which of the following is a reliable way to observe that a chemical changed occurred when two substances come in contact with one another?

I don't see any of "the following".

However, production of heat, change of color, change in odor, formation of gas, formation of precipitate, are reliable indicators of a chemical change.

Well, one reliable way is to invite all the chemicals to a fancy party and see if they've changed into different outfits! 🎉 But on a more serious note, there are a few ways to observe chemical changes. You can look for the formation of a precipitate (when a solid forms in a liquid), the production of gas (bubbles!), a change in color (like when your favorite shirt mistakenly meets bleach), or even a temperature change (although be careful not to burn yourself, unless you're a fan of hot hands!). So keep an eye out for these signs and witness the magic of chemical transformations! ✨

There are several reliable ways to observe a chemical change when two substances come in contact with each other. Here are a few methods:

1. Color Change: If there is a noticeable change in color after the substances are mixed, it could indicate a chemical change. For example, if a colorless substance turns blue or a white substance turns yellow, it suggests a chemical reaction has occurred.

2. Formation of Precipitate: If two clear solutions are mixed and a solid substance forms, called a precipitate, it suggests a chemical change. The precipitate is evidence of a reaction between the two substances.

3. Gas Formation: If there is a release of gas, such as bubbles or fizzing, it indicates a chemical change. Examples include the release of carbon dioxide gas during the reaction of an acid and a carbonate compound.

4. Temperature Change: A significant change in temperature, either an increase or decrease, can be a sign of a chemical change. For example, if two substances are mixed, and the mixture becomes noticeably hotter or colder, it suggests a chemical reaction has occurred.

5. Odor Change: A distinct change in smell after the substances are mixed could indicate a chemical change. New odors may be produced as a result of the reaction.

It's important to note that these observations alone do not confirm a chemical change conclusively. To make a more accurate determination, additional experiments and analyses might be required.

A reliable way to observe a chemical change when two substances come in contact with each other is to look for one or more of the following indicators:

1. Formation of a precipitate: A precipitate is a solid substance that forms when two solutions are mixed. If you notice the appearance of a solid material in the mixture, it suggests that a chemical reaction has taken place.

2. Color change: If there is a noticeable change in the color of the mixture after the substances are combined, it could indicate a chemical change. For example, a clear solution turning yellow or a colorless solution becoming red could be signs of a chemical reaction.

3. Evolution of gas: The release of gas bubbles or the observation of gas being produced is a strong indication that a chemical change has occurred. You might observe effervescence, bubbling, or the formation of a gas that can be seen or smelled.

4. Temperature change: In some cases, a chemical reaction can lead to a significant change in temperature. This change could be an increase (exothermic) or a decrease (endothermic) in temperature. If you can feel a noticeable difference in the warmth or coolness of the substances, it suggests a chemical change has taken place.

5. Change in odor: A change in smell can also be an indicator of a chemical reaction. If there is a noticeable difference in the odor of the substances or the formation of a new smell, it could indicate a chemical change has occurred.

It is important to note that these indicators are not definitive proof of a chemical change on their own. To confirm a chemical reaction, it is often necessary to perform further tests or analyze the reaction using scientific methods and techniques.