Invisible Heart Activity (Valentine’s Day STEM for Kids)

The magic behind this Invisible Heart Activity is actually homemade invisible ink. Although this recipe is usually used for creating a secret message and hidden pictures, I wanted to use this simple chemical reaction for a Valentine’s STEM activity instead. 

I used this DIY invisible ink to create a version of the cardiovascular system that would magically appear when my kids painted over their little human figures. It was a great way to show the kids some of the inner workings of the human body. As well as gives you the perfect opportunity to talk about the heart, blood vessels, and why they are so important.

This is a great activity if you’re exploring a Valentine’s day or Human Body theme in your classroom. Read the step-by-step instructions below and learn how to make invisible ink for younger children.

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Disclosure: Adult supervision is required for all activities at all times. Some of the links provided in this blog are affiliate links. I will be paid a small commission if you use this link to make a purchase.

More Valentine STEM Activities to try

If you’re looking for more Valentine’s day science experiments, then check out these fun ideas. Kids of all ages will have a great time learning with these hands-on educational activities.

1. Free Valentines addition worksheet
2. Free Valentine’s Pattern printable for kids
3. Valentines Sort By Size
4. Valentine’s Candy Hearts Building STEM
5. Light Up Circuit Valentine’s

For more ideas, take a look at these 8 Valentines STEM for Kids.

Materials Needed

1. 1 Tbsp Baking Soda
2. 1/2 cup warm water
3. 4 squirts of hand sanitizer
4. Pinch of Turmeric
5. Watercolor Paper
6. Paintbrushes or q-tips

Watch the Video to See it in Action

How to make an Invisible Heart Activity

While I used homemade invisible ink to make a cardiovascular system, remember that this recipe is still a great way to make hidden love notes. Write kind words in a note to your kids to make this special day a whole lot of fun.

Make the Invisible Ink

1. Combine the baking soda and warm water in a bowl and mix until the baking soda has dissolved. 

Using warm or hot water will help the baking soda to dissolve more than cold water will.

2. Wait for the water to cool down a bit before using it to make your picture.

This isn’t essential but it is helpful. If you wait for the solution to cool down first, some of the hot water will have evaporated away. Leaving behind a supersaturated solution. In the end, this just means that the color of your blood vessels will be more intense.

It also means that it is safer for your kids to use. I let my kids make their own hidden message while I was making the body pictures. That way they could enjoy all of the steps of the entire process without ruining the surprise reaction in the final step.

Make the Human Body Outline

3. Draw the outline of a human silhouette onto the watercolor paper.

If you are doing this activity with a class, then I recommend having one large body outline for small groups of children. That way 4 – 6 kids can work together on one picture, instead of having their own. 

It’s the perfect way to cut down preparation required for this STEM lesson, while still allowing the kids to have fantastic fun.

4. Use a paintbrush to paint a heart shape, and blood vessels throughout the body.5. Let your picture dry for 10 – 15 minutes.

The amount of time it takes to dry will depend on the weather and how much baking soda solution you used. If you are doing this activity in a cold climate, it will probably take longer to dry.

Make the Reagent

5. To make the reagent mix the hand sanitizer and turmeric together.

Remember that turmeric stains, so be careful that the kids little hands don’t spread it everywhere.

6. Paint the reagent over the picture to reveal the hidden blood vessels underneath.

This is the best part because you get to see the invisible ink show. You’ll find that the ink turns red while the rest of the paper turns yellow. It’s such a fun way to get kids excited about science and learning.

Tips and Tricks

I love how well this activity worked out, especially since you only need a few simple supplies to do it. However, I would like to change a couple of things. 

1. I wish that I had used a smaller paintbrush to make the blood vessels, or that I had made them less condensed.

2. I also wish that I had made the heart bigger, or at least more obvious. So that when I talked to the kids about the cardiovascular system, the heart would stand out and highlight that it is the major organ.

In the end, we all had so much fun seeing the surprise pictures created with invisible ink. This is definitely an activity I will repeat next valentines day.

Are you going to try this heart activity for kids?
Don’t forget to Pin the idea for later.

More Play Based Learning for Kids

25 Fun Valentine’s Day Activities for Toddlers and Preschoolers

42 STEM Activities for Kids