I just bought a Bi-Carb Soda baking soda, Is this the one use for baking cakes? How if I mixed it to my cake?

No. Baking soda is a mixture of two ingredients, soda and cream of tartar which combine to make a raising agent. Baking soda is sometimes used in recipes - I have a date cake recipe where you soak the dates in the soda and hot water first. Baking soda dissolved in warm water is also good for wiping down fridges without leaving a smell.

That is a best by statement, yes it is ok to use.

Cake mix should have its own leavening, unless it is stale.

it is the ingredient that makes soda water
you use a very small quantity in a jug of water
it is a good drink for mild reflux

Baking soda.

Not sure about the yeast. If it is present it should be listed in the ingredients.

I suspect there will be no yeast in cake mix but if there is it will be dried yeast or be in a separate sachet.

I suspect the mix uses either self-raising flour or plain flour and a raising agent such as baking powder or bicarbonate of soda.

If it has been correctly stored and not allowed to become damp it could be used successfully several years after the best before date, though it is unlikely to be as flavoursome it won't harm you.

If the pack contents have become damp or they do not smell wholesome throw it away otherwise try your hand at baking a cake.

Ingredients

Original recipe makes 1 cake

• 2 cups white sugar
• 1 3/4 cups all-purpose flour
• 3/4 cup unsweetened cocoa powder
• 1 1/2 teaspoons baking soda
• 1 1/2 teaspoons baking powder
• 1 teaspoon salt

• 2 eggs
• 1 cup milk
• 1/2 cup vegetable oil
• 2 teaspoons vanilla extract
• 1 cup boiling water

• PREP20 mins
• COOK3 hrs
• READY IN3 hrs 50 mins

Directions

1. Spray crock of a large slow cooker with cooking spray.
2. In a medium bowl, whisk the sugar, flour, cocoa, baking soda, baking powder and salt. In a separate small bowl, whisk together the eggs, milk, oil and vanilla until well combined. Whisk in the boiling water. Pour the wet ingredients into the dry and mix well.
3. Pour the cake batter into the prepared slow cooker.
4. Set slow cooker to "Low". Cook until cake has no wet spots on top and has pulled away from the sides of the crock, about 3 hours. Turn off slow cooker and allow cake to rest for 30 minutes before serving.

There are lots of causes of cakes issues, from something simple like over cooking to over beating of egg whites.

Take a look at this link for a good guide to cake issues;

http://www.great-recipes.com/baking-tips/cake-baking-tips.html

helllo dis.... There are many things that can cause cakes to fall in the middle.... Jumping, etc., around the baking area is unlikely to be a problem except with very delicate sponge or angel food cakes. More likely, cakes fall when the crust appears to be done, but the batter is not baked through the middle. An uneven baking temperature is a frequent culprit; check the temperature of your oven with a separate baking thermometer, and be certain that your oven holds a steady heat through the entire baking period. Some older ovens preheat properly, then cycle off and drop the temperature after 15 or 20 minutes, which causes the uncooked portion of the cake to fall.

Less likely but possible problems could be inaccurate proportions of baking powder or baking soda if the cake is made from scratch. This could occur with cake mixes if sour milk or buttermilk is used instead of regular milk; the additional acid in sour milk requires additional baking soda to rise properly. Eggs that are not beaten properly, or perhaps beaten too much for the type of cake being made, might also cause problems. Joe

This is just one of many I found at this website http://www.lynettee.com/kidfood10.htm

Children's Chocolate Cake Mix
1 cup sugar
3 tablespoons unsweetened cocoa powder
1 1/2 cups all-purpose flour
1 teaspoon baking soda
1/2 teaspoon salt
1/3 cup vegetable shortening
In a medium bowl, combine sugar, cocoa powder, flour, baking soda and salt. Stir with a wire whisk until blended. With a pastry blender, cut in shortening until evenly distributed and mixture resembles corn meal. Spoon about 1/3 cup of the mixture into each of 11 small containers with tight fitting lids or Ziploc bags. Seal containers. Label with date and contents. Store in a cool dry place. Use within 12 weeks. Makes 11 packages of Children's Chocolate Cake Mix for Easy Bake style oven.

To Use:
1 package cake mix
4 tsp. water
Mix together and stir with a fork or spoon until blended and smooth. Pour mixture into greased and floured 4 inch round miniature baking pan. Follow directions for child's oven, or bake in mom's preheated 375* oven for 12 to 13 minutes. Remove from oven and cool in pan on a rack for 5 minutes.
Invert onto a small plate and remove pan. When cool, frost with Children's Chocolate Frosting. Serves 2 children.

Enjoy!