3M science at home: acids and bases

Acids and Bases

How can we tell whether something is an acid or a base?

Key Concepts

  • acids icon
  • bases icon
  • pH indicators icon
    pH indicators

  • Introduction

    When we study acids, many people immediately consider things like lemons and limes. Citrus fruit as a relatively high acid content compared to other things like milk, for instance. Stomach acid is a stronger example – it takes a powerful acid to help break down our food for digesting. If the pH balance of your stomach was less acidic, you would have a difficult time digesting solid food. Did you know that water – the same stuff that comes out of your tap, the local lake, and even the ocean – can all contain acid at varying levels? Scientists have discovered ways of measuring levels of acidity, or pH balance, using materials that change color based on a substance’s chemistry. Acids can be present naturally (like with citrus) or through chemical engineering.

  • Background

    The pH scale is a number that falls from one to fourteen. Zero to seven indicates an acid, with zero being the strongest acid. Hydrogen ions are numerous in acids. Eight to fourteen indicates a base, with fourteen being the strongest base. Hydroxide ions are numerous in bases. While acids are certainly dangerous at extreme levels, bases can also be dangerous at high levels. The body needs a neutral (in the middle) pH and controls the amount of acids and bases through processes involving the kidneys, lungs and blood so that they can function properly. What does pH indicate if it falls somewhere between the extremes? Does it take expensive equipment to test for acids? How do we know if an unknown substance is acidic or not? In today’s experiment, you will function as a chemical engineer and differentiate between acidic and basic materials.

  • Preparation

    1. To make the indicator, take one or two leaves from the cabbage and put them in the blender.
    2. Add about 1 cup of water for each leaf you use. Make sure to put the lid on the blender!
    3. Blend the cabbage into the water for about 30 seconds.
    4. Pour the liquid through a strainer into a container that is big enough for all the liquid.
    5. Compost or throw away the cabbage bits.
  • Procedure

    1. Now that you have an indicator liquid, you can use it to test how acidic or basic something is. Collect some things to test. Lemons, limes, oranges, and kiwis are good fruits to test. You should also collect some items like soap, vinegar, baking soda, sugar, and salt.
    2. Make a prediction: Which things that you collected are acidic, and which things are basic? Which things might be neutral? Try sorting things into groups based on your prediction.
    3. Pour a little of the indicator into a smaller container – something like an ice cube tray will let you compare lots of tests next to each other, or several small cups works well too.
    4. If you are testing a powder, like baking soda, sugar, or salt, stir a little bit of it into the indicator in your container. If you are testing a liquid, add a few drops to the indicator and stir it in.
    5. Observe what happens to the indicator. Try testing lots of different substances and see what happens.
  • Observation and Result

    You should see the indicator change color depending on what substance was added to it. If a substance is acidic, it will make the indicator turn pink or red. If a substance is basic, the indicator will turn lighter blue or green. Cabbage juice works well as an indicator because it contains a pigment called anthocyanin, which dissolves in water, and looks different when exposed to acids and bases. If the acid or base is strong, more of it will interact with the anthocyanin and the color will change more dramatically. If the acid or base is weaker (close to neutral), then the pigment will change less.

  • Clean Up

    Don’t forget to clean up when you are done. Any of the suggested testing substances can be washed down the drain or thrown away. Any leftover indicator can be poured down the drain. Wash any containers that you used, as well as the blender and strainer. Put any leftover substances you tested back where you found them.

  • More to Explore

    One fun thing to explore with pH indicators is the fact that liquids can become more acidic or more basic based on gases that get mixed into them. One way to mix a gas into a liquid is to seal it into a bottle and shake it. That’s not the only way though, and in fact blowing bubbles can work really well. If you blow bubbles into water with your cabbage indicator, does it change color? Whether it does or doesn’t, what does that say about the effect of the gasses you exhale? Do they make the water more acidic or basic, or have no effect? If there is a change, think about what you know about the gasses that we exhale, and see if you can come to any conclusions about their effect on a liquid. Of course, you can do some research and see if your conclusions match what other scientists have found.

    One other thing to know is that cabbage isn’t the only plant with anthocyanins. In fact, they’re very common in lots of plants, and even a lot of different foods. Fruits and seeds that are purple or purple-ish are a good place to look, such as various berries, grapes, purple corn, or black beans. Some red and blue foods also have them, like raspberries, cherries, and blue corn. Another place to look is leaves that turn red or purple-ish in the fall, since those colors are a sign of anthocyanins. Similarly, flowers with these colors are also a good place to look. Some of these plants might not work as well as cabbage for making a pH indicator, especially if the plant itself makes the liquid acidic or basic, but you can use the same process you used here to try them out.

  • Safety First & Adult Supervision

    • Follow the experiment’s instructions carefully.
    • A responsible adult should assist with each experiment.
    • While science experiments at home are exciting ways to learn about science hands-on, please note that some may require participants to take extra safety precautions and/or make a mess.
    • Adults should handle or assist with potentially harmful materials or sharp objects.
    • Adult should review each experiment and determine what the appropriate age is for the student’s participation in each activity before conducting any experiment.

