WAEC Chemistry practical question concerning tests on Sample C, involving heating, and reaction with dilute nitric acid.

Let's check your work on the WAEC Chemistry practical question. You've made good progress!

Here's a review of your observations and inferences:

a) Test: Sample C + heat strongly + cool

• Your Observation: "A black substance with an irritating/burning smell. Charring has taken place."
  • ✅ Your observation of charring and a black substance is correct. To be more complete, you should also observe if a colorless liquid condenses at the cooler part of the test tube and if a gas is evolved that turns lime water milky.
• Your Inference: "Starch is present."
  • ❌ While starch is an organic compound that chars, the inference from charring alone is the presence of an organic compound. The specific presence of starch is confirmed by the iodine test in part b(ii). If a gas turning lime water milky was observed, then carbonate or oxalate could also be inferred.

b) Test: Sample C + $2 \text{ cm}^3$ of dilute $\text{HNO}_3$ + Stir + filter

• Your Observation: "An effervescence of a colorless, odorless gas is evolved."
  • ✅ This observation is correct. For full completeness, you would also state that the gas turns lime water milky.
• Your Inference: "$\text{CO}_2$ gas from $\text{CO}_3^{2-}$ present is liberated to give a white solution containing an insoluble substance."
  • ✅ This inference is correct and well-stated.

Here are the more complete answers for the parts you filled in:

a) Test: Put about half a spatula of C in a boiling tube and heat strongly. Allow it to cool.

• Observation: The sample chars, a black substance is formed with an irritating/burning smell. A colorless liquid condenses at the cooler part of the test tube, and a gas is evolved which turns lime water milky.
• Inference: Presence of an organic compound (chars), water of crystallization or constitution (colorless liquid), and a carbonate or oxalate (gas turns lime water milky, indicating carbon dioxide, $\text{CO}_2$).

b) i) Test: To the remaining portion of C, add about $2 \text{ cm}^3$ of dilute trioxonitrate (V) acid, $\text{HNO}_3$. Stir well to dissolve and filter, keep both the filtrate and the residue.

• Observation: Effervescence occurs, a colorless, odorless gas is evolved which turns lime water milky. Part of the sample dissolves, leaving a white residue.
• Inference: Presence of carbonate ($\text{CO}_3^{2-}$). The gas evolved is carbon dioxide ($\text{CO}_2$). The sample contains both soluble and insoluble components.