3 Materials Dissolving in water When you add sugar to a cup of tea, the sugar seems to disappear. You know it is still there because you can taste it. The sugar hasn t vanished, it has UdissolvedU. The substance that dissolves is called the UsoluteU (sugar). The liquid that it dissolves in is called the UsolventU (water). The mixture of solute dissolved in solvent is called a UsolutionU (tea). How to increase the rate of dissolving? To make the sugar dissolve faster, you can do the following: 76
How much sugar can you dissolve in water? Could you go on adding more and more, making the solution stronger and stronger? The answer is no. You would find that no more sugar will dissolve, no matter how long you stirred the mixture. When this happens, the solution is UsaturatedU. So there is a limit to how much solute can dissolve in a solvent. UMaking a saturated solution of salt in wateru. A solution is saturated when no more solute can be dissolved and some solid remains at the bottom. 77
Factors affecting dissolving Aunt Fatima likes sugar in her tea, but wants the sugar to dissolve quickly. She has heard that some types of sugar dissolve more quickly than others. Help her. Activity 1: Does the size of the sugar make a difference to how quickly sugar dissolve? Materials : 2 beakers, Spoon, Stop clock, Sugar cubes, Sugar grains, Hot water Procedure: 1- Weigh out 10 g of sugar cubes and sugar grains. 2- Measure 50 ml of water into each of two containers. 3-Add the cubes to one container and the grains to the other container. 4-Stir both containers at the same speed. 5-Time how long it takes for the sugar to dissolve in both containers. 78
Type of sugar Sugar grains Sugar cubes Time taken to dissolve (second) What do you observe? ------------------------------ Conclusion: --------------------------------------- Activity 2: Does stirring make a difference to how quickly sugar dissolves? Materials: Sugar 2 beakers Water Glass stirring rod Procedure: 1-Measure 50 ml of water into each one of the two containers. 2-Add equal amounts sugar to each container of water. 3-Stir the first container but leave the other unstirred. 79
4-Time how long it takes for sugar to dissolve in both containers. Time taken to dissolve (second) sugar stirred sugar not stirred What do you observe? ------------------------------ Conclusion: ---------------------------------------- ------------------------------------------------ Activity 3: How temperature affects how quickly sugar dissolves? Plan an investigation to find out how the changes of water temperature affect how quickly sugar dissolves. Prediction: I expect that: ------------------------------------- ------------------------------------------------- 80
What I will do -------------------------------------------------- -------------------------------------------------- -------------------------------------------------- -------------------------------------------------- -------------------------------------------------- -------------------------------------------------- Results: Temperature of water ( C) Time taken to dissolve (second ) Use your results to draw either a bar chart or a line graph. (Do this on metric paper) 81
Conclusion: My results show that -------------------------------- ------------------------------------------------- Activity 4: Does the type of solute affect how much will dissolve? Material: Bicarbonate of soda Salt, Sugar Six tea spoons Three clear plastic tumblers Water Sticky label and pen Procedure: 1- Label each of the tumblers (sugar), (salt), etc. half fill them with water and place a teaspoon in each. 2- Add a teaspoon full of sugar to the tumbler labeled sugar stir until the solid has dissolved. Now repeat this step in the other tumblers using bicarbonate of soda and salt. 82
3-Add more solid to each tumbler until no more will dissolve, count how many spoonfuls of solid dissolve in each tumbler. Results Substance No. of Spoonfulls What do you observe? ------------------------------------------------- ------------------------------------------------- Conclusion: ------------------- ------------------------------ ------------------------------------------------- So, factors affecting dissolving are: 1- Heating 2- Size of grains 3- Stirring 83
Activity: Which gas dissolved in air? Material: Tap water beaker Bunsen burner Producer: 1- Heat some tap water in a glass beaker. 2- Observe what Observation: 1- Before the water boiling. ------------------------------------------- 2- What do you think the bubbles contain? ----------------------------------------- 3- How can you investigate the type of gas produced? --------------------------------------------- 4- Collect the gas produce? What is the smell of the collected gas? ----------------------------------------------- 5- Where do you think the gas came from? ----------------------------------------------- Both nitrogen and oxygen come from the air, but oxygen is more soluble than nitrogen, so the gas that is boiled out of the liquid may be as much as 50% oxygen. The dissolved oxygen is essential for the survival of many organisms which live in water. 84
6.11.6 Materials Different solvents dissolve different substances Water is a very common solvent, but it is not the only one. There are many substances that are insoluble in water, but dissolve in other liquids. If you get grease or oil on your clothes, you can t wash it out with Uonly wateru because grease and oil are insoluble in water. But grease and oil dissolve in methylated spirits. Methylated spirits can be used to clean grease off things. Sugar and salt dissolve in water but will not dissolve in petrol. 85
Some substances, such as nail varnish, do not dissolve in water. Nail varnish is insoluble in water. If you want to remove nail varnish then you have to use a liquid called acetone. This is another type of solvent. The nail varnish is soluble in acetone. Which solvent will you l use? A marker pen has been used to write on the desk, how can you remove it? Which liquid removes the dye in the picture? ------------------------------------- Does the dye dissolve in water? -------------------------------------- 86
Which liquid is a good solvent for dye? -------------------------------------- Some felt-tip pens are filled with water proof ink. You can use them to label beakers or test tubes. The ink is insoluble in water, so it does not dissolve when it gets wet. To remove the ink you have to use alcohol, as a solvent since the ink is soluble in alcohol. 87
Key words Solvent Solute Solution Suspended React Do you know? Some clothes have to be dry-cleaned. The clothes are sprayed with a special solvent that dissolves grease and dirt. The solvent then evaporates, leaving the clothes clean. This is useful for cleaning fabrics that would be damaged by soaking them in water detergent. Key idea Some solids dissolve in water to form a solution. The water is the solvent, and the solid is the solute. If no more solid will dissolve, the solution is saturated. There is a limit to how much solute a solvent can take. You can speed up dissolving by crushing the solid, heating or stirring the liquid. 88
Key question 1-Write the scientific term for: a- The solid that dissolves. ( ) b- The liquid that does the dissolving. ( ) 2-There are three things you can do to speed up dissolving. What are they? 1- ----------- 2- --------- 3- ------------ 3-Some breakfast cereals have a frosting of fine grains of sugar. Why do these cereals taste sweeter than ordinary cornflakes that have been sprinkled with granulated sugar? ------------------------------------------ 4- A beaker contains water. It is on a balance. The balance reads 200.0 g. Nisreen adds 10.5 g of salt to the water. The salt dissolves. When all the salt has dissolved, what is the reading on the balance? -------- g Nisreen wants to get all of the solid salt back from the water. Describe how she could do this. ----------------------------------- 89