Next Generation Science Standard (NGSS) Supported - Disciplinary Core Ideas

This experiment was selected for Science at Home because it teaches NGSS Disciplinary Core Ideas, which have broad importance within or across multiple science or engineering disciplines.

Learn more about how this experiment is based in NGSS Disciplinary Core Ideas.

Physical Science (PS) 1: Matter and Its Interactions

Grades K-2

  • 2-PS1-1. Different kinds of matter exist and can be described by its observable properties.
  • 2-PS1-2. Different properties are suited to different purposes.

Grades 3-5

  • 5-PS1-1. Matter of any type can be subdivided into particles that are too small to see, but event then the matter still exists and can be detected by other means.

Grades 6-8

  • MS-PS1-1. Atoms form molecules that range in size from two to thousands of atoms. Molecules may be extended structures with repeating subunits.
  • MS-PS1-2. Each pure substance has characteristic physical and chemical properties that can be used to identify it.
  • MS-PS1-4. In a solid, atoms are closely spaced and may vibrate in position but do not change relative locations.

Grades 9-12

  • HS-PS1-1. Each atom has a charged substructure consisting of a nucleus, which is made of protons and neutrons, surrounded by electrons.
  • HS-PS1-2. The periodic table orders elements horizontally by the number of protons in the atom’s nucleus and places those with similar chemical properties in columns. The repeating patterns of this table reflect pattern of outer electron states.
  • HS-PS1-3. The structure and interactions of matter at the bulk scale are determined by electrical forces within and between atoms.
  • HS-PS1-4. Stable forms of matter are those in which the electric and magnetic field energy is minimized.

PS2: Motion and Stability: Forces and Interactions

Grades K-2

  • K-PS2-1. Pushes and pulls can have different strengths and directions.
  • K-PS2-2. Pushing or pulling on an object can change the speed or direction of its motion and can start or stop it.

Grades 3-5

  • 3-PS2-1. Each force acts on one particular object and has both strength and a direction. An object at rest typically has multiple forces acting on it, but they add to give zero net force on the object. Forces that do not sum to zero can cause changes in the object’s speed or direction of motion.
  • 3-PS2-2. The patterns of an object’s motion in various situations can be observed and measured; when that past motion exhibits a regular pattern, future motion can be predicted from it.

Grades 6-8

  • MS-PS2-1. For any pair of interacting objects, the force exerted by the first object on the second object is equal in strength to the force that the second object exerts on the first, but in the opposite direction.
  • MS-PS2-2. The motion of an object is determined by the sum of the forces acting on it; if the total force on the object is not zero its motion will change. The greater the mass of the object, the greater the force needed to achieve the same change in motion. For any given object, a larger force causes a larger change in motion.

Grades 9-12

  • HS-PS2-1. Newton’s second law accurately predicts changes in the motion of macroscopic objects.
  • HS-PS2-2. Momentum is defined for a particular frame of reference; it is the mass times the velocity of the object. In any system, total momentum is always conserved.
  • HS-PS2-3. If a system interacts with objects outside itself, the total momentum of the system can change; however, any such change is balanced by changes in the moment of objects outside the system.

Grades K-2

  • K-PS2-1. When objects touch or collide, they push on one another and can change motion.

Grades 3-5

  • 3-PS2-1. Objects in contact exert forces on each other.
  • 3-PS2-3. Electric and magnetic forces between a pair of objects do not require the objects to be in contact.
  • 3-PS2-4. The sizes of the forces in each situation depend on the properties of the objects and their distance apart and, for forces between two magnets, on their orientation relative to each other.

Grades 6-8

  • MS-PS2-3. Electric and magnetic forces can be attractive or repulsive, and their sizes depend on the magnitudes of the charges, currents, or magnetic strengths involved and on the distance between the interacting objects.
  • MS-PS2-3. Forces that act at a distance can be explained by fields that extend through space and can be mapped by their effect on a test object.

Grades 9-12

  • HS-PS2-4. Forces at a distance are explained by fields permeating space than can transfer energy through space.
  • HS-PS2-5. Magnets or electric currents cause magnetic field; electric charges or changing magnetic fields cause electric fields.
  • HS-PS2-6. Attraction and repulsion between electric charges at the atomic scale explain the structure, properties, and the constant forces between material objects.

Grades K-2

  • K-PS2-1. A bigger push or pull makes things go faster.

Grades 3-5

  • 4-PS3-1. When objects collide, the contact forces transfer energy so as to change the objects’ motions.

Grades 6-8

  • MS-PS3-2. When two objects interact, each one exerts a force on the other that can cause energy to be transferred to or from the object.

Grades 9-12

  • HS-PS3-5. When two objects interacting through a field change relative position, the energy stored in the field is changes.