Periodicity of Metals - Teacher

Periodicity of Metals (With Alkaline Earth Metals)What is that white gunk on the sides of my glass after I wash it?

Standards Addressed: 3a, 3d, 3e and 7b.

Purchasing (prices from 2012):FLINN SCIENTIFIC

C0345 Calcium, turnings, 10g 6.05L0024 Lithium, stick, 10g 30.71M0212 Magnesium, granules, 100g 15.85S0232 Sodium, reagent, 25g 19.21U0001 Universal Indicator, 100mL 7.85

GALLIUMSOURCE.com

020 Potassium, 10g 85.00050 Strontium, 100g 45.00

Notes:1. Magnesium and calcium turnings are easily dispensed, although the calcium must be

protected from excessive oxidation. Strontium granules (as supplied from Aldrich) are too big and very time intensive to cut. We are currently looking for a source of strontium ribbon.

2. Calcium and strontium both react with water almost instantaneously with water, so the added vinegar makes a slight difference in reactivity.

3. Although students can safely work with lithium under no circumstances should they work with sodium or potassium.

4. Sodium and potassium should always be purchased in very small quantities. Flinn Scientific sells precut demonstration sized pieces of sodium and potassium. Sodium and potassium demonstration pieces should be about the size of a lentil.

5. This demonstration should be practiced several times before being done in front of the class. If the metal sample is to big it will pop/explode out of the beaker sending molten metal a few feet in all directions. The molten metal is severely caustic. Many instructors place a safety shield around the demonstration to keep the metal fragments away from students.

6. If a piece the size of a walnut is used the metal will violently explode sending molten metal and glass (from the beaker) ten to twenty feet in all directions. The demonstration is safe, if small pieces (the size of a lentil) are used.

© Stanford UniversityPage 1 of 6

Teacher Notes

Periodicity of Metals (With Alkaline Earth Metals)What is that white gunk on the sides of my glass after I wash it?

Background:Many people these days filter their water or drink bottled water. Why? You may also have heard of “hard” water. What does that mean and how does it happen. Water is filtered or softened in order to reduce the amount of the effect of dissolved minerals in the water. Some of the most common of these are the alkaline earth metals, particularly magnesium and calcium. These are the elements responsible for that white film that ends up on glassware after washing. It is the result of the reaction of pure metallic elements with water to produce relatively insoluble compounds of the metals called hydroxides.

Purpose:We will investigate three elements in the same family of the periodic table. Calcium, magnesium and strontium are in the alkali-earth metal family. Look at a periodic table and locate these three elements in relation to each other.

Materials required (student group) magnesium, calcium, strontium

turnings ~0.1g piece of lithium Vinegar Universal indicator solution deionized water balloon

rubber stopper with attached delivery pipette

50mL graduated cylinder 5- 100mL beakers mass balance 3- 50 mL beakers Stopwatch or timer of some sort

Materials required (instructor):Small pieces of sodium and potassium for demonstration.2 large beakers for sodium and potassium demonstration.

Part A: Alkaline Earth MetalsQ1. Examine the three different metal samples. Identify some similarities and differences amongst the three samples. They are solid and fairly hard. They look different.

Procedure:1. Into each of the three 50 mL beakers measure 35mL of deionized water2. Add 20 drops of universal indicator and 5 drops of vinegar to each of the beakers from

step 1.3. Place 2 large pieces of calcium into the first 100mL beaker.4. Place 2 similar sized pieces of magnesium into the second 100mL beaker.5. Using tweezers, obtain 2 similar sized pieces of strontium and use a tissue to clean the

oil off of them. Place the cleaned strontium in the third 100mL beaker. At this point you should have three dry 100mL beakers containing different metal samples and three


Teacher Key

identical 50mL each containing 35mL of water, 5drops of vinegar and 10 drops of indicator. The smaller 50mL beakers should be red at this point. Confirm this before moving on.

6. This step requires some teamwork. Read completely before executing. One team member should be responsible for timing and announcing start of this step. Each of the other team members should be responsible for one of the pairs of metals and solution. On the timers mark all team members should pour their vinegar solution into the 100mL beaker containing their metal sample. The timer should start timing at this point as well. Each team member should start swirling as soon as the vinegar solution is poured into the metal. When a purple-blue color forms the reaction is considered over. Record the time at this point. Fill in the table below.

Element Indicator Response Time

Observations (Evidence of Reaction) Reactivity Ranking

Magnesium 64,800 seconds Fine stream of bubbles 3

Calcium 20 seconds Bubbles after 10 seconds 2

Strontium 8 seconds Bubbles immediately 1

Q2 What evidence do you have of a reaction for all three metals? They all produced bubbles and they changed the indicator blue, although the magnesium took a day.

Q3 Describe the difference between the three metals in terms of the reaction evidence in Q2. Strontium produced bubbles of gas immediately and the indicator changed color in 8 seconds. The calcium reacted the same way, but was a little slower. The magnesium produced a fine stream of barely visible bubbles and took a day to change the color of the indicator.

Q4 Rank the three metals in terms of the speed of their response time for the color change and the evidence you identified in Q2. Do these rankings match? Strontium changed the indicator quickly and the magnesium was the slowest. Strontium produced bubbles quickly and the magnesium was very slow. In both cases the calcium was intermediate. In both cases, Strontium ranked #1, calcium #2 and finally magnesium #3.

Q5 What is the ratio of response time between strontium and calcium. 8:20 or 2:5.

7. At this point you may have noticed that the magnesium still hasn’t changed color and may be wondering if it really is reacting in the same manner as the other two. Remember that in the background section we defined a buffer as a barrier so we’re going to check now if the magnesium reacts with water in the absence of the barrier. Pour about 40mL of deionized water into a new 100mL beaker and add 10 drops of the


indicator solution. Use forceps (tweezers) to pick one of the pieces of magnesium out of the reaction beaker and transfer it to the new water and indicator solution.

8. Observe the solution near the piece of magnesium. Does it change color in the same manner as the calcium and strontium did earlier?Yes, the Mg changed after a day.

Part B: Solubility of Alkaline Earth Metal HydroxidesSolubility can most simply be imagined as how much of one material will dissolve into a given amount of solvent. One unit for measuring this is parts per million (ppm) meaning one particle of the dissolved thing in every million particles in the mixture. Solubilities for the products of the reaction in Part A are as follows: magnesium hydroxide—2 ppm, calcium hydroxide--- 193 ppm, and strontium hydroxide---775 ppm. These metals may react with water easily, but the resulting compounds often fall out of solution and remain as white films, rimes or crusts on pipes, glassware or pretty much anything they come in contact with.

Q6 Examine the solutions containing the calcium and strontium. Comment on the clarity of each solution (is it clear and transparent or cloudy) and the solubility of the hydroxides of each metal. The strontium solution is clear and blue, while the calcium is blue and cloudy.

9. Cover the strontium containing beaker with parafilm leaving a small opening that a delivery pipette might fit through. (This is to form a splash shield for step 11)

10. Inflate a balloon and without letting it deflate insert the delivery pipette or rubber stopper into the inflation tube of the balloon.

11. Insert the delivery tip of the delivery pipette into the strontium containing solution while maintaining a seal on the balloon and then slowly release the air in the balloon as bubbles into the solution.

Q7 What happens to the clarity of the strontium solution when the carbon dioxide in your breath is bubbled through it? It also becomes cloudy.

Q8 What are the ratios of solubilities for each of the hydroxides? Magnesium (2ppm), Calcium (193ppm) and Strontium (775ppm), so the ratio is 2 : 193 : 775.

What metallic elements do you use at home? Iron is used as an inexpensive, strong metal that kitchen pans are made of. Iron will easily rust, if it is damp and stay wet so it is said to be moderately reactive.Gold and silver are used for jewelry, because they do not rust or tarnish, like iron does, and so are considered unreactive. Some metals are even more reactive than iron. Is it possible to predict how reactive a metallic element is compared to another? The far left side of the periodic table contains elements that are very reactive. They react with oxygen and water.


Q9 How do the ratios of solubilities compare to the ratios of response times from Part A?The strontium was clear, which meant that the hydroxide was soluble, which makes sense with a solubility of 775 ppm. The calcium was cloudy, because not all of it dissolved (193ppm).

Q10 Using the idea that a faster response time indicates more reactivity. Rank the reactivity of the Alkaline earth metals and compare that to solubility of the hydroxides. Is there a correlation? Yes, the more reactive the alkaline-earth metal, the more soluble the hydroxide.

Part C: Alkali MetalsQ11. Let’s shift our attention from the alkali-earth metals to the alkali metals. Locate the elements lithium, potassium and sodium on the periodic table. Based on your conclusion with your work involving calcium, magnesium and strontium predict a ranking for the reactivity of lithium, sodium and potassium.

12. Obtain a 100mL beaker and fill it half full with water and add ten drops of universal indicator solution. Drop a piece of lithium metal into the water and record your results in the table below.13. Your instructor will demonstrate the reactions of sodium and potassium metals with water. Record your observations of sodium and water reacting with each other in the table below. Repeat with potassium and water.

Element Indicator Response Time

Observations Reactivity Ranking

Lithium 4 seconds Produces bubbles very quickly. 3

Sodium Immediate Floats on water and moves around fizzing 2

Potassium Immediate Same as sodium, but catches on fire. 1

Q12. Was your prediction ranking the reactivites of lithium, sodium and potassium correct? Explain. My prediction was correct. Strontium was the most reactive in the alkaline-earth metal family and it was at the bottom of the column. Magnesium was the least reactive and at the top of the column. I assumed the same thing would happen in the alkali metal family. Potassium would be the most reactive and lithium the least reactive of the three metals.

Q13. For each of the following pairs of metals describe how the reactivity changes from the alkali earth metals (2nd column) to the alkali metals (1st column).

Magnesium to sodium Sodium was the most reactiveCalcium to potassium Potassium was the most reactive

Q14. Based on the results of this activity write a rule explaining how the reactivity of metals is related to their position on the periodic table. (Specifically relate change in reactivity to


movement both horizontally and vertically.) As you move down a column the reactivity of the element increases. As you move horizontally to the left the reactivity of the element increases.


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Periodicity of Metals - Teacher