How to determine if an unknown is potassium chloride or potassium bromate?

In class, we were given unknown substances. I've narrowed the possible substances down to either potassium chloride or potassium bromate.
We were given several acids, indicators, and other chemicals (silver nitrate, calcium hydroxide, ammonium hydroxide) to test with. How could one tell them apart? Maybe with a precipitate reaction?

• Do you actually mean bromate and not bromide? Check out the solubilities of those compounds themselves and see if that gets you somewhere. – ericksonla Jun 8 '18 at 16:40

Noting the solubilities of potassium chloride and potassium bromate gives a nice point in the identification but apart from it, there are certain chemical test specific for certain anion that gives pinpoint information about the salt.

First is the chromyl chloride test for chloride ions. Perform the test and you will identify potassium chloride (potassium bromate will give -ve result to the test). Second is conc. $\ce{H2SO4}$ test. Potassium chloride will gives vapors of hydrogen chloride. Potassium bromate will give bromine gas.

$$\ce{KCl + H2SO4 -> HCl ^ + KHSO4}$$ $$\ce{4KBrO3 + 2H2SO4 -> 2Br2 ^ + 2O2 ^ + 4K+ + 2SO4^{2-} + 2H2O}$$

Third test is the heat test. Potassium chloride is stable to heat while potassium bromate is prone to decomposition $\ce{\approx 370°C}$.

$$\ce{2KBrO3 ->[\Delta] 2KBr + 3O2 ^}$$

About the silver nitrate test pointed by @Dallon Penney, precipitation will occur in both cases albeit depending on the concentration of respective ions, it seems that the precipitate forming has different properties. Silver chloride exist as a white, curdy precipitate, insoluble in water and in dilute nitric acid while silver bromate exist as a white crystalline precipitate soluble in hot water and sparingly soluble in nitric acid. Both precipitates however are soluble in ammonia solution.

Solubility of Silver Bromate in water: 0.167 g/100 mL

Solubility of Silver Chloride in water: 0.00052 g/100 g at 50 °C.

Assuming room temperature conditions, if you have potassium chloride in your solution it will form a white precipitate when silver nitrate is added due to the formation of silver chloride:

Ag+(aq) + Cl-(aq) -> AgCl(s)

Unless the concentration of potassium bromate in your solution is over 0.167g/100 mL, adding silver nitrate will not produce any precipitate.

• It was quite nice until the last paragraph: the mass concentration of KBrO3 does not actually matter all that much, because it is AgBrO3 precipitating out, not KBrO3. Also you have a mixture of ions. Solubility products work much better here than mass concentrations – orthocresol Jun 8 '18 at 23:09

Put the unknown on filter paper and heat it. Sodium bromate, $\ce{NaBrO3}$, is a strong oxidizer and it's effect in speeding combustion should be obvious. $\ce{KCl}$, on the other hand, helps keep air off the surface and retards burning.