In this activity, students carry out a chemical reaction in which two reactants (baking soda and hydrochloric acid) produce three products (sodium chloride, carbon dioxide gas, and water) and determine that the solid product (sodium chloride) is not the same as the solid reactant (baking soda). The hydrochloric acid, the water, and the carbon dioxide gas are not accounted for by mass; the water and gas are driven off. Conservation of mass is not illustrated here.
In this lab activity, students will learn not only how to synthesize a compound but will also learn some basic tests to determine that the product is different from the starting chemicals (reactants).
||A chemical reaction in which the reactants have chemical properties that are different from those of the products can be identified through a number of chemical tests including solubility and chemical tests (reactions) of the reactants and products.
The following are the materials required for one team.
- 2.5 g sodium hydrogen carbonate (baking soda NaHCO3)
- 5.0 to 6.0 mL of 6M hydrochloric acid (HCl)
- Optional: 0.5M silver nitrate (AgNO3) solution in 25 mL dropper bottle
- Evaporating dish (ceramic), small jar or beaker (100 mL capacity, maximum)
- Test tube, 13 x 100 mm, with stopper for acid
- Dropper pipet, long enough to reach down into the acid solution
- Goggles, gloves (if available), and lab apron (if available).
||Students must be able to work carefully and safely in the laboratory. They must be able to read and interpret the instructions in the lab procedure.
||Student activities span two days, one day (Part I) to set up and carry out the reaction; the next day (Part II) to weigh the dry product, test the reactant and product, collect data, and confer with partners.
- Students must wear goggles at all times and inexpensive plastic gloves (available at any paint store), if available.
- You should dispense the hydrochloric acid to students in test tubes with stoppers.
- You should evaporate liquid from the dishes or jars away from the students.
|Student Ability Level and Grouping
||Middle school students working in pairs or in groups of 3 to 4.
- If there is limited equipment, such as balances, put the baking soda into individual containers (small cups) ahead of time for the students, just as you will be doing with the hydrochloric acid. Weigh the baking soda and label the containers with the mass of the baking soda (2.5 g).
- Set up an evaporation device that has good ventilation. Use a hot plate with the jars or dishes elevated slightly above the plate, using an oven rack. This will prevent cracking of the dishes or jars from the direct heat. Remember that there will be some acidic fumesso don’t use a toaster oven, unless you want to have it cleaned! Heating the evaporating dishes in a fume hood would be ideal; heating them near an open window is advisable.
- The chemical reaction in this exercise is:
Baking soda + hydrochloric acid = carbon dioxide gas + sodium chloride + water
NaHCO3 + HCl = CO2 + NaCl + H2O
- The solubility of the baking soda is greater than that of sodium chloride (main component of table salt), the only solid product.
- The solid product, sodium chloride, will not react with the hydrochloric acid.
- Testing baking soda and sodium chloride with silver nitrate solution produces a white, cloudy suspension (precipitate) with sodium chloride only.
- Comparing mass of baking soda used with the mass of sodium chloride produced could show greater mass of original baking soda than sodium chloride produced (84:58). But this will not be reliable for several reasons, including the extent to which the sodium chloride is dried and the skill of the students in massing their materials. Conservation of mass will not be evident since the mass of hydrochloric acid solution, carbon dioxide gas, and evaporated water are not accounted for. A discussion of these points might be helpful with the students as part of their basic physical science knowledge.
- To prepare 6M hydrochloric acid: Combine equal volumes of concentrated acid (12M) and water. Make sure you are wearing goggles. Acid is added to water, not the reverse! (“Do as you oughter... add acid to water.”) The solution will become hot. After it cools, it should be capped.
- To prepare 0.5M silver nitrate solution: Dissolve 12.8 g silver nitrate in distilled water to 150 mL volume.
- Prepare the setup for evaporation of reaction solution (hotplate and ventilation).
- If you do not have chemical-resistant countertops, cover table tops with newspaper to protect the surface.
- If students are to weigh out the reactants, they should be familiar with, or instructed in, the use of the balance.
- Students should also be cautioned about the safe use of acid.
||This is an opportunity for students to present their results and their evidence in support of having produced substances different from the starting reactants. Do students agree on the evidence? Why or why not?
||Evaluate the student lab notes and discussion with regard to what evidence supports their conclusions as to what products have been found.
This activity meets the following National Science Education Standards (Grades 5-8) and Curriculum Standards for Social Studies.
National Science Education Standards (Grades 5-8)
Unifying Concepts and Processes
- Evidence, models, and explanation
- Change, constancy, and measurement
- Form and function
Science as Inquiry
- Abilities necessary to do scientific inquiry
Understandings about scientific inquiry
- Properties and changes of properties in matter
Curriculum Standards for Social Studies