To test the strength of baking powder -

Mix 1 teaspoon baking powder with 1/3 cup hot water.
If it bubbles it's still active and it will still make your baked goods rise.

To test the strength of baking soda -

The mixture will bubble immediately if the baking soda is still fresh and effective.

To test the rising ability of of dry yeast -

Add 1 teaspoon sugar to one cup of water.
Sprinkle a small amount of dry yeast on top of the water.
If it bubbles in 5 to 10 minutes, the yeast is still fresh and active.

The expiration date on a packet of yeast is the date past which the activity may have diminished.
You can still safely use it after the expiration date, but the time required for rising will be longer.

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