Hypothesis If we make invisible ink with other citrus fruits rather than just lemon, then we can have different types/shades/colors of invisible ink because other fruits have higher/lower Ph levels; which might affect the ink.

How does it happen? We are able to create invisible ink with lemons because lemons are organic, and that's the reason they have carbon compounds. When there is no heat those carbon compounds are colorless but when they are exposed to heat they oxidize, which then brings the darker color from the carbon components and you are able to see your secret message!

What happens to natural substances when they are heated ? When natural substances are heated they usually absorb heat causing particles to move faster, which also causes them to spread apart leading to physical change. Physical changes depend on the type of natural substances and the amount of heat, they normally include chemical change, temperature rise and change in state like melting and evaporating.

What role do sugars in fruit juice play in the darkening of invisible ink when heated?  Sugars in fruit juices are heat-sensitive compounds, so when heat is close to sugar molecules they break down and dehydrate, which then turns into complex Carmel compounds that bring the brown color.

What is acidity ? Acidity is how much a substance is acidic (sour); This usually gets measured by pH scale. pH scale is a scale to measure how much something is acidic or basic , it starts from zero and ends at fourteen. Zero to six is acidic (tastes sour), seven is neutral, and eight to fourteen is alkaline (basic). pH scale was invented by Søren P.L. Sørensen, who was a Danish chemist. He invented the pH scale in 1909 in Denmark. His purpose to create the pH scale was to quantify the alkalinity/acidity of solutions for brewing and biochemistry. The name pH stands for potential hydrogen.

Different fruits have different acidity levels because of environmental factors, and types of acids. Types of acids affect fruits because different fruits contain different acids like citric, malic, and tartaric; and specific combination of these acids decides the pH levels and the taste. The environment also affects the pH level because the soil/climate/influences the fruit's acidity. For example, when lemon trees are grown in highly acidic soil (Sandy loam soil), lemons  have a higher acidic level.

Why does heat trigger reactions? Heat triggers reactions when close to a substance because it increases the kinetic energy in the molecules, this makes the molecules move faster and collide with immense force.  The molecules overcome the activation energy barrier by breaking the existing bonds and forming new ones. This is how a heat triggering reaction occurs.

Heat is able to trigger two types of reaction, which are endothermic and exothermic. The endothermic reaction absorbs the energy from their surroundings,  causing the temperature around it to drop, while making its temperature rise. An example of an endothermic reaction is ice melting. The exothermic reaction releases energy into its surroundings while making its temperature low, and making the surrounding temperature rise. An example of an exothermic reaction is freezing water. The invisible ink experiment is an endothermic reaction. It is an endothermic reaction because it requires external energy to break bonds that already exist, which then causes the organic/invisible components to decompose them self because of the heat. This is abortion of energy which makes it an endothermic reaction.

What is in the fruits that make it acidic and allow us to use it as invisible ink? All fruits  (pomelo, lime, orange, and grapefruit) have citric acid, ascorbic acid and malic acid. These acids make the fruits acidic. Acids also help with making invisible ink because they have carbon components which weaken paper’s fiber, so when heat is applied to the paper it burns the areas of paper that are rich in carbon first, revealing the secret message.

The manipulated variable in this experiment is the ingredients (fruits) for the invisible ink.  The responding variable is going to be if I was able to create different types of invisible ink with other substances. The control variable is paper, water, temperature, iron, and Q-tip.

Procedure First gather all the materials. Then Squeeze the juice from the lemon. After take Q-tip and dip it in the lemon juice. After using the same Q-tip write a message on the thick white paper and let it dry completely.  When it is dry use the iron on top of the paper to reveal the secret message. Repeat it with other fruits!

Observation  Doing the experiment, the observation I made was that some of the fruits produced a strong smell like burning toast. Another observation that I made was the longer you put the Iron on the paper the more texture gets created.

Analyze The data showed that when it was fresh juices, it gave a faster reaction, and when I tried it an hour later it gave a slower reaction ( it took more time to dry and appear when heat was applied). The data also showed that texture and the shade of each invisible ink (fruit juice) did not change in any of the trials.

Conclusion To conclude, I was able to create different types of invisible ink. As I mentioned in my hypothesis, having different citrus fruits can create invisible ink in different shades because they'll have different pH levels. Which is what exactly happened, I was able to create different shades of invisible ink; where I observed that fruits with higher pH level had a darker shades of brown and the fruits that had lower pH level had lighter shade of brown. This happened because when heat was applied to fruit juices, the carbon compounds burned and I had lower/higher pH levels which gave different shades of brown.

Where do we use it ?        Invisible ink gets used for many reasons like security, and education. In security, invisible ink usually gets used by printing hidden features on important items like jewelry, tickets, prescription bottles, ids, currency, and other valuable documents. We print these hidden codes because you can’t see them with a naked eye but can see them with a UV scanner, verifier, and black light, so they give us the advantage of traceability and anti-counterfeit protection. It gives us traceability because of these hidden codes we will be able to track the original item from the supply chain. Also by printing these hidden features we can offer anti-counterfeit protection to businesses. In education, invisible ink is used for teaching chemistry and history. We learn chemistry from invisible ink by observing how carbon compounds and heat react when they come together. Invisible ink also teaches us history by telling how people used it during the wars for secret communication.

Sources of error There were two sources of error in my experiment, the first one was me forgetting to plug in the iron when I was making invisible ink for pomelo, which led to putting a greater amount of juices, making it look dirtier and perplexing. The second source of error was when I was writing with invisible ink (fruit juices) I wrote some letters that were large and other letters that were little, which is why some letters took way more time to dry than others

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Invisible Ink with Lemon | Chemistry Experiment | Science Buddies | https://www.sciencebuddies.org/ Accessed 3/29 Dec. 2025 ;27 Feb.2026

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UV-Readable Inks: Invisible Printing with Powerful Applications | What is UV-Readable ink ? | https://www.needham-ink.com/blog/uv-ink-uses#:~:text=Because%20these%20inks%20lack%20visible,an%20extra%20layer%20of%20security accessed- Dec 30, 2025/Dec 31, 2025/Jan 1, 2026/Jan 2, 2026/Jan 3, 2026/Feb 27, 2026

Fruit and vegetables: enzymic browning | Institute of Food Science and Technology | What is enzymic browning? | https://www.ifst.org/lovefoodlovescience/resources/fruit-and-vegetables-enzymic-browning#:~:text=What%20is%20enzymic%20browning?,and%20act%20as%20biological%20catalysts accessed - Jan 23, 2026/Jan 24, 2026/Jan 25, 2026/Feb 12, 2026   Invisible Ink STEM Activity: Spy Science Fun | I'm the Chef Too.| Oxidation/pH Indicators | https://www.imthecheftoo.com/blogs/stem-for-kids/invisible-ink-stem-activity-spy-science-fun?srsltid=AfmBOooX41MCsekRRb7OTcRIGMo2QVogPT8G8pcyzhseZ3kYNNBNtD0h#:~:text=Oxidation%20(Heat%2DActivated%20Inks):,phenolphthalein%20are%20common%20pH%20indicators accessed- Jan 24/25, 2026

I would like to acknowledge Manreet, Mr.Lee, my sister, and my parents. I would like to acknowledge Manreet for organizing science fair club for us, I would also like to appreciate her for guiding and helping me through out my project. I would like to acknowledge Mr.Lee for sharing his classroom with us so we can work on our projects. I would like to acknowledge my sister for helping me take photos while I was experimenting. I would like to thank my parents for buying the materials.