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When two solutions interact and create a new solid that isn't soluble in the liquid, it's known as a precipitate. This is a tell-tale sign that a chemical reaction has occurred. The new solid forms because the products of the reaction aren’t willing to stay dissolved in the liquid like the original substances were. Precipitate formation is a fascinating process that clearly shows a chemical transformation.
For example, if you mix barium chloride ( BaCl_2 ) with sodium sulfate ( Na_2SO_4 ), they create a white solid called barium sulfate ( BaSO_4 ). This white substance, the precipitate, separates from the solution and settles out.
A similar indication of a reaction is the formation of a gas. Take the case where hydrochloric acid ( HCl ) reacts with sodium bicarbonate ( NaHCO_3 ). This reaction liberates carbon dioxide ( CO_2 ), visible as bubbles forming in the mixture.

• Precipitate: A sign of an insoluble product forming.
• Gas formation: Indicates reaction via bubble observation.

Color changes are another fascinating indicator that a chemical reaction has taken place. Often, when chemicals react, the resulting products have different attributes than the starting materials, including color. The molecular changes alter how substances absorb and reflect light, which is what we see as color.
One classic demonstration of this is the reaction between potassium iodide ( KI ) and lead(II) nitrate ( Pb(NO_3)_2 ). When these two come into contact, a bright yellow solid called lead(II) iodide ( PbI_2 ) forms, clearly visible as a vivid color change.

• Color change: Caused by different light absorption/reflection.
• Color change example: KI and Pb(NO_3)_2 mix to form yellow PbI_2.

These changes can make chemistry visually exciting and provide clear evidence that chemical processes are at play.

Temperature changes offer another tangible clue that a chemical reaction has occurred. These changes can be either exothermic, where the reaction emits heat, or endothermic, where it absorbs heat.
Exothermic reactions, like magnesium ( Mg ) burning in air to form magnesium oxide ( MgO ), release heat, often making the surroundings feel warmer. You might sense this as warmth on your skin during a lab experiment.
In contrast, endothermic reactions absorb heat. Mix ammonium chloride ( NH_4Cl ) with water, and the reaction cools the solution, causing it to feel unusually cold.

• Exothermic: Releases heat, warms surroundings.
• Endothermic: Absorbs heat, cools surroundings.

These temperature changes can be easily measured and often accompany the sound, light, or even the fizzing seen in reactions